    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TupleSections       #-}
    {-# LANGUAGE RecursiveDo         #-}
    {-# LANGUAGE MultiWayIf          #-}
    {-# LANGUAGE RecordWildCards     #-}

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

    -- | Type subsumption and unification
    module GHC.Tc.Utils.Unify (
      -- Full-blown subsumption
      tcWrapResult, tcWrapResultO, tcWrapResultMono,
      tcSubType, tcSubTypeSigma, tcSubTypePat, tcSubTypeDS,
      tcSubTypeAmbiguity, tcSubMult,
      checkConstraints, checkTvConstraints,
      buildImplicationFor, buildTvImplication, emitResidualTvConstraint,

      -- Skolemisation
      DeepSubsumptionFlag(..), getDeepSubsumptionFlag,
      tcSkolemise, tcSkolemiseCompleteSig, tcSkolemiseExpectedType,

      -- Various unifications
      unifyType, unifyKind, unifyInvisibleType, unifyExpectedType,
      unifyTypeAndEmit, promoteTcType,
      swapOverTyVars, touchabilityAndShapeTest,
      UnifyEnv(..), updUEnvLoc, setUEnvRole,
      uType,

      --------------------------------
      -- Holes
      tcInfer,
      matchExpectedListTy,
      matchExpectedTyConApp,
      matchExpectedAppTy,
      matchExpectedFunTys,
      matchExpectedFunKind,
      matchActualFunTy, matchActualFunTys,

      checkTyEqRhs, recurseIntoTyConApp,
      PuResult(..), failCheckWith, okCheckRefl, mapCheck,
      TyEqFlags(..), TyEqFamApp(..), AreUnifying(..), LevelCheck(..), FamAppBreaker,
      famAppArgFlags, simpleUnifyCheck, checkPromoteFreeVars,
      ) where

    import GHC.Prelude

    import GHC.Hs

    import GHC.Tc.Utils.Concrete ( hasFixedRuntimeRep, hasFixedRuntimeRep_syntactic )
    import GHC.Tc.Utils.Env
    import GHC.Tc.Utils.Instantiate
    import GHC.Tc.Utils.Monad
    import GHC.Tc.Utils.TcMType
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Types.Constraint
    import GHC.Tc.Types.Origin
    import GHC.Tc.Zonk.TcType

    import GHC.Core.Type
    import GHC.Core.TyCo.Rep
    import GHC.Core.TyCo.FVs( isInjectiveInType )
    import GHC.Core.TyCo.Ppr( debugPprType {- pprTyVar -} )
    import GHC.Core.TyCon
    import GHC.Core.Coercion
    import GHC.Core.Multiplicity
    import GHC.Core.Reduction

    import qualified GHC.LanguageExtensions as LangExt

    import GHC.Builtin.Types
    import GHC.Types.Name
    import GHC.Types.Id( idType )
    import GHC.Types.Var as Var
    import GHC.Types.Var.Set
    import GHC.Types.Var.Env
    import GHC.Types.Basic
    import GHC.Types.Unique.Set (nonDetEltsUniqSet)

    import GHC.Utils.Error
    import GHC.Utils.Misc
    import GHC.Utils.Outputable as Outputable
    import GHC.Utils.Panic

    import GHC.Driver.DynFlags
    import GHC.Data.Bag
    import GHC.Data.FastString( fsLit )

    import Control.Monad
    import Data.Monoid as DM ( Any(..) )
    import qualified Data.Semigroup as S ( (<>) )

    {- *********************************************************************
    *                                                                      *
                  matchActualFunTys
    *                                                                      *
    ********************************************************************* -}

    -- | 'matchActualFunTy' looks for just one function arrow,
    -- returning an uninstantiated sigma-type.
    --
    -- Invariant: the returned argument type has a syntactically fixed
    -- RuntimeRep in the sense of Note [Fixed RuntimeRep]
    -- in GHC.Tc.Utils.Concrete.
    --
    -- See Note [Return arguments with a fixed RuntimeRep].
    matchActualFunTy
      :: ExpectedFunTyOrigin
          -- ^ See Note [Herald for matchExpectedFunTys]
      -> Maybe TypedThing
          -- ^ The thing with type TcSigmaType
      -> (Arity, TcType)
          -- ^ Total number of value args in the call, and
          --   the original function type
          -- (Both are used only for error messages)
      -> TcRhoType
          -- ^ Type to analyse: a TcRhoType
      -> TcM (HsWrapper, Scaled TcSigmaTypeFRR, TcSigmaType)
    -- This function takes in a type to analyse (a RhoType) and returns
    -- an argument type and a result type (splitting apart a function arrow).
    -- The returned argument type is a SigmaType with a fixed RuntimeRep;
    -- as explained in Note [Return arguments with a fixed RuntimeRep].
    --
    -- See Note [matchActualFunTy error handling] for the first three arguments

    -- If   (wrap, arg_ty, res_ty) = matchActualFunTy ... fun_ty
    -- then wrap :: fun_ty ~> (arg_ty -> res_ty)
    -- and NB: res_ty is an (uninstantiated) SigmaType

    matchActualFunTy :: ExpectedFunTyOrigin
    -> Maybe TypedThing
    -> (Int, TcType)
    -> TcType
    -> TcM (HsWrapper, Scaled TcType, TcType)
    matchActualFunTy ExpectedFunTyOrigin
    herald Maybe TypedThing
    mb_thing (Int, TcType)
    err_info TcType
    fun_ty
      = Bool
    -> SDoc
    -> TcM (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TcType -> Bool
    isRhoTy TcType
    fun_ty) (TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    fun_ty) (TcM (HsWrapper, Scaled TcType, TcType)
     -> TcM (HsWrapper, Scaled TcType, TcType))
    -> TcM (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a b. (a -> b) -> a -> b
    $
        TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    go TcType
    fun_ty
      where
        -- Does not allocate unnecessary meta variables: if the input already is
        -- a function, we just take it apart.  Not only is this efficient,
        -- it's important for higher rank: the argument might be of form
        --              (forall a. ty) -> other
        -- If allocated (fresh-meta-var1 -> fresh-meta-var2) and unified, we'd
        -- hide the forall inside a meta-variable
        go :: TcRhoType   -- The type we're processing, perhaps after
                          -- expanding type synonyms
           -> TcM (HsWrapper, Scaled TcSigmaTypeFRR, TcSigmaType)
        go :: TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    go TcType
    ty | Just TcType
    ty' <- TcType -> Maybe TcType
    coreView TcType
    ty = TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    go TcType
    ty'

        go (FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w, ft_arg :: TcType -> TcType
    ft_arg = TcType
    arg_ty, ft_res :: TcType -> TcType
    ft_res = TcType
    res_ty })
          = Bool
    -> TcM (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a. HasCallStack => Bool -> a -> a
    assert (FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af) (TcM (HsWrapper, Scaled TcType, TcType)
     -> TcM (HsWrapper, Scaled TcType, TcType))
    -> TcM (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a b. (a -> b) -> a -> b
    $
          do { HasDebugCallStack => FixedRuntimeRepContext -> TcType -> TcM ()
    FixedRuntimeRepContext -> TcType -> TcM ()
    hasFixedRuntimeRep_syntactic (ExpectedFunTyOrigin -> Int -> FixedRuntimeRepContext
    FRRExpectedFunTy ExpectedFunTyOrigin
    herald Int
    1) TcType
    arg_ty
             ; (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, TcType -> TcType -> Scaled TcType
    forall a. TcType -> a -> Scaled a
    Scaled TcType
    w TcType
    arg_ty, TcType
    res_ty) }

        go ty :: TcType
    ty@(TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
    forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case cts of
                   Indirect TcType
    ty' -> TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    go TcType
    ty'
                   MetaDetails
    Flexi        -> TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    defer TcType
    ty }

           -- In all other cases we bale out into ordinary unification
           -- However unlike the meta-tyvar case, we are sure that the
           -- number of arguments doesn't match arity of the original
           -- type, so we can add a bit more context to the error message
           -- (cf #7869).
           --
           -- It is not always an error, because specialized type may have
           -- different arity, for example:
           --
           -- > f1 = f2 'a'
           -- > f2 :: Monad m => m Bool
           -- > f2 = undefined
           --
           -- But in that case we add specialized type into error context
           -- anyway, because it may be useful. See also #9605.
        go TcType
    ty = (TidyEnv -> ZonkM (TidyEnv, SDoc))
    -> TcM (HsWrapper, Scaled TcType, TcType)
    -> TcM (HsWrapper, Scaled TcType, TcType)
    forall a. (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM (TcType -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    mk_ctxt TcType
    ty) (TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    defer TcType
    ty)

        ------------
        defer :: TcType -> TcM (HsWrapper, Scaled TcType, TcType)
    defer TcType
    fun_ty
          = do { arg_ty <- ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
    new_check_arg_ty ExpectedFunTyOrigin
    herald Int
    1
               ; res_ty <- newOpenFlexiTyVarTy
               ; let unif_fun_ty = [Scaled TcType] -> TcType -> TcType
    HasDebugCallStack => [Scaled TcType] -> TcType -> TcType
    mkScaledFunTys [Scaled TcType
    arg_ty] TcType
    res_ty
               ; co <- unifyType mb_thing fun_ty unif_fun_ty
               ; return (mkWpCastN co, arg_ty, res_ty) }

        ------------
        mk_ctxt :: TcType -> TidyEnv -> ZonkM (TidyEnv, SDoc)
        mk_ctxt :: TcType -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    mk_ctxt TcType
    _res_ty = ExpectedFunTyOrigin
    -> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    mkFunTysMsg ExpectedFunTyOrigin
    herald (Int, TcType)
    err_info

    {- Note [matchActualFunTy error handling]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    matchActualFunTy is made much more complicated by the
    desire to produce good error messages. Consider the application
        f @Int x y
    In GHC.Tc.Gen.Head.tcInstFun we instantiate the function type, one
    argument at a time.  It must instantiate any type/dictionary args,
    before looking for an arrow type.

    But if it doesn't find an arrow type, it wants to generate a message
    like "f is applied to two arguments but its type only has one".
    To do that, it needs to know about the args that tcArgs has already
    munched up -- hence passing in n_val_args_in_call and arg_tys_so_far;
    and hence also the accumulating so_far arg to 'go'.

    This allows us (in mk_ctxt) to construct f's /instantiated/ type,
    with just the values-arg arrows, which is what we really want
    in the error message.

    Ugh!
    -}

    -- | Like 'matchExpectedFunTys', but used when you have an "actual" type,
    -- for example in function application.
    --
    -- INVARIANT: the returned argument types all have a syntactically fixed RuntimeRep
    -- in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
    -- See Note [Return arguments with a fixed RuntimeRep].
    matchActualFunTys :: ExpectedFunTyOrigin -- ^ See Note [Herald for matchExpectedFunTys]
                      -> CtOrigin
                      -> Arity
                      -> TcSigmaType
                      -> TcM (HsWrapper, [Scaled TcSigmaTypeFRR], TcRhoType)
    -- If    matchActualFunTys n ty = (wrap, [t1,..,tn], res_ty)
    -- then  wrap : ty ~> (t1 -> ... -> tn -> res_ty)
    --       and res_ty is a RhoType
    -- NB: the returned type is top-instantiated; it's a RhoType
    matchActualFunTys :: ExpectedFunTyOrigin
    -> CtOrigin
    -> Int
    -> TcType
    -> TcM (HsWrapper, [Scaled TcType], TcType)
    matchActualFunTys ExpectedFunTyOrigin
    herald CtOrigin
    ct_orig Int
    n_val_args_wanted TcType
    top_ty
      = Int
    -> [Scaled TcType]
    -> TcType
    -> TcM (HsWrapper, [Scaled TcType], TcType)
    go Int
    n_val_args_wanted [] TcType
    top_ty
      where
        go :: Int
    -> [Scaled TcType]
    -> TcType
    -> TcM (HsWrapper, [Scaled TcType], TcType)
    go Int
    n [Scaled TcType]
    so_far TcType
    fun_ty
          | Bool -> Bool
    not (TcType -> Bool
    isRhoTy TcType
    fun_ty)
          = do { (wrap1, rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    topInstantiate CtOrigin
    ct_orig TcType
    fun_ty
               ; (wrap2, arg_tys, res_ty) <- go n so_far rho
               ; return (wrap2 <.> wrap1, arg_tys, res_ty) }

        go Int
    0 [Scaled TcType]
    _ TcType
    fun_ty = (HsWrapper, [Scaled TcType], TcType)
    -> TcM (HsWrapper, [Scaled TcType], TcType)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, [], TcType
    fun_ty)

        go Int
    n [Scaled TcType]
    so_far TcType
    fun_ty
          = do { (wrap_fun1, arg_ty1, res_ty1) <- ExpectedFunTyOrigin
    -> Maybe TypedThing
    -> (Int, TcType)
    -> TcType
    -> TcM (HsWrapper, Scaled TcType, TcType)
    matchActualFunTy
                                                     ExpectedFunTyOrigin
    herald Maybe TypedThing
    forall a. Maybe a
    Nothing
                                                     (Int
    n_val_args_wanted, TcType
    top_ty)
                                                     TcType
    fun_ty
               ; (wrap_res, arg_tys, res_ty)   <- go (n-1) (arg_ty1:so_far) res_ty1
               ; let wrap_fun2 = HsWrapper -> HsWrapper -> Scaled TcType -> TcType -> HsWrapper
    mkWpFun HsWrapper
    idHsWrapper HsWrapper
    wrap_res Scaled TcType
    arg_ty1 TcType
    res_ty
               -- NB: arg_ty1 comes from matchActualFunTy, so it has
               -- a syntactically fixed RuntimeRep as needed to call mkWpFun.
               ; return (wrap_fun2 <.> wrap_fun1, arg_ty1:arg_tys, res_ty) }

    {-
    ************************************************************************
    *                                                                      *
              Skolemisation and matchExpectedFunTys
    *                                                                      *
    ************************************************************************

    Note [Skolemisation overview]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose f :: (forall a. a->a) -> blah, and we have the application (f e)
    Then we want to typecheck `e` pushing in the type `forall a. a->a`. But we
    need to be careful:

    * Roughly speaking, in (tcPolyExpr e (forall a b. rho)), we skolemise `a` and `b`,
      and then call (tcExpr e rho)

    * But not quite!  We must be careful if `e` is a type lambda (\ @p @q -> blah).
      Then we want to line up the skolemised variables `a`,`b`
      with `p`,`q`, so we can't just call (tcExpr (\ @p @q -> blah) rho)

    * A very similar situation arises with
         (\ @p @q -> blah) :: forall a b. rho
      Again, we must line up `p`, `q` with the skolemised `a` and `b`.

    * Another similar situation arises with
        g :: forall a b. rho
        g @p @q x y = ....
      Here again when skolemising `a` and `b` we must be careful to match them up
      with `p` and `q`.

    OK, so how exactly do we check @p binders in lambdas?  First note that we only
    we only attempt to deal with @p binders when /checking/. We don't do inference for
    (\ @a -> blah), not yet anyway.

    For checking, there are two cases to consider:
      * Function LHS, where the function has a type signature
                      f :: forall a. a -> forall b. [b] -> blah
                      f @p x @q y = ...

      * Lambda        \ @p x @q y -> ...
                      \cases { @p x @q y -> ... }
        (\case p behaves like \cases { p -> ... }, and p is always a term pattern.)

    Both ultimately handled by matchExpectedFunTys.

    * Function LHS case is handled by `GHC.Tc.Gen.Bind.tcPolyCheck`:
      * It calls `tcSkolemiseCompleteSig`
      * Passes the skolemised variables into `tcFunBindMatches`
      * Which uses `matchExpectedFunTys` to decompose the function type to
        match the arguments
      * And then passes the (skolemised-variables ++ arg tys) on to `tcMatches`

    * For the Lambda case there are two sub-cases:
       * An expression with a type signature: (\ @a x y -> blah) :: hs_ty
         This is handled by `GHC.Tc.Gen.Head.tcExprWithSig`, which kind-checks
         the signature and hands off to `tcExprPolyCheck` vai `tcPolyLExprSig`
         Note that the foralls at the top of hs_ty scope over the expression.

       * A higher order call: h e, where h :: poly_ty -> blah
         This is handlded by `GHC.Tc.Gen.Expr.tcPolyExpr`, which (in the
         checking case) again hands off to `tcExprPolyCheck`.  Here there is
         no type-variable scoping to worry about.

      So both sub-cases end up in `GHC.Tc.Gen.Expr.tcPolyExprCheck`
      * This skolemises the /top-level/ invisible binders, but remembers
        the binders as [ExpPatType]
      * Then it looks for a lambda, and if so, calls `tcLambdaMatches` passing in
        the skolemised binders so they can be matched up with the lambda binders.
      * Otherwise it does deep-skolemisation if DeepSubsumption is on,
        and then calls tcExpr to typecheck `e`

      The outer skolemisation in tcPolyExprCheck is done using
        * tcSkolemiseCompleteSig when there is a user-written signature
        * tcSkolemiseGeneral when the polytype just comes from the context e.g. (f e)
      The former just calls the latter, so the two cases differ only slightly:
        * Both do shallow skolemisation
        * Both go via checkConstraints, which uses implicationNeeded to decide whether
          to build an implication constraint even if there /are/ no skolems.
          See Note [When to build an implication] below.

      The difference between the two cases is that `tcSkolemiseCompleteSig`
      also brings the outer type variables into scope.  It would do no
      harm to do so in both cases, but I found that (to my surprise) doing
      so caused a non-trivial (1%-ish) perf hit on the compiler.

    * `tcFunBindMatches` and `tcLambdaMatches` both use `matchExpectedFunTys`, which
      ensures that any trailing invisible binders are skolemised; and does so deeply
      if DeepSubsumption is on.

      This corresponds to the plan: "skolemise at the '=' of a function binding or
      at the '->' of a lambda binding".  (See #17594 and "Plan B2".)

    Some wrinkles

    (SK1) tcSkolemiseGeneral and tcSkolemiseCompleteSig make fresh type variables
          See Note [Instantiate sig with fresh variables]

    (SK2) All skolemisation (even without DeepSubsumption) builds just one implication
          constraint for a nested forall like:
              forall a. Eq a => forall b. Ord b => blah
          The implication constraint will look like
              forall a b. (Eq a, Ord b) => <constraints>
          See the loop in GHC.Tc.Utils.Instantiate.topSkolemise.
          This is just an optimisation; it would be fine to generate one implication
          constraint for each nesting layer.

    Some examples:

    *     f :: forall a b. blah
          f @p x = rhs
      `tcPolyCheck` calls `tcSkolemiseCompleteSig` to skolemise the signature, and
      then calls `tcFunBindMatches` passing in [a_sk, b_sk], the skolemsed
      variables. The latter ultimately calls `tcMatches`, and thence `tcMatchPats`.
      The latter matches up the `a_sk` with `@p`, and discards the `b_sk`.

    *     f :: forall (a::Type) (b::a). blah
          f @(p::b) x = rhs
      `tcSkolemiseCompleteSig` brings `a` and `b` into scope, bound to `a_sk` and `b_sk` resp.
      When `tcMatchPats` typechecks the pattern `@(p::b)` it'll find that `b` is in
      scope (as a result of tcSkolemiseCompleteSig) which is a bit strange.  But
      it'll then unify the kinds `Type ~ b`, which will fail as it should.

    *     f :: Int -> forall (a::Type) (b::a). blah
          f x  @p = rhs
      `matchExpectedFunTys` does shallow skolemisation eagerly, so we'll skolemise the
      forall a b.  Then `tcMatchPats` will bind [p :-> a_sk], and discard `b_sk`.
      Discarding the `b_sk` means that
          f x @p = \ @q -> blah
      or  f x @p = let .. in \ @q -> blah
      will both be rejected: this is Plan B2: skolemise at the "=".

    * Suppose DeepSubsumption is on
        f :: forall a. a -> forall b. b -> b -> forall z. z
        f @p x @q y = rhs
      The `tcSkolemiseCompleteSig` uses shallow skolemisation, so it only skolemises
      and brings into scope [a :-> a_sk]. Then `matchExpectedFunTys` skolemises the
      forall b, because it needs to expose two value arguments.  Finally
      `matchExpectedFunTys` concludes with deeply skolemising the remaining type.

      So we end up with `[p :-> a_sk, q :-> b_sk]`.  Notice that we must not
      deeply-skolemise /first/ or we'd get the tyvars [a_sk, b_sk, c_sk] which would
      not line up with the patterns [@p, x, @q, y]
    -}

    tcSkolemiseGeneral
      :: DeepSubsumptionFlag
      -> UserTypeCtxt
      -> TcType -> TcType   -- top_ty and expected_ty
            -- Here, top_ty      is the type we started to skolemise; used only in SigSkol
            -- -     expected_ty is the type we are actually skolemising
            -- matchExpectedFunTys walks down the type, skolemising as it goes,
            -- keeping the same top_ty, but successively smaller expected_tys
      -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
      -> TcM (HsWrapper, result)
    tcSkolemiseGeneral :: forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseGeneral DeepSubsumptionFlag
    ds_flag UserTypeCtxt
    ctxt TcType
    top_ty TcType
    expected_ty [(Name, TcInvisTVBinder)] -> TcType -> TcM result
    thing_inside
      | DeepSubsumptionFlag -> TcType -> Bool
    definitely_mono DeepSubsumptionFlag
    ds_flag TcType
    expected_ty
        -- Fast path for a very very common case: no skolemisation to do
        -- But still call checkConstraints in case we need an implication regardless
      = do { let sig_skol :: SkolemInfoAnon
    sig_skol = UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
    SigSkol UserTypeCtxt
    ctxt TcType
    top_ty []
           ; (ev_binds, result) <- SkolemInfoAnon
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    forall result.
    SkolemInfoAnon
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    checkConstraints SkolemInfoAnon
    sig_skol [] [] (TcM result -> TcM (TcEvBinds, result))
    -> TcM result -> TcM (TcEvBinds, result)
    forall a b. (a -> b) -> a -> b
    $
                                   [(Name, TcInvisTVBinder)] -> TcType -> TcM result
    thing_inside [] TcType
    expected_ty
           ; return (mkWpLet ev_binds, result) }

      | Bool
    otherwise
      = do { -- rec {..}: see Note [Keeping SkolemInfo inside a SkolemTv]
             --           in GHC.Tc.Utils.TcType
           ; rec { (wrap, tv_prs, given, rho_ty) <- case ds_flag of
                        DeepSubsumptionFlag
    Deep    -> SkolemInfo
    -> TcType
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    deeplySkolemise SkolemInfo
    skol_info TcType
    expected_ty
                        DeepSubsumptionFlag
    Shallow -> SkolemInfo
    -> TcType
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    topSkolemise SkolemInfo
    skol_info TcType
    expected_ty
                 ; let sig_skol = UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
    SigSkol UserTypeCtxt
    ctxt TcType
    top_ty (((Name, TcInvisTVBinder) -> (Name, TcTyVar))
    -> [(Name, TcInvisTVBinder)] -> [(Name, TcTyVar)]
    forall a b. (a -> b) -> [a] -> [b]
    map ((TcInvisTVBinder -> TcTyVar)
    -> (Name, TcInvisTVBinder) -> (Name, TcTyVar)
    forall a b. (a -> b) -> (Name, a) -> (Name, b)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap TcInvisTVBinder -> TcTyVar
    forall tv argf. VarBndr tv argf -> tv
    binderVar) [(Name, TcInvisTVBinder)]
    tv_prs)
                 ; skol_info <- mkSkolemInfo sig_skol }

           ; let skol_tvs = ((Name, TcInvisTVBinder) -> TcTyVar)
    -> [(Name, TcInvisTVBinder)] -> [TcTyVar]
    forall a b. (a -> b) -> [a] -> [b]
    map (TcInvisTVBinder -> TcTyVar
    forall tv argf. VarBndr tv argf -> tv
    binderVar (TcInvisTVBinder -> TcTyVar)
    -> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
    -> (Name, TcInvisTVBinder)
    -> TcTyVar
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (Name, TcInvisTVBinder) -> TcInvisTVBinder
    forall a b. (a, b) -> b
    snd) [(Name, TcInvisTVBinder)]
    tv_prs
           ; traceTc "tcSkolemiseGeneral" (pprUserTypeCtxt ctxt <+> ppr skol_tvs <+> ppr given)
           ; (ev_binds, result) <- checkConstraints sig_skol skol_tvs given $
                                   thing_inside tv_prs rho_ty

           ; return (wrap <.> mkWpLet ev_binds, result) }
             -- The ev_binds returned by checkConstraints is very
             -- often empty, in which case mkWpLet is a no-op

    tcSkolemiseCompleteSig :: TcCompleteSig
                           -> ([ExpPatType] -> TcRhoType -> TcM result)
                           -> TcM (HsWrapper, result)
    -- ^ The wrapper has type: spec_ty ~> expected_ty
    -- See Note [Skolemisation] for the differences between
    -- tcSkolemiseCompleteSig and tcTopSkolemise

    tcSkolemiseCompleteSig :: forall result.
    TcCompleteSig
    -> ([ExpPatType] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseCompleteSig (CSig { sig_bndr :: TcCompleteSig -> TcTyVar
    sig_bndr = TcTyVar
    poly_id, sig_ctxt :: TcCompleteSig -> UserTypeCtxt
    sig_ctxt = UserTypeCtxt
    ctxt, sig_loc :: TcCompleteSig -> SrcSpan
    sig_loc = SrcSpan
    loc })
                           [ExpPatType] -> TcType -> TcM result
    thing_inside
      = do { cur_loc <- TcRn SrcSpan
    getSrcSpanM
           ; let poly_ty = TcTyVar -> TcType
    idType TcTyVar
    poly_id
           ; setSrcSpan loc $   -- Sets the location for the implication constraint
             tcSkolemiseGeneral Shallow ctxt poly_ty poly_ty $ \[(Name, TcInvisTVBinder)]
    tv_prs TcType
    rho_ty ->
             SrcSpan -> TcM result -> TcM result
    forall a. SrcSpan -> TcRn a -> TcRn a
    setSrcSpan SrcSpan
    cur_loc (TcM result -> TcM result) -> TcM result -> TcM result
    forall a b. (a -> b) -> a -> b
    $ -- Revert to the original location
             [(Name, TcTyVar)] -> TcM result -> TcM result
    forall r. [(Name, TcTyVar)] -> TcM r -> TcM r
    tcExtendNameTyVarEnv (((Name, TcInvisTVBinder) -> (Name, TcTyVar))
    -> [(Name, TcInvisTVBinder)] -> [(Name, TcTyVar)]
    forall a b. (a -> b) -> [a] -> [b]
    map ((TcInvisTVBinder -> TcTyVar)
    -> (Name, TcInvisTVBinder) -> (Name, TcTyVar)
    forall a b. (a -> b) -> (Name, a) -> (Name, b)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap TcInvisTVBinder -> TcTyVar
    forall tv argf. VarBndr tv argf -> tv
    binderVar) [(Name, TcInvisTVBinder)]
    tv_prs) (TcM result -> TcM result) -> TcM result -> TcM result
    forall a b. (a -> b) -> a -> b
    $
             [ExpPatType] -> TcType -> TcM result
    thing_inside (((Name, TcInvisTVBinder) -> ExpPatType)
    -> [(Name, TcInvisTVBinder)] -> [ExpPatType]
    forall a b. (a -> b) -> [a] -> [b]
    map (TcInvisTVBinder -> ExpPatType
    mkInvisExpPatType (TcInvisTVBinder -> ExpPatType)
    -> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
    -> (Name, TcInvisTVBinder)
    -> ExpPatType
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (Name, TcInvisTVBinder) -> TcInvisTVBinder
    forall a b. (a, b) -> b
    snd) [(Name, TcInvisTVBinder)]
    tv_prs) TcType
    rho_ty }

    tcSkolemiseExpectedType :: TcSigmaType
                            -> ([ExpPatType] -> TcRhoType -> TcM result)
                            -> TcM (HsWrapper, result)
    -- Just like tcSkolemiseCompleteSig, except that we don't have a user-written
    -- type signature, we only have a type comimg from the context.
    -- Eg. f :: (forall a. blah) -> blah
    --     In the call (f e) we will call tcSkolemiseExpectedType on (forall a.blah)
    --     before typececking `e`
    tcSkolemiseExpectedType :: forall result.
    TcType
    -> ([ExpPatType] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseExpectedType TcType
    exp_ty [ExpPatType] -> TcType -> TcM result
    thing_inside
      = DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseGeneral DeepSubsumptionFlag
    Shallow UserTypeCtxt
    GenSigCtxt TcType
    exp_ty TcType
    exp_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
     -> TcM (HsWrapper, result))
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    forall a b. (a -> b) -> a -> b
    $ \[(Name, TcInvisTVBinder)]
    tv_prs TcType
    rho_ty ->
        [ExpPatType] -> TcType -> TcM result
    thing_inside (((Name, TcInvisTVBinder) -> ExpPatType)
    -> [(Name, TcInvisTVBinder)] -> [ExpPatType]
    forall a b. (a -> b) -> [a] -> [b]
    map (TcInvisTVBinder -> ExpPatType
    mkInvisExpPatType (TcInvisTVBinder -> ExpPatType)
    -> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
    -> (Name, TcInvisTVBinder)
    -> ExpPatType
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (Name, TcInvisTVBinder) -> TcInvisTVBinder
    forall a b. (a, b) -> b
    snd) [(Name, TcInvisTVBinder)]
    tv_prs) TcType
    rho_ty

    tcSkolemise :: DeepSubsumptionFlag -> UserTypeCtxt -> TcSigmaType
                -> (TcRhoType -> TcM result)
                -> TcM (HsWrapper, result)
    tcSkolemise :: forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> (TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemise DeepSubsumptionFlag
    ds_flag UserTypeCtxt
    ctxt TcType
    expected_ty TcType -> TcM result
    thing_inside
      = DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseGeneral DeepSubsumptionFlag
    ds_flag UserTypeCtxt
    ctxt TcType
    expected_ty TcType
    expected_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
     -> TcM (HsWrapper, result))
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    forall a b. (a -> b) -> a -> b
    $ \[(Name, TcInvisTVBinder)]
    _ TcType
    rho_ty ->
        TcType -> TcM result
    thing_inside TcType
    rho_ty

    checkConstraints :: SkolemInfoAnon
                     -> [TcTyVar]           -- Skolems
                     -> [EvVar]             -- Given
                     -> TcM result
                     -> TcM (TcEvBinds, result)
    -- checkConstraints is careful to build an implication even if
    -- `skol_tvs` and `given` are both empty, under certain circumstances
    -- See Note [When to build an implication]
    checkConstraints :: forall result.
    SkolemInfoAnon
    -> [TcTyVar] -> [TcTyVar] -> TcM result -> TcM (TcEvBinds, result)
    checkConstraints SkolemInfoAnon
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given TcM result
    thing_inside
      = do { implication_needed <- SkolemInfoAnon -> [TcTyVar] -> [TcTyVar] -> TcM Bool
    implicationNeeded SkolemInfoAnon
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given

           ; if implication_needed
             then do { (tclvl, wanted, result) <- pushLevelAndCaptureConstraints thing_inside
                     ; (implics, ev_binds) <- buildImplicationFor tclvl skol_info skol_tvs given wanted
                     ; traceTc "checkConstraints" (ppr tclvl $$ ppr skol_tvs)
                     ; emitImplications implics
                     ; return (ev_binds, result) }

             else -- Fast path.  We check every function argument with tcCheckPolyExpr,
                  -- which uses tcTopSkolemise and hence checkConstraints.
                  -- So this fast path is well-exercised
                  do { res <- thing_inside
                     ; return (emptyTcEvBinds, res) } }

    checkTvConstraints :: SkolemInfo
                       -> [TcTyVar]          -- Skolem tyvars
                       -> TcM result
                       -> TcM result

    checkTvConstraints :: forall result. SkolemInfo -> [TcTyVar] -> TcM result -> TcM result
    checkTvConstraints SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcM result
    thing_inside
      = do { (tclvl, wanted, result) <- TcM result -> TcM (TcLevel, WantedConstraints, result)
    forall a. TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndCaptureConstraints TcM result
    thing_inside
           ; emitResidualTvConstraint skol_info skol_tvs tclvl wanted
           ; return result }

    emitResidualTvConstraint :: SkolemInfo -> [TcTyVar]
                             -> TcLevel -> WantedConstraints -> TcM ()
    emitResidualTvConstraint :: SkolemInfo -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM ()
    emitResidualTvConstraint SkolemInfo
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      | Bool -> Bool
    not (WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted) Bool -> Bool -> Bool
    ||
        SkolemInfoAnon -> Bool
    checkTelescopeSkol SkolemInfoAnon
    skol_info_anon
      = -- checkTelescopeSkol: in this case, /always/ emit this implication
        -- even if 'wanted' is empty. We need the implication so that we check
        -- for a bad telescope. See Note [Skolem escape and forall-types] in
        -- GHC.Tc.Gen.HsType
        do { implic <- SkolemInfoAnon
    -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication SkolemInfoAnon
    skol_info_anon [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
           ; emitImplication implic }

      | Bool
    otherwise  -- Empty 'wanted', emit nothing
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      where
         skol_info_anon :: SkolemInfoAnon
    skol_info_anon = SkolemInfo -> SkolemInfoAnon
    getSkolemInfo SkolemInfo
    skol_info

    buildTvImplication :: SkolemInfoAnon -> [TcTyVar]
                       -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication :: SkolemInfoAnon
    -> [TcTyVar] -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication SkolemInfoAnon
    skol_info [TcTyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      = Bool -> SDoc -> TcM Implication -> TcM Implication
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr ((TcTyVar -> Bool) -> [TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all (TcTyVar -> Bool
    isSkolemTyVar (TcTyVar -> Bool) -> (TcTyVar -> Bool) -> TcTyVar -> Bool
    forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
    <||> TcTyVar -> Bool
    isTyVarTyVar) [TcTyVar]
    skol_tvs) ([TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcTyVar]
    skol_tvs) (TcM Implication -> TcM Implication)
    -> TcM Implication -> TcM Implication
    forall a b. (a -> b) -> a -> b
    $
        do { ev_binds <- TcM EvBindsVar
    newNoTcEvBinds  -- Used for equalities only, so all the constraints
                                         -- are solved by filling in coercion holes, not
                                         -- by creating a value-level evidence binding
           ; implic   <- newImplication

           ; let implic' = Implication
    implic { ic_tclvl     = tclvl
                                  , ic_skols     = skol_tvs
                                  , ic_given_eqs = NoGivenEqs
                                  , ic_wanted    = wanted
                                  , ic_binds     = ev_binds
                                  , ic_info      = skol_info }

           ; checkImplicationInvariants implic'
           ; return implic' }

    implicationNeeded :: SkolemInfoAnon -> [TcTyVar] -> [EvVar] -> TcM Bool
    -- See Note [When to build an implication]
    implicationNeeded :: SkolemInfoAnon -> [TcTyVar] -> [TcTyVar] -> TcM Bool
    implicationNeeded SkolemInfoAnon
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given
      | [TcTyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    skol_tvs
      , [TcTyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    given
      , Bool -> Bool
    not (SkolemInfoAnon -> Bool
    alwaysBuildImplication SkolemInfoAnon
    skol_info)
      = -- Empty skolems and givens
        do { tc_lvl <- TcM TcLevel
    getTcLevel
           ; if not (isTopTcLevel tc_lvl)  -- No implication needed if we are
             then return False             -- already inside an implication
             else
        do { dflags <- getDynFlags       -- If any deferral can happen,
                                         -- we must build an implication
           ; return (gopt Opt_DeferTypeErrors dflags ||
                     gopt Opt_DeferTypedHoles dflags ||
                     gopt Opt_DeferOutOfScopeVariables dflags) } }

      | Bool
    otherwise     -- Non-empty skolems or givens
      = Bool -> TcM Bool
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True   -- Definitely need an implication

    alwaysBuildImplication :: SkolemInfoAnon -> Bool
    -- See Note [When to build an implication]
    alwaysBuildImplication :: SkolemInfoAnon -> Bool
    alwaysBuildImplication SkolemInfoAnon
    _ = Bool
    False

    {-  Commmented out for now while I figure out about error messages.
        See #14185

    alwaysBuildImplication (SigSkol ctxt _ _)
      = case ctxt of
          FunSigCtxt {} -> True  -- RHS of a binding with a signature
          _             -> False
    alwaysBuildImplication (RuleSkol {})      = True
    alwaysBuildImplication (InstSkol {})      = True
    alwaysBuildImplication (FamInstSkol {})   = True
    alwaysBuildImplication _                  = False
    -}

    buildImplicationFor :: TcLevel -> SkolemInfoAnon -> [TcTyVar]
                       -> [EvVar] -> WantedConstraints
                       -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor :: TcLevel
    -> SkolemInfoAnon
    -> [TcTyVar]
    -> [TcTyVar]
    -> WantedConstraints
    -> TcM (Bag Implication, TcEvBinds)
    buildImplicationFor TcLevel
    tclvl SkolemInfoAnon
    skol_info [TcTyVar]
    skol_tvs [TcTyVar]
    given WantedConstraints
    wanted
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted Bool -> Bool -> Bool
    && [TcTyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    given
                 -- Optimisation : if there are no wanteds, and no givens
                 -- don't generate an implication at all.
                 -- Reason for the (null given): we don't want to lose
                 -- the "inaccessible alternative" error check
      = (Bag Implication, TcEvBinds) -> TcM (Bag Implication, TcEvBinds)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Bag Implication
    forall a. Bag a
    emptyBag, TcEvBinds
    emptyTcEvBinds)

      | Bool
    otherwise
      = Bool
    -> SDoc
    -> TcM (Bag Implication, TcEvBinds)
    -> TcM (Bag Implication, TcEvBinds)
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr ((TcTyVar -> Bool) -> [TcTyVar] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all (TcTyVar -> Bool
    isSkolemTyVar (TcTyVar -> Bool) -> (TcTyVar -> Bool) -> TcTyVar -> Bool
    forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
    <||> TcTyVar -> Bool
    isTyVarTyVar) [TcTyVar]
    skol_tvs) ([TcTyVar] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcTyVar]
    skol_tvs) (TcM (Bag Implication, TcEvBinds)
     -> TcM (Bag Implication, TcEvBinds))
    -> TcM (Bag Implication, TcEvBinds)
    -> TcM (Bag Implication, TcEvBinds)
    forall a b. (a -> b) -> a -> b
    $
          -- Why allow TyVarTvs? Because implicitly declared kind variables in
          -- non-CUSK type declarations are TyVarTvs, and we need to bring them
          -- into scope as a skolem in an implication. This is OK, though,
          -- because TyVarTvs will always remain tyvars, even after unification.
        do { ev_binds_var <- TcM EvBindsVar
    newTcEvBinds
           ; implic <- newImplication
           ; let implic' = Implication
    implic { ic_tclvl  = tclvl
                                  , ic_skols  = skol_tvs
                                  , ic_given  = given
                                  , ic_wanted = wanted
                                  , ic_binds  = ev_binds_var
                                  , ic_info   = skol_info }
           ; checkImplicationInvariants implic'

           ; return (unitBag implic', TcEvBinds ev_binds_var) }

    {- Note [When to build an implication]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have some 'skolems' and some 'givens', and we are
    considering whether to wrap the constraints in their scope into an
    implication.  We must /always/ do so if either 'skolems' or 'givens' are
    non-empty.  But what if both are empty?  You might think we could
    always drop the implication.  Other things being equal, the fewer
    implications the better.  Less clutter and overhead.  But we must
    take care:

    * If we have an unsolved [W] g :: a ~# b, and -fdefer-type-errors,
      we'll make a /term-level/ evidence binding for 'g = error "blah"'.
      We must have an EvBindsVar those bindings!, otherwise they end up as
      top-level unlifted bindings, which are verboten. This only matters
      at top level, so we check for that
      See also Note [Deferred errors for coercion holes] in GHC.Tc.Errors.
      cf #14149 for an example of what goes wrong.

    * This is /necessary/ for top level but may be /desirable/ even for
      nested bindings, because if the deferred coercion is bound too far
      out it will be reported even if that thunk (say) is not evaluated.

    * If you have
         f :: Int;  f = f_blah
         g :: Bool; g = g_blah
      If we don't build an implication for f or g (no tyvars, no givens),
      the constraints for f_blah and g_blah are solved together.  And that
      can yield /very/ confusing error messages, because we can get
          [W] C Int b1    -- from f_blah
          [W] C Int b2    -- from g_blan
      and fundeps can yield [W] b1 ~ b2, even though the two functions have
      literally nothing to do with each other.  #14185 is an example.
      Building an implication keeps them separate.

    Note [Herald for matchExpectedFunTys]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The 'herald' always looks like:
       "The equation(s) for 'f' have"
       "The abstraction (\x.e) takes"
       "The section (+ x) expects"
       "The function 'f' is applied to"

    This is used to construct a message of form

       The abstraction `\Just 1 -> ...' takes two arguments
       but its type `Maybe a -> a' has only one

       The equation(s) for `f' have two arguments
       but its type `Maybe a -> a' has only one

       The section `(f 3)' requires 'f' to take two arguments
       but its type `Int -> Int' has only one

       The function 'f' is applied to two arguments
       but its type `Int -> Int' has only one

    When visible type applications (e.g., `f @Int 1 2`, as in #13902) enter the
    picture, we have a choice in deciding whether to count the type applications as
    proper arguments:

       The function 'f' is applied to one visible type argument
         and two value arguments
       but its type `forall a. a -> a` has only one visible type argument
         and one value argument

    Or whether to include the type applications as part of the herald itself:

       The expression 'f @Int' is applied to two arguments
       but its type `Int -> Int` has only one

    The latter is easier to implement and is arguably easier to understand, so we
    choose to implement that option.

    Note [matchExpectedFunTys]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    matchExpectedFunTys checks that a sigma has the form
    of an n-ary function.  It passes the decomposed type to the
    thing_inside, and returns a wrapper to coerce between the two types

    It's used wherever a language construct must have a functional type,
    namely:
            A lambda expression
            A function definition
         An operator section

    This function must be written CPS'd because it needs to fill in the
    ExpTypes produced for arguments before it can fill in the ExpType
    passed in.

    Note [Return arguments with a fixed RuntimeRep]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The functions

      - matchExpectedFunTys,
      - matchActualFunTy,
      - matchActualFunTys,

    peel off argument types, as explained in Note [matchExpectedFunTys].
    It's important that these functions return argument types that have
    a fixed runtime representation, otherwise we would be in violation
    of the representation-polymorphism invariants of
    Note [Representation polymorphism invariants] in GHC.Core.

    This is why all these functions have an additional invariant,
    that the argument types they return all have a syntactically fixed RuntimeRep,
    in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.

    Example:

      Suppose we have

        type F :: Type -> RuntimeRep
        type family F a where { F Int = LiftedRep }

        type Dual :: Type -> Type
        type family Dual a where
          Dual a = a -> ()

        f :: forall (a :: TYPE (F Int)). Dual a
        f = \ x -> ()

      The body of `f` is a lambda abstraction, so we must be able to split off
      one argument type from its type. This is handled by `matchExpectedFunTys`
      (see 'GHC.Tc.Gen.Match.tcLambdaMatches'). We end up with desugared Core that
      looks like this:

        f :: forall (a :: TYPE (F Int)). Dual (a |> (TYPE F[0]))
        f = \ @(a :: TYPE (F Int)) ->
              (\ (x :: (a |> (TYPE F[0]))) -> ())
              `cast`
              (Sub (Sym (Dual[0] <(a |> (TYPE F[0]))>)))

      Two important transformations took place:

        1. We inserted casts around the argument type to ensure that it has
           a fixed runtime representation, as required by invariant (I1) from
           Note [Representation polymorphism invariants] in GHC.Core.
        2. We inserted a cast around the whole lambda to make everything line up
           with the type signature.
    -}

    -- | Use this function to split off arguments types when you have an
    -- \"expected\" type.
    --
    -- This function skolemises at each polytype.
    --
    -- Invariant: this function only applies the provided function
    -- to a list of argument types which all have a syntactically fixed RuntimeRep
    -- in the sense of Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
    -- See Note [Return arguments with a fixed RuntimeRep].
    matchExpectedFunTys :: forall a.
                           ExpectedFunTyOrigin  -- See Note [Herald for matchExpectedFunTys]
                        -> UserTypeCtxt
                        -> VisArity
                        -> ExpSigmaType
                        -> ([ExpPatType] -> ExpRhoType -> TcM a)
                        -> TcM (HsWrapper, a)
    -- If    matchExpectedFunTys n ty = (wrap, _)
    -- then  wrap : (t1 -> ... -> tn -> ty_r) ~> ty,
    --   where [t1, ..., tn], ty_r are passed to the thing_inside
    --
    -- Unconditionally concludes by skolemising any trailing invisible
    -- binders and, if DeepSubsumption is on, it does so deeply.
    --
    -- Postcondition:
    --   If exp_ty is Check {}, then [ExpPatType] and ExpRhoType results are all Check{}
    --   If exp_ty is Infer {}, then [ExpPatType] and ExpRhoType results are all Infer{}
    matchExpectedFunTys :: forall a.
    ExpectedFunTyOrigin
    -> UserTypeCtxt
    -> Int
    -> ExpRhoType
    -> ([ExpPatType] -> ExpRhoType -> TcM a)
    -> TcM (HsWrapper, a)
    matchExpectedFunTys ExpectedFunTyOrigin
    herald UserTypeCtxt
    _ Int
    arity (Infer InferResult
    inf_res) [ExpPatType] -> ExpRhoType -> TcM a
    thing_inside
      = do { arg_tys <- (Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType))
    -> [Int] -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled ExpRhoType]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM (ExpectedFunTyOrigin
    -> Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
    new_infer_arg_ty ExpectedFunTyOrigin
    herald) [Int
    1 .. Int
    arity]
           ; res_ty  <- newInferExpType
           ; result  <- thing_inside (map ExpFunPatTy arg_tys) res_ty
           ; arg_tys <- mapM (\(Scaled TcType
    m ExpRhoType
    t) -> TcType -> TcType -> Scaled TcType
    forall a. TcType -> a -> Scaled a
    Scaled TcType
    m (TcType -> Scaled TcType) -> TcM TcType -> TcM (Scaled TcType)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> ExpRhoType -> TcM TcType
    forall (m :: * -> *). MonadIO m => ExpRhoType -> m TcType
    readExpType ExpRhoType
    t) arg_tys
           ; res_ty  <- readExpType res_ty
           ; co <- fillInferResult (mkScaledFunTys arg_tys res_ty) inf_res
           ; return (mkWpCastN co, result) }

    matchExpectedFunTys ExpectedFunTyOrigin
    herald UserTypeCtxt
    ctx Int
    arity (Check TcType
    top_ty) [ExpPatType] -> ExpRhoType -> TcM a
    thing_inside
      = Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    check Int
    0 [] TcType
    top_ty
      where
        check :: VisArity -> [ExpPatType] -> TcSigmaType -> TcM (HsWrapper, a)
        -- `check` is called only in the Check{} case
        -- It collects rev_pat_tys in reversed order
        -- n_so_far is the number of /visible/ arguments seen so far:
        --     i.e. length (filterOut isExpForAllPatTyInvis rev_pat_tys)

        -- Do shallow skolemisation if there are top-level invisible quantifiers
        check :: Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty
          | TcType -> Bool
    isSigmaTy TcType
    ty  -- Type has invisible quantifiers
          = do { (wrap_gen, (wrap_res, result))
                     <- DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)]
        -> TcType -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> TcM (HsWrapper, (HsWrapper, a))
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseGeneral DeepSubsumptionFlag
    Shallow UserTypeCtxt
    ctx TcType
    top_ty TcType
    ty (([(Name, TcInvisTVBinder)]
      -> TcType -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
     -> TcM (HsWrapper, (HsWrapper, a)))
    -> ([(Name, TcInvisTVBinder)]
        -> TcType -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> TcM (HsWrapper, (HsWrapper, a))
    forall a b. (a -> b) -> a -> b
    $ \[(Name, TcInvisTVBinder)]
    tv_bndrs TcType
    ty' ->
                        let rev_pat_tys' :: [ExpPatType]
    rev_pat_tys' = [ExpPatType] -> [ExpPatType]
    forall a. [a] -> [a]
    reverse (((Name, TcInvisTVBinder) -> ExpPatType)
    -> [(Name, TcInvisTVBinder)] -> [ExpPatType]
    forall a b. (a -> b) -> [a] -> [b]
    map (TcInvisTVBinder -> ExpPatType
    mkInvisExpPatType (TcInvisTVBinder -> ExpPatType)
    -> ((Name, TcInvisTVBinder) -> TcInvisTVBinder)
    -> (Name, TcInvisTVBinder)
    -> ExpPatType
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . (Name, TcInvisTVBinder) -> TcInvisTVBinder
    forall a b. (a, b) -> b
    snd) [(Name, TcInvisTVBinder)]
    tv_bndrs)
                                           [ExpPatType] -> [ExpPatType] -> [ExpPatType]
    forall a. [a] -> [a] -> [a]
    ++ [ExpPatType]
    rev_pat_tys
                        in Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    check Int
    n_so_far [ExpPatType]
    rev_pat_tys' TcType
    ty'
               ; return (wrap_gen <.> wrap_res, result) }

        -- (n_so_far == arity): no more args
        -- rho_ty has no top-level quantifiers
        -- If there is deep subsumption, do deep skolemisation
        check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    rho_ty
          | Int
    n_so_far Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    arity
          = do { let pat_tys :: [ExpPatType]
    pat_tys = [ExpPatType] -> [ExpPatType]
    forall a. [a] -> [a]
    reverse [ExpPatType]
    rev_pat_tys
               ; ds_flag <- TcM DeepSubsumptionFlag
    getDeepSubsumptionFlag
               ; case ds_flag of
                   DeepSubsumptionFlag
    Shallow -> do { res <- [ExpPatType] -> ExpRhoType -> TcM a
    thing_inside [ExpPatType]
    pat_tys (TcType -> ExpRhoType
    mkCheckExpType TcType
    rho_ty)
                                 ; return (idHsWrapper, res) }
                   DeepSubsumptionFlag
    Deep    -> DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemiseGeneral DeepSubsumptionFlag
    Deep UserTypeCtxt
    ctx TcType
    top_ty TcType
    rho_ty (([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> ([(Name, TcInvisTVBinder)] -> TcType -> TcM a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a b. (a -> b) -> a -> b
    $ \[(Name, TcInvisTVBinder)]
    _ TcType
    rho_ty ->
                              -- "_" drop the /deeply/-skolemise binders
                              -- They do not line up with binders in the Match
                              [ExpPatType] -> ExpRhoType -> TcM a
    thing_inside [ExpPatType]
    pat_tys (TcType -> ExpRhoType
    mkCheckExpType TcType
    rho_ty) }

        -- NOW do coreView.  We didn't do it before, so that we do not unnecessarily
        -- unwrap a synonym in the returned rho_ty
        check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty
          | Just TcType
    ty' <- TcType -> Maybe TcType
    coreView TcType
    ty = Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty'

        -- Decompose /visible/ (forall a -> blah), to give an ExpForAllPat
        -- NB: invisible binders are handled by tcSplitSigmaTy/tcTopSkolemise above
        -- NB: visible foralls "count" for the Arity argument; they correspond
        --     to syntactically visible patterns in the source program
        -- See Note [Visible type application and abstraction] in GHC.Tc.Gen.App
        check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty
          | Just (Bndr TcTyVar
    tv ForAllTyFlag
    vis, TcType
    body_ty) <- TcType -> Maybe (ForAllTyBinder, TcType)
    splitForAllForAllTyBinder_maybe TcType
    ty
          = Bool
    -> SDoc
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (ForAllTyFlag -> Bool
    isVisibleForAllTyFlag ForAllTyFlag
    vis) (TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty) (IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a b. (a -> b) -> a -> b
    $
            -- isSigmaTy case above has dealt with /invisible/ quantifiers,
            -- so this one must be /visible/ (= Required)
            do { let init_subst :: Subst
    init_subst = InScopeSet -> Subst
    mkEmptySubst (VarSet -> InScopeSet
    mkInScopeSet (TcType -> VarSet
    tyCoVarsOfType TcType
    ty))
                 -- rec {..}: see Note [Keeping SkolemInfo inside a SkolemTv]
                 --           in GHC.Tc.Utils.TcType
               ; rec { (subst', [tv']) <- tcInstSkolTyVarsX skol_info init_subst [tv]
                     ; let tv_prs = [(TcTyVar -> Name
    tyVarName TcTyVar
    tv, TcTyVar
    tv')]
                     ; skol_info <- mkSkolemInfo (SigSkol ctx top_ty tv_prs) }
               ; let body_ty' = HasDebugCallStack => Subst -> TcType -> TcType
    Subst -> TcType -> TcType
    substTy Subst
    subst' TcType
    body_ty
                     pat_ty   = ForAllTyBinder -> ExpPatType
    ExpForAllPatTy (ForAllTyFlag -> TcTyVar -> ForAllTyBinder
    forall vis. vis -> TcTyVar -> VarBndr TcTyVar vis
    mkForAllTyBinder ForAllTyFlag
    Required TcTyVar
    tv')
               ; (ev_binds, (wrap_res, result)) <- checkConstraints (getSkolemInfo skol_info) [tv'] [] $
                                                   check (n_so_far+1) (pat_ty : rev_pat_tys) body_ty'
               ; let wrap_gen = TcTyVar -> TcType -> HsWrapper
    mkWpVisTyLam TcTyVar
    tv' TcType
    body_ty' HsWrapper -> HsWrapper -> HsWrapper
    <.> TcEvBinds -> HsWrapper
    mkWpLet TcEvBinds
    ev_binds
               ; return (wrap_gen <.> wrap_res, result) }

        check Int
    n_so_far [ExpPatType]
    rev_pat_tys (FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_mult :: TcType -> TcType
    ft_mult = TcType
    mult
                                          , ft_arg :: TcType -> TcType
    ft_arg = TcType
    arg_ty, ft_res :: TcType -> TcType
    ft_res = TcType
    res_ty })
          = Bool
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a. HasCallStack => Bool -> a -> a
    assert (FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af) (IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a b. (a -> b) -> a -> b
    $
            do { let arg_pos :: Int
    arg_pos = Int
    n_so_far Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    1
               ; (arg_co, arg_ty) <- HasDebugCallStack =>
    FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
    FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
    hasFixedRuntimeRep (ExpectedFunTyOrigin -> Int -> FixedRuntimeRepContext
    FRRExpectedFunTy ExpectedFunTyOrigin
    herald Int
    arg_pos) TcType
    arg_ty
               ; (wrap_res, result) <- check arg_pos
                                             (mkCheckExpFunPatTy (Scaled mult arg_ty) : rev_pat_tys)
                                             res_ty
               ; let wrap_arg = Coercion -> HsWrapper
    mkWpCastN Coercion
    arg_co
                     fun_wrap = HsWrapper -> HsWrapper -> Scaled TcType -> TcType -> HsWrapper
    mkWpFun HsWrapper
    wrap_arg HsWrapper
    wrap_res (TcType -> TcType -> Scaled TcType
    forall a. TcType -> a -> Scaled a
    Scaled TcType
    mult TcType
    arg_ty) TcType
    res_ty
               ; return (fun_wrap, result) }

        check Int
    n_so_far [ExpPatType]
    rev_pat_tys ty :: TcType
    ty@(TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
    forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case cts of
                   Indirect TcType
    ty' -> Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty'
                   MetaDetails
    Flexi        -> Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    defer Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    ty }

           -- In all other cases we bale out into ordinary unification
           -- However unlike the meta-tyvar case, we are sure that the
           -- number of arguments doesn't match arity of the original
           -- type, so we can add a bit more context to the error message
           -- (cf #7869).
           --
           -- It is not always an error, because specialized type may have
           -- different arity, for example:
           --
           -- > f1 = f2 'a'
           -- > f2 :: Monad m => m Bool
           -- > f2 = undefined
           --
           -- But in that case we add specialized type into error context
           -- anyway, because it may be useful. See also #9605.
        check Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    res_ty
          = (TidyEnv -> ZonkM (TidyEnv, SDoc))
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a. (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM (ExpectedFunTyOrigin
    -> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    mkFunTysMsg ExpectedFunTyOrigin
    herald (Int
    arity, TcType
    top_ty))  (IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a))
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    forall a b. (a -> b) -> a -> b
    $
            Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    defer Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    res_ty

        ------------
        defer :: VisArity -> [ExpPatType] -> TcRhoType -> TcM (HsWrapper, a)
        defer :: Int
    -> [ExpPatType]
    -> TcType
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, a)
    defer Int
    n_so_far [ExpPatType]
    rev_pat_tys TcType
    fun_ty
          = do { more_arg_tys <- (Int -> TcM (Scaled TcType))
    -> [Int] -> IOEnv (Env TcGblEnv TcLclEnv) [Scaled TcType]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM (ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
    new_check_arg_ty ExpectedFunTyOrigin
    herald) [Int
    n_so_far Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    1 .. Int
    arity]
               ; let all_pats = [ExpPatType] -> [ExpPatType]
    forall a. [a] -> [a]
    reverse [ExpPatType]
    rev_pat_tys [ExpPatType] -> [ExpPatType] -> [ExpPatType]
    forall a. [a] -> [a] -> [a]
    ++ (Scaled TcType -> ExpPatType) -> [Scaled TcType] -> [ExpPatType]
    forall a b. (a -> b) -> [a] -> [b]
    map Scaled TcType -> ExpPatType
    mkCheckExpFunPatTy [Scaled TcType]
    more_arg_tys
               ; res_ty <- newOpenFlexiTyVarTy
               ; result <- thing_inside all_pats (mkCheckExpType res_ty)

               ; co <- unifyType Nothing (mkScaledFunTys more_arg_tys res_ty) fun_ty
               ; return (mkWpCastN co, result) }

    new_infer_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled ExpSigmaTypeFRR)
    new_infer_arg_ty :: ExpectedFunTyOrigin
    -> Int -> IOEnv (Env TcGblEnv TcLclEnv) (Scaled ExpRhoType)
    new_infer_arg_ty ExpectedFunTyOrigin
    herald Int
    arg_pos -- position for error messages only
      = do { mult     <- TcType -> TcM TcType
    newFlexiTyVarTy TcType
    multiplicityTy
           ; inf_hole <- newInferExpTypeFRR (FRRExpectedFunTy herald arg_pos)
           ; return (mkScaled mult inf_hole) }

    new_check_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
    new_check_arg_ty :: ExpectedFunTyOrigin -> Int -> TcM (Scaled TcType)
    new_check_arg_ty ExpectedFunTyOrigin
    herald Int
    arg_pos -- Position for error messages only
      = do { mult   <- TcType -> TcM TcType
    newFlexiTyVarTy TcType
    multiplicityTy
           ; arg_ty <- newOpenFlexiFRRTyVarTy (FRRExpectedFunTy herald arg_pos)
           ; return (mkScaled mult arg_ty) }

    mkFunTysMsg :: ExpectedFunTyOrigin
                -> (VisArity, TcType)
                -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    -- See Note [Reporting application arity errors]
    mkFunTysMsg :: ExpectedFunTyOrigin
    -> (Int, TcType) -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    mkFunTysMsg ExpectedFunTyOrigin
    herald (Int
    n_vis_args_in_call, TcType
    fun_ty) TidyEnv
    env
      = do { (env', fun_ty) <- TidyEnv -> TcType -> ZonkM (TidyEnv, TcType)
    zonkTidyTcType TidyEnv
    env TcType
    fun_ty

           ; let (pi_ty_bndrs, _) = splitPiTys fun_ty
                 n_fun_args = (PiTyBinder -> Bool) -> [PiTyBinder] -> Int
    forall a. (a -> Bool) -> [a] -> Int
    count PiTyBinder -> Bool
    isVisiblePiTyBinder [PiTyBinder]
    pi_ty_bndrs
                 msg | Int
    n_vis_args_in_call Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    <= Int
    n_fun_args  -- Enough args, in the end
                     = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "In the result of a function call"
                     | Bool
    otherwise
                     = SDoc -> Int -> SDoc -> SDoc
    hang (SDoc
    full_herald SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    comma)
                          Int
    2 ([SDoc] -> SDoc
    forall doc. IsLine doc => [doc] -> doc
    sep [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "but its type" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> SDoc -> SDoc
    quotes (TcType -> SDoc
    pprSigmaType TcType
    fun_ty)
                                 , if Int
    n_fun_args Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0 then String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "has none"
                                   else String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "has only" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Int -> SDoc
    speakN Int
    n_fun_args])

           ; return (env', msg) }
     where
      full_herald :: SDoc
    full_herald = ExpectedFunTyOrigin -> SDoc
    pprExpectedFunTyHerald ExpectedFunTyOrigin
    herald
                SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Int -> SDoc -> SDoc
    speakNOf Int
    n_vis_args_in_call (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "visible argument")
                 -- What are "visible" arguments? See Note [Visibility and arity] in GHC.Types.Basic

    {- Note [Reporting application arity errors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider      f :: Int -> Int -> Int
    and the call  foo = f 3 4 5
    We'd like to get an error like:

        • Couldn't match expected type ‘t0 -> t’ with actual type ‘Int’
        • The function ‘f’ is applied to three visible arguments,           -- What are "visible" arguments?
            but its type ‘Int -> Int -> Int’ has only two                   -- See Note [Visibility and arity] in GHC.Types.Basic

    That is what `mkFunTysMsg` tries to do.  But what is the "type of the function".
    Most obviously, we can report its full, polymorphic type; that is simple and
    explicable.  But sometimes a bit odd.  Consider
        f :: Bool -> t Int Int
        foo = f True 5 10
    We get this error:
        • Couldn't match type ‘Int’ with ‘t0 -> t’
          Expected: Int -> t0 -> t
            Actual: Int -> Int
        • The function ‘f’ is applied to three visible arguments,
            but its type ‘Bool -> t Int Int’ has only one

    That's not /quite/ right beause we can instantiate `t` to an arrow and get
    two arrows (but not three!).  With that in mind, one could consider reporting
    the /instantiated/ type, and GHC used to do so.  But it's more work, and in
    some ways more confusing, especially when nested quantifiers are concerned, e.g.
        f :: Bool -> forall t. t Int Int

    So we just keep it simple and report the original function type.

    ************************************************************************
    *                                                                      *
                        Other matchExpected functions
    *                                                                      *
    ********************************************************************* -}

    matchExpectedListTy :: TcRhoType -> TcM (TcCoercionN, TcRhoType)
    -- Special case for lists
    matchExpectedListTy :: TcType -> TcM (Coercion, TcType)
    matchExpectedListTy TcType
    exp_ty
     = do { (co, [elt_ty]) <- TyCon -> TcType -> TcM (Coercion, [TcType])
    matchExpectedTyConApp TyCon
    listTyCon TcType
    exp_ty
          ; return (co, elt_ty) }

    ---------------------
    matchExpectedTyConApp :: TyCon                -- T :: forall kv1 ... kvm. k1 -> ... -> kn -> *
                          -> TcRhoType            -- orig_ty
                          -> TcM (TcCoercionN,    -- T k1 k2 k3 a b c ~N orig_ty
                                  [TcSigmaType])  -- Element types, k1 k2 k3 a b c

    -- It's used for wired-in tycons, so we call checkWiredInTyCon
    -- Precondition: never called with FunTyCon
    -- Precondition: input type :: *
    -- Postcondition: (T k1 k2 k3 a b c) is well-kinded

    matchExpectedTyConApp :: TyCon -> TcType -> TcM (Coercion, [TcType])
    matchExpectedTyConApp TyCon
    tc TcType
    orig_ty
      = Bool
    -> SDoc -> TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType])
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TyCon -> Bool
    isAlgTyCon TyCon
    tc) (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc) (TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType]))
    -> TcM (Coercion, [TcType]) -> TcM (Coercion, [TcType])
    forall a b. (a -> b) -> a -> b
    $
        TcType -> TcM (Coercion, [TcType])
    go TcType
    orig_ty
      where
        go :: TcType -> TcM (Coercion, [TcType])
    go TcType
    ty
           | Just TcType
    ty' <- TcType -> Maybe TcType
    coreView TcType
    ty
           = TcType -> TcM (Coercion, [TcType])
    go TcType
    ty'

        go ty :: TcType
    ty@(TyConApp TyCon
    tycon [TcType]
    args)
           | TyCon
    tc TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tycon  -- Common case
           = (Coercion, [TcType]) -> TcM (Coercion, [TcType])
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType -> Coercion
    mkNomReflCo TcType
    ty, [TcType]
    args)

        go (TyVarTy TcTyVar
    tv)
           | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
           = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
    forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
    readMetaTyVar TcTyVar
    tv
                ; case cts of
                    Indirect TcType
    ty -> TcType -> TcM (Coercion, [TcType])
    go TcType
    ty
                    MetaDetails
    Flexi       -> TcM (Coercion, [TcType])
    defer }

        go TcType
    _ = TcM (Coercion, [TcType])
    defer

        -- If the common case does not occur, instantiate a template
        -- T k1 .. kn t1 .. tm, and unify with the original type
        -- Doing it this way ensures that the types we return are
        -- kind-compatible with T.  For example, suppose we have
        --       matchExpectedTyConApp T (f Maybe)
        -- where data T a = MkT a
        -- Then we don't want to instantiate T's data constructors with
        --    (a::*) ~ Maybe
        -- because that'll make types that are utterly ill-kinded.
        -- This happened in #7368
        defer :: TcM (Coercion, [TcType])
    defer
          = do { (_, arg_tvs) <- [TcTyVar] -> TcM (Subst, [TcTyVar])
    newMetaTyVars (TyCon -> [TcTyVar]
    tyConTyVars TyCon
    tc)
               ; traceTc "matchExpectedTyConApp" (ppr tc $$ ppr (tyConTyVars tc) $$ ppr arg_tvs)
               ; let args = [TcTyVar] -> [TcType]
    mkTyVarTys [TcTyVar]
    arg_tvs
                     tc_template = TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc [TcType]
    args
               ; co <- unifyType Nothing tc_template orig_ty
               ; return (co, args) }

    ----------------------
    matchExpectedAppTy :: TcRhoType                         -- orig_ty
                       -> TcM (TcCoercion,                   -- m a ~N orig_ty
                               (TcSigmaType, TcSigmaType))  -- Returns m, a
    -- If the incoming type is a mutable type variable of kind k, then
    -- matchExpectedAppTy returns a new type variable (m: * -> k); note the *.

    matchExpectedAppTy :: TcType -> TcM (Coercion, (TcType, TcType))
    matchExpectedAppTy TcType
    orig_ty
      = TcType -> TcM (Coercion, (TcType, TcType))
    go TcType
    orig_ty
      where
        go :: TcType -> TcM (Coercion, (TcType, TcType))
    go TcType
    ty
          | Just TcType
    ty' <- TcType -> Maybe TcType
    coreView TcType
    ty = TcType -> TcM (Coercion, (TcType, TcType))
    go TcType
    ty'

          | Just (TcType
    fun_ty, TcType
    arg_ty) <- TcType -> Maybe (TcType, TcType)
    tcSplitAppTy_maybe TcType
    ty
          = (Coercion, (TcType, TcType)) -> TcM (Coercion, (TcType, TcType))
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType -> Coercion
    mkNomReflCo TcType
    orig_ty, (TcType
    fun_ty, TcType
    arg_ty))

        go (TyVarTy TcTyVar
    tv)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    tv
          = do { cts <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
    forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
    readMetaTyVar TcTyVar
    tv
               ; case cts of
                   Indirect TcType
    ty -> TcType -> TcM (Coercion, (TcType, TcType))
    go TcType
    ty
                   MetaDetails
    Flexi       -> TcM (Coercion, (TcType, TcType))
    defer }

        go TcType
    _ = TcM (Coercion, (TcType, TcType))
    defer

        -- Defer splitting by generating an equality constraint
        defer :: TcM (Coercion, (TcType, TcType))
    defer
          = do { ty1 <- TcType -> TcM TcType
    newFlexiTyVarTy TcType
    kind1
               ; ty2 <- newFlexiTyVarTy kind2
               ; co <- unifyType Nothing (mkAppTy ty1 ty2) orig_ty
               ; return (co, (ty1, ty2)) }

        orig_kind :: TcType
    orig_kind = HasDebugCallStack => TcType -> TcType
    TcType -> TcType
    typeKind TcType
    orig_ty
        kind1 :: TcType
    kind1 = HasDebugCallStack => TcType -> TcType -> TcType
    TcType -> TcType -> TcType
    mkVisFunTyMany TcType
    liftedTypeKind TcType
    orig_kind
        kind2 :: TcType
    kind2 = TcType
    liftedTypeKind    -- m :: * -> k
                                  -- arg type :: *

    {- **********************************************************************
    *
                          fillInferResult
    *
    ********************************************************************** -}

    {- Note [inferResultToType]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    expTypeToType and inferResultType convert an InferResult to a monotype.
    It must be a monotype because if the InferResult isn't already filled in,
    we fill it in with a unification variable (hence monotype).  So to preserve
    order-independence we check for mono-type-ness even if it *is* filled in
    already.

    See also Note [TcLevel of ExpType] in GHC.Tc.Utils.TcType, and
    Note [fillInferResult].
    -}

    -- | Fill an 'InferResult' with the given type.
    --
    -- If @co = fillInferResult t1 infer_res@, then @co :: t1 ~# t2@,
    -- where @t2@ is the type stored in the 'ir_ref' field of @infer_res@.
    --
    -- This function enforces the following invariants:
    --
    --  - Level invariant.
    --    The stored type @t2@ is at the same level as given by the
    --    'ir_lvl' field.
    --  - FRR invariant.
    --    Whenever the 'ir_frr' field is not @Nothing@, @t2@ is guaranteed
    --    to have a syntactically fixed RuntimeRep, in the sense of
    --    Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete.
    fillInferResult :: TcType -> InferResult -> TcM TcCoercionN
    fillInferResult :: TcType -> InferResult -> TcM Coercion
    fillInferResult TcType
    act_res_ty (IR { ir_uniq :: InferResult -> Unique
    ir_uniq = Unique
    u
                                   , ir_lvl :: InferResult -> TcLevel
    ir_lvl  = TcLevel
    res_lvl
                                   , ir_frr :: InferResult -> Maybe FixedRuntimeRepContext
    ir_frr  = Maybe FixedRuntimeRepContext
    mb_frr
                                   , ir_ref :: InferResult -> IORef (Maybe TcType)
    ir_ref  = IORef (Maybe TcType)
    ref })
      = do { mb_exp_res_ty <- IORef (Maybe TcType)
    -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    forall (m :: * -> *) a. MonadIO m => TcRef a -> m a
    readTcRef IORef (Maybe TcType)
    ref
           ; case mb_exp_res_ty of
                Just TcType
    exp_res_ty
                   -- We progressively refine the type stored in 'ref',
                   -- for example when inferring types across multiple equations.
                   --
                   -- Example:
                   --
                   --  \ x -> case y of { True -> x ; False -> 3 :: Int }
                   --
                   -- When inferring the return type of this function, we will create
                   -- an 'Infer' 'ExpType', which will first be filled by the type of 'x'
                   -- after typechecking the first equation, and then filled again with
                   -- the type 'Int', at which point we want to ensure that we unify
                   -- the type of 'x' with 'Int'. This is what is happening below when
                   -- we are "joining" several inferred 'ExpType's.
                   -> do { String -> SDoc -> TcM ()
    traceTc String
    "Joining inferred ExpType" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
                           Unique -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Unique
    u SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    colon SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    act_res_ty SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Char -> SDoc
    forall doc. IsLine doc => Char -> doc
    char Char
    '~' SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    exp_res_ty
                         ; cur_lvl <- TcM TcLevel
    getTcLevel
                         ; unless (cur_lvl `sameDepthAs` res_lvl) $
                           ensureMonoType act_res_ty
                         ; unifyType Nothing act_res_ty exp_res_ty }
                Maybe TcType
    Nothing
                   -> do { String -> SDoc -> TcM ()
    traceTc String
    "Filling inferred ExpType" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
                           Unique -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Unique
    u SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    ":=" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    act_res_ty

                         -- Enforce the level invariant: ensure the TcLevel of
                         -- the type we are writing to 'ref' matches 'ir_lvl'.
                         ; (prom_co, act_res_ty) <- TcLevel -> TcType -> TcM (Coercion, TcType)
    promoteTcType TcLevel
    res_lvl TcType
    act_res_ty

                         -- Enforce the FRR invariant: ensure the type has a syntactically
                         -- fixed RuntimeRep (if necessary, i.e. 'mb_frr' is not 'Nothing').
                         ; (frr_co, act_res_ty) <-
                             case mb_frr of
                               Maybe FixedRuntimeRepContext
    Nothing       -> (Coercion, TcType) -> TcM (Coercion, TcType)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType -> Coercion
    mkNomReflCo TcType
    act_res_ty, TcType
    act_res_ty)
                               Just FixedRuntimeRepContext
    frr_orig -> HasDebugCallStack =>
    FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
    FixedRuntimeRepContext -> TcType -> TcM (Coercion, TcType)
    hasFixedRuntimeRep FixedRuntimeRepContext
    frr_orig TcType
    act_res_ty

                         -- Compose the two coercions.
                         ; let final_co = Coercion
    prom_co Coercion -> Coercion -> Coercion
    `mkTransCo` Coercion
    frr_co

                         ; writeTcRef ref (Just act_res_ty)

                         ; return final_co }
         }

    {- Note [fillInferResult]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    When inferring, we use fillInferResult to "fill in" the hole in InferResult
       data InferResult = IR { ir_uniq :: Unique
                             , ir_lvl  :: TcLevel
                             , ir_ref  :: IORef (Maybe TcType) }

    There are two things to worry about:

    1. What if it is under a GADT or existential pattern match?
       - GADTs: a unification variable (and Infer's hole is similar) is untouchable
       - Existentials: be careful about skolem-escape

    2. What if it is filled in more than once?  E.g. multiple branches of a case
         case e of
            T1 -> e1
            T2 -> e2

    Our typing rules are:

    * The RHS of a existential or GADT alternative must always be a
      monotype, regardless of the number of alternatives.

    * Multiple non-existential/GADT branches can have (the same)
      higher rank type (#18412).  E.g. this is OK:
          case e of
            True  -> hr
            False -> hr
      where hr:: (forall a. a->a) -> Int
      c.f. Section 7.1 of "Practical type inference for arbitrary-rank types"
           We use choice (2) in that Section.
           (GHC 8.10 and earlier used choice (1).)

      But note that
          case e of
            True  -> hr
            False -> \x -> hr x
      will fail, because we still /infer/ both branches, so the \x will get
      a (monotype) unification variable, which will fail to unify with
      (forall a. a->a)

    For (1) we can detect the GADT/existential situation by seeing that
    the current TcLevel is greater than that stored in ir_lvl of the Infer
    ExpType.  We bump the level whenever we go past a GADT/existential match.

    Then, before filling the hole use promoteTcType to promote the type
    to the outer ir_lvl.  promoteTcType does this
      - create a fresh unification variable alpha at level ir_lvl
      - emits an equality alpha[ir_lvl] ~ ty
      - fills the hole with alpha
    That forces the type to be a monotype (since unification variables can
    only unify with monotypes); and catches skolem-escapes because the
    alpha is untouchable until the equality floats out.

    For (2), we simply look to see if the hole is filled already.
      - if not, we promote (as above) and fill the hole
      - if it is filled, we simply unify with the type that is
        already there

    There is one wrinkle.  Suppose we have
       case e of
          T1 -> e1 :: (forall a. a->a) -> Int
          G2 -> e2
    where T1 is not GADT or existential, but G2 is a GADT.  Then suppose the
    T1 alternative fills the hole with (forall a. a->a) -> Int, which is fine.
    But now the G2 alternative must not *just* unify with that else we'd risk
    allowing through (e2 :: (forall a. a->a) -> Int).  If we'd checked G2 first
    we'd have filled the hole with a unification variable, which enforces a
    monotype.

    So if we check G2 second, we still want to emit a constraint that restricts
    the RHS to be a monotype. This is done by ensureMonoType, and it works
    by simply generating a constraint (alpha ~ ty), where alpha is a fresh
    unification variable.  We discard the evidence.

    -}

    {-
    ************************************************************************
    *                                                                      *
                    Subsumption checking
    *                                                                      *
    ************************************************************************

    Note [Subsumption checking: tcSubType]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    All the tcSubType calls have the form
                    tcSubType actual_ty expected_ty
    which checks
                    actual_ty <= expected_ty

    That is, that a value of type actual_ty is acceptable in
    a place expecting a value of type expected_ty.  I.e. that

        actual ty   is more polymorphic than   expected_ty

    It returns a wrapper function
            co_fn :: actual_ty ~ expected_ty
    which takes an HsExpr of type actual_ty into one of type
    expected_ty.

    Note [Ambiguity check and deep subsumption]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       f :: (forall b. Eq b => a -> a) -> Int

    Does `f` have an ambiguous type?   The ambiguity check usually checks
    that this definition of f' would typecheck, where f' has the exact same
    type as f:
       f' :: (forall b. Eq b => a -> a) -> Intp
       f' = f

    This will be /rejected/ with DeepSubsumption but /accepted/ with
    ShallowSubsumption.  On the other hand, this eta-expanded version f''
    would be rejected both ways:
       f'' :: (forall b. Eq b => a -> a) -> Intp
       f'' x = f x

    This is squishy in the same way as other examples in GHC.Tc.Validity
    Note [The squishiness of the ambiguity check]

    The situation in June 2022.  Since we have SimpleSubsumption at the moment,
    we don't want introduce new breakage if you add -XDeepSubsumption, by
    rejecting types as ambiguous that weren't ambiguous before.  So, as a
    holding decision, we /always/ use SimpleSubsumption for the ambiguity check
    (erring on the side accepting more programs). Hence tcSubTypeAmbiguity.
    -}

    -----------------
    -- tcWrapResult needs both un-type-checked (for origins and error messages)
    -- and type-checked (for wrapping) expressions
    tcWrapResult :: HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
                 -> TcM (HsExpr GhcTc)
    tcWrapResult :: HsExpr GhcRn
    -> HsExpr GhcTc -> TcType -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcWrapResult HsExpr GhcRn
    rn_expr = CtOrigin
    -> HsExpr GhcRn
    -> HsExpr GhcTc
    -> TcType
    -> ExpRhoType
    -> TcM (HsExpr GhcTc)
    tcWrapResultO (HsExpr GhcRn -> CtOrigin
    exprCtOrigin HsExpr GhcRn
    rn_expr) HsExpr GhcRn
    rn_expr

    tcWrapResultO :: CtOrigin -> HsExpr GhcRn -> HsExpr GhcTc -> TcSigmaType -> ExpRhoType
                   -> TcM (HsExpr GhcTc)
    tcWrapResultO :: CtOrigin
    -> HsExpr GhcRn
    -> HsExpr GhcTc
    -> TcType
    -> ExpRhoType
    -> TcM (HsExpr GhcTc)
    tcWrapResultO CtOrigin
    orig HsExpr GhcRn
    rn_expr HsExpr GhcTc
    expr TcType
    actual_ty ExpRhoType
    res_ty
      = do { String -> SDoc -> TcM ()
    traceTc String
    "tcWrapResult" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Actual:  " SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    actual_ty
                                          , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Expected:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> ExpRhoType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpRhoType
    res_ty ])
           ; wrap <- CtOrigin
    -> UserTypeCtxt
    -> Maybe TypedThing
    -> TcType
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    orig UserTypeCtxt
    GenSigCtxt (TypedThing -> Maybe TypedThing
    forall a. a -> Maybe a
    Just (TypedThing -> Maybe TypedThing) -> TypedThing -> Maybe TypedThing
    forall a b. (a -> b) -> a -> b
    $ HsExpr GhcRn -> TypedThing
    HsExprRnThing HsExpr GhcRn
    rn_expr) TcType
    actual_ty ExpRhoType
    res_ty
           ; return (mkHsWrap wrap expr) }

    tcWrapResultMono :: HsExpr GhcRn -> HsExpr GhcTc
                     -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                     -> ExpRhoType  -- Expected
                     -> TcM (HsExpr GhcTc)
    -- A version of tcWrapResult to use when the actual type is a
    -- rho-type, so nothing to instantiate; just go straight to unify.
    -- It means we don't need to pass in a CtOrigin
    tcWrapResultMono :: HsExpr GhcRn
    -> HsExpr GhcTc -> TcType -> ExpRhoType -> TcM (HsExpr GhcTc)
    tcWrapResultMono HsExpr GhcRn
    rn_expr HsExpr GhcTc
    expr TcType
    act_ty ExpRhoType
    res_ty
      = Bool -> SDoc -> TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc)
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TcType -> Bool
    isRhoTy TcType
    act_ty) (TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    act_ty SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ HsExpr GhcRn -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr HsExpr GhcRn
    rn_expr) (TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc))
    -> TcM (HsExpr GhcTc) -> TcM (HsExpr GhcTc)
    forall a b. (a -> b) -> a -> b
    $
        do { co <- HsExpr GhcRn -> TcType -> ExpRhoType -> TcM Coercion
    unifyExpectedType HsExpr GhcRn
    rn_expr TcType
    act_ty ExpRhoType
    res_ty
           ; return (mkHsWrapCo co expr) }

    unifyExpectedType :: HsExpr GhcRn
                      -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                      -> ExpRhoType  -- Expected
                      -> TcM TcCoercionN
    unifyExpectedType :: HsExpr GhcRn -> TcType -> ExpRhoType -> TcM Coercion
    unifyExpectedType HsExpr GhcRn
    rn_expr TcType
    act_ty ExpRhoType
    exp_ty
      = case ExpRhoType
    exp_ty of
          Infer InferResult
    inf_res -> TcType -> InferResult -> TcM Coercion
    fillInferResult TcType
    act_ty InferResult
    inf_res
          Check TcType
    exp_ty  -> Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType (TypedThing -> Maybe TypedThing
    forall a. a -> Maybe a
    Just (TypedThing -> Maybe TypedThing) -> TypedThing -> Maybe TypedThing
    forall a b. (a -> b) -> a -> b
    $ HsExpr GhcRn -> TypedThing
    HsExprRnThing HsExpr GhcRn
    rn_expr) TcType
    act_ty TcType
    exp_ty

    ------------------------
    tcSubTypePat :: CtOrigin -> UserTypeCtxt
                -> ExpSigmaType -> TcSigmaType -> TcM HsWrapper
    -- Used in patterns; polarity is backwards compared
    --   to tcSubType
    -- If wrap = tc_sub_type_et t1 t2
    --    => wrap :: t1 ~> t2
    tcSubTypePat :: CtOrigin -> UserTypeCtxt -> ExpRhoType -> TcType -> TcM HsWrapper
    tcSubTypePat CtOrigin
    inst_orig UserTypeCtxt
    ctxt (Check TcType
    ty_actual) TcType
    ty_expected
      = (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type TcType -> TcType -> TcM Coercion
    unifyTypeET CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected

    tcSubTypePat CtOrigin
    _ UserTypeCtxt
    _ (Infer InferResult
    inf_res) TcType
    ty_expected
      = do { co <- TcType -> InferResult -> TcM Coercion
    fillInferResult TcType
    ty_expected InferResult
    inf_res
                   -- In patterns we do not instantatiate

           ; return (mkWpCastN (mkSymCo co)) }

    ---------------
    tcSubType :: CtOrigin -> UserTypeCtxt
              -> TcSigmaType  -- ^ Actual
              -> ExpRhoType   -- ^ Expected
              -> TcM HsWrapper
    -- Checks that 'actual' is more polymorphic than 'expected'
    tcSubType :: CtOrigin -> UserTypeCtxt -> TcType -> ExpRhoType -> TcM HsWrapper
    tcSubType CtOrigin
    orig UserTypeCtxt
    ctxt TcType
    ty_actual ExpRhoType
    ty_expected
      = TcType -> ExpRhoType -> TcM HsWrapper -> TcM HsWrapper
    forall a. TcType -> ExpRhoType -> TcM a -> TcM a
    addSubTypeCtxt TcType
    ty_actual ExpRhoType
    ty_expected (TcM HsWrapper -> TcM HsWrapper) -> TcM HsWrapper -> TcM HsWrapper
    forall a b. (a -> b) -> a -> b
    $
        do { String -> SDoc -> TcM ()
    traceTc String
    "tcSubType" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [UserTypeCtxt -> SDoc
    pprUserTypeCtxt UserTypeCtxt
    ctxt, TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_actual, ExpRhoType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpRhoType
    ty_expected])
           ; CtOrigin
    -> UserTypeCtxt
    -> Maybe TypedThing
    -> TcType
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    orig UserTypeCtxt
    ctxt Maybe TypedThing
    forall a. Maybe a
    Nothing TcType
    ty_actual ExpRhoType
    ty_expected }

    ---------------
    tcSubTypeDS :: HsExpr GhcRn
                -> TcRhoType   -- Actual -- a rho-type not a sigma-type
                -> ExpRhoType  -- Expected
                -> TcM HsWrapper
    -- Similar signature to unifyExpectedType; does deep subsumption
    -- Only one call site, in GHC.Tc.Gen.App.tcApp
    tcSubTypeDS :: HsExpr GhcRn -> TcType -> ExpRhoType -> TcM HsWrapper
    tcSubTypeDS HsExpr GhcRn
    rn_expr TcType
    act_rho ExpRhoType
    res_ty
      = case ExpRhoType
    res_ty of
          Check TcType
    exp_rho -> DeepSubsumptionFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> TcM HsWrapper
    tc_sub_type_ds DeepSubsumptionFlag
    Deep (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType Maybe TypedThing
    m_thing) CtOrigin
    orig
                                          UserTypeCtxt
    GenSigCtxt TcType
    act_rho TcType
    exp_rho

          Infer InferResult
    inf_res -> do { co <- TcType -> InferResult -> TcM Coercion
    fillInferResult TcType
    act_rho InferResult
    inf_res
                              ; return (mkWpCastN co) }
      where
        orig :: CtOrigin
    orig    = HsExpr GhcRn -> CtOrigin
    exprCtOrigin HsExpr GhcRn
    rn_expr
        m_thing :: Maybe TypedThing
    m_thing = TypedThing -> Maybe TypedThing
    forall a. a -> Maybe a
    Just (HsExpr GhcRn -> TypedThing
    HsExprRnThing HsExpr GhcRn
    rn_expr)

    ---------------
    tcSubTypeNC :: CtOrigin          -- ^ Used when instantiating
                -> UserTypeCtxt      -- ^ Used when skolemising
                -> Maybe TypedThing -- ^ The expression that has type 'actual' (if known)
                -> TcSigmaType       -- ^ Actual type
                -> ExpRhoType        -- ^ Expected type
                -> TcM HsWrapper
    tcSubTypeNC :: CtOrigin
    -> UserTypeCtxt
    -> Maybe TypedThing
    -> TcType
    -> ExpRhoType
    -> TcM HsWrapper
    tcSubTypeNC CtOrigin
    inst_orig UserTypeCtxt
    ctxt Maybe TypedThing
    m_thing TcType
    ty_actual ExpRhoType
    res_ty
      = case ExpRhoType
    res_ty of
          Check TcType
    ty_expected -> (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType Maybe TypedThing
    m_thing) CtOrigin
    inst_orig UserTypeCtxt
    ctxt
                                           TcType
    ty_actual TcType
    ty_expected

          Infer InferResult
    inf_res -> do { (wrap, rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    topInstantiate CtOrigin
    inst_orig TcType
    ty_actual
                                       -- See Note [Instantiation of InferResult]
                              ; co <- fillInferResult rho inf_res
                              ; return (mkWpCastN co <.> wrap) }

    ---------------
    tcSubTypeSigma :: CtOrigin       -- where did the actual type arise / why are we
                                     -- doing this subtype check?
                   -> UserTypeCtxt   -- where did the expected type arise?
                   -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
    -- External entry point, but no ExpTypes on either side
    -- Checks that actual <= expected
    -- Returns HsWrapper :: actual ~ expected
    tcSubTypeSigma :: CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tcSubTypeSigma CtOrigin
    orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected
      = (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType Maybe TypedThing
    forall a. Maybe a
    Nothing) CtOrigin
    orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected

    ---------------
    tcSubTypeAmbiguity :: UserTypeCtxt   -- Where did this type arise
                       -> TcSigmaType -> TcSigmaType -> TcM HsWrapper
    -- See Note [Ambiguity check and deep subsumption]
    tcSubTypeAmbiguity :: UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tcSubTypeAmbiguity UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected
      = DeepSubsumptionFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> TcM HsWrapper
    tc_sub_type_ds DeepSubsumptionFlag
    Shallow (Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType Maybe TypedThing
    forall a. Maybe a
    Nothing)
                               (UserTypeCtxt -> CtOrigin
    AmbiguityCheckOrigin UserTypeCtxt
    ctxt)
                               UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected

    ---------------
    addSubTypeCtxt :: TcType -> ExpType -> TcM a -> TcM a
    addSubTypeCtxt :: forall a. TcType -> ExpRhoType -> TcM a -> TcM a
    addSubTypeCtxt TcType
    ty_actual ExpRhoType
    ty_expected TcM a
    thing_inside
     | TcType -> Bool
    isRhoTy TcType
    ty_actual        -- If there is no polymorphism involved, the
     , ExpRhoType -> Bool
    isRhoExpTy ExpRhoType
    ty_expected   -- TypeEqOrigin stuff (added by the _NC functions)
     = TcM a
    thing_inside             -- gives enough context by itself
     | Bool
    otherwise
     = (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM a -> TcM a
    forall a. (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM TidyEnv -> ZonkM (TidyEnv, SDoc)
    mk_msg TcM a
    thing_inside
      where
        mk_msg :: TidyEnv -> ZonkM (TidyEnv, SDoc)
    mk_msg TidyEnv
    tidy_env
          = do { (tidy_env, ty_actual)   <- TidyEnv -> TcType -> ZonkM (TidyEnv, TcType)
    zonkTidyTcType TidyEnv
    tidy_env TcType
    ty_actual
               ; ty_expected             <- readExpType ty_expected
                       -- A worry: might not be filled if we're debugging. Ugh.
               ; (tidy_env, ty_expected) <- zonkTidyTcType tidy_env ty_expected
               ; let msg = [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "When checking that:")
                                     Int
    4 (TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_actual)
                                , Int -> SDoc -> SDoc
    nest Int
    2 (SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "is more polymorphic than:")
                                             Int
    2 (TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_expected)) ]
               ; return (tidy_env, msg) }

    {- Note [Instantiation of InferResult]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We now always instantiate before filling in InferResult, so that
    the result is a TcRhoType: see #17173 for discussion.

    For example:

    1. Consider
        f x = (*)
       We want to instantiate the type of (*) before returning, else we
       will infer the type
         f :: forall {a}. a -> forall b. Num b => b -> b -> b
       This is surely confusing for users.

       And worse, the monomorphism restriction won't work properly. The MR is
       dealt with in simplifyInfer, and simplifyInfer has no way of
       instantiating. This could perhaps be worked around, but it may be
       hard to know even when instantiation should happen.

    2. Another reason.  Consider
           f :: (?x :: Int) => a -> a
           g y = let ?x = 3::Int in f
       Here want to instantiate f's type so that the ?x::Int constraint
      gets discharged by the enclosing implicit-parameter binding.

    3. Suppose one defines plus = (+). If we instantiate lazily, we will
       infer plus :: forall a. Num a => a -> a -> a. However, the monomorphism
       restriction compels us to infer
          plus :: Integer -> Integer -> Integer
       (or similar monotype). Indeed, the only way to know whether to apply
       the monomorphism restriction at all is to instantiate

    There is one place where we don't want to instantiate eagerly,
    namely in GHC.Tc.Module.tcRnExpr, which implements GHCi's :type
    command. See Note [Implementing :type] in GHC.Tc.Module.
    -}

    ---------------
    tc_sub_type :: (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
                -> CtOrigin       -- Used when instantiating
                -> UserTypeCtxt   -- Used when skolemising
                -> TcSigmaType    -- Actual; a sigma-type
                -> TcSigmaType    -- Expected; also a sigma-type
                -> TcM HsWrapper
    -- Checks that actual_ty is more polymorphic than expected_ty
    -- If wrap = tc_sub_type t1 t2
    --    => wrap :: t1 ~> t2
    --
    -- The "how to unify argument" is always a call to `uType TypeLevel orig`,
    -- but with different ways of constructing the CtOrigin `orig` from
    -- the argument types and context.

    ----------------------
    tc_sub_type :: (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type TcType -> TcType -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected
      = do { ds_flag <- TcM DeepSubsumptionFlag
    getDeepSubsumptionFlag
           ; tc_sub_type_ds ds_flag unify inst_orig ctxt ty_actual ty_expected }

    ----------------------
    tc_sub_type_ds :: DeepSubsumptionFlag
                   -> (TcType -> TcType -> TcM TcCoercionN)
                   -> CtOrigin -> UserTypeCtxt -> TcSigmaType
                   -> TcSigmaType -> TcM HsWrapper
    -- tc_sub_type_ds is the main subsumption worker function
    -- It takes an explicit DeepSubsumptionFlag
    tc_sub_type_ds :: DeepSubsumptionFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> TcM HsWrapper
    tc_sub_type_ds DeepSubsumptionFlag
    ds_flag TcType -> TcType -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected
      | TcType -> Bool
    definitely_poly TcType
    ty_expected   -- See Note [Don't skolemise unnecessarily]
      , DeepSubsumptionFlag -> TcType -> Bool
    definitely_mono DeepSubsumptionFlag
    ds_flag TcType
    ty_actual
      = do { String -> SDoc -> TcM ()
    traceTc String
    "tc_sub_type (drop to equality)" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_actual   =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_actual
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_expected =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_expected ]
           ; Coercion -> HsWrapper
    mkWpCastN (Coercion -> HsWrapper) -> TcM Coercion -> TcM HsWrapper
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$>
             TcType -> TcType -> TcM Coercion
    unify TcType
    ty_actual TcType
    ty_expected }

      | Bool
    otherwise   -- This is the general case
      = do { String -> SDoc -> TcM ()
    traceTc String
    "tc_sub_type (general case)" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_actual   =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_actual
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_expected =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_expected ]

           ; (sk_wrap, inner_wrap)
               <- case DeepSubsumptionFlag
    ds_flag of
                    DeepSubsumptionFlag
    Shallow -> -- Shallow: skolemise, instantiate and unify
                               DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> (TcType -> TcM HsWrapper)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper)
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> (TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemise DeepSubsumptionFlag
    Shallow UserTypeCtxt
    ctxt TcType
    ty_expected ((TcType -> TcM HsWrapper)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper))
    -> (TcType -> TcM HsWrapper)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper)
    forall a b. (a -> b) -> a -> b
    $ \TcType
    sk_rho ->
                               do { (wrap, rho_a) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    topInstantiate CtOrigin
    inst_orig TcType
    ty_actual
                                  ; cow           <- unify rho_a sk_rho
                                  ; return (mkWpCastN cow <.> wrap) }
                    DeepSubsumptionFlag
    Deep -> -- Deep: we have co/contra work to do
                            DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> (TcType -> TcM HsWrapper)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper)
    forall result.
    DeepSubsumptionFlag
    -> UserTypeCtxt
    -> TcType
    -> (TcType -> TcM result)
    -> TcM (HsWrapper, result)
    tcSkolemise DeepSubsumptionFlag
    Deep UserTypeCtxt
    ctxt TcType
    ty_expected ((TcType -> TcM HsWrapper)
     -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper))
    -> (TcType -> TcM HsWrapper)
    -> IOEnv (Env TcGblEnv TcLclEnv) (HsWrapper, HsWrapper)
    forall a b. (a -> b) -> a -> b
    $ \TcType
    sk_rho ->
                            (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type_deep TcType -> TcType -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    sk_rho

           ; return (sk_wrap <.> inner_wrap) }

    ----------------------
    definitely_mono :: DeepSubsumptionFlag -> TcType -> Bool
    definitely_mono :: DeepSubsumptionFlag -> TcType -> Bool
    definitely_mono DeepSubsumptionFlag
    ds_flag TcType
    ty
      = case DeepSubsumptionFlag
    ds_flag of
          DeepSubsumptionFlag
    Shallow -> TcType -> Bool
    isRhoTy TcType
    ty      -- isRhoTy: no top level forall or (=>)
          DeepSubsumptionFlag
    Deep    -> TcType -> Bool
    isDeepRhoTy TcType
    ty  -- "deep" version: no nested forall or (=>)

    definitely_poly :: TcType -> Bool
    -- A very conservative test:
    -- see Note [Don't skolemise unnecessarily]
    definitely_poly :: TcType -> Bool
    definitely_poly TcType
    ty
      | ([TcTyVar]
    tvs, [TcType]
    theta, TcType
    tau) <- TcType -> ([TcTyVar], [TcType], TcType)
    tcSplitSigmaTy TcType
    ty
      , (TcTyVar
    tv:[TcTyVar]
    _) <- [TcTyVar]
    tvs   -- At least one tyvar
      , [TcType] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcType]
    theta      -- No constraints; see (DP1)
      , TcTyVar
    tv TcTyVar -> TcType -> Bool
    `isInjectiveInType` TcType
    tau
           -- The tyvar actually occurs (DP2),
           -- and occurs in an injective position (DP3).
      = Bool
    True
      | Bool
    otherwise
      = Bool
    False

    {- Note [Don't skolemise unnecessarily]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we are trying to solve
         ty_actual   <= ty_expected
        (Char->Char) <= (forall a. a->a)
    We could skolemise the 'forall a', and then complain
    that (Char ~ a) is insoluble; but that's a pretty obscure
    error.  It's better to say that
        (Char->Char) ~ (forall a. a->a)
    fails.

    If we prematurely go to equality we'll reject a program we should
    accept (e.g. #13752).  So the test (which is only to improve error
    message) is very conservative:

     * ty_actual   is /definitely/ monomorphic: see `definitely_mono`
       This definitely_mono test comes in "shallow" and "deep" variants

     * ty_expected is /definitely/ polymorphic: see `definitely_poly`
       This definitely_poly test is more subtle than you might think.
       Here are three cases where expected_ty looks polymorphic, but
       isn't, and where it would be /wrong/ to switch to equality:

       (DP1)  (Char->Char) <= (forall a. (a~Char) => a -> a)

       (DP2)  (Char->Char) <= (forall a. Char -> Char)

       (DP3)  (Char->Char) <= (forall a. F [a] Char -> Char)
                              where type instance F [x] t = t

    Note [Wrapper returned from tcSubMult]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There is no notion of multiplicity coercion in Core, therefore the wrapper
    returned by tcSubMult (and derived functions such as tcCheckUsage and
    checkManyPattern) is quite unlike any other wrapper: it checks whether the
    coercion produced by the constraint solver is trivial, producing a type error
    if it is not. This is implemented via the WpMultCoercion wrapper, as desugared
    by GHC.HsToCore.Binds.dsHsWrapper, which does the reflexivity check.

    This wrapper needs to be placed in the term; otherwise, checking of the
    eventual coercion won't be triggered during desugaring. But it can be put
    anywhere, since it doesn't affect the desugared code.

    Why do we check this in the desugarer? It's a convenient place, since it's
    right after all the constraints are solved. We need the constraints to be
    solved to check whether they are trivial or not.

    An alternative would be to have a kind of constraint which can
    only produce trivial evidence. This would allow such checks to happen
    in the constraint solver (#18756).
    This would be similar to the existing setup for Concrete, see
      Note [The Concrete mechanism] in GHC.Tc.Utils.Concrete
        (PHASE 1 in particular).
    -}

    tcSubMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
    tcSubMult :: CtOrigin -> TcType -> TcType -> TcM HsWrapper
    tcSubMult CtOrigin
    origin TcType
    w_actual TcType
    w_expected
      | Just (TcType
    w1, TcType
    w2) <- TcType -> Maybe (TcType, TcType)
    isMultMul TcType
    w_actual =
      do { w1 <- CtOrigin -> TcType -> TcType -> TcM HsWrapper
    tcSubMult CtOrigin
    origin TcType
    w1 TcType
    w_expected
         ; w2 <- tcSubMult origin w2 w_expected
         ; return (w1 <.> w2) }
      -- Currently, we consider p*q and sup p q to be equal.  Therefore, p*q <= r is
      -- equivalent to p <= r and q <= r.  For other cases, we approximate p <= q by p
      -- ~ q.  This is not complete, but it's sound. See also Note [Overapproximating
      -- multiplicities] in Multiplicity.
    tcSubMult CtOrigin
    origin TcType
    w_actual TcType
    w_expected =
      case TcType -> TcType -> IsSubmult
    submult TcType
    w_actual TcType
    w_expected of
        IsSubmult
    Submult -> HsWrapper -> TcM HsWrapper
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return HsWrapper
    WpHole
        IsSubmult
    Unknown -> CtOrigin -> TcType -> TcType -> TcM HsWrapper
    tcEqMult CtOrigin
    origin TcType
    w_actual TcType
    w_expected

    tcEqMult :: CtOrigin -> Mult -> Mult -> TcM HsWrapper
    tcEqMult :: CtOrigin -> TcType -> TcType -> TcM HsWrapper
    tcEqMult CtOrigin
    origin TcType
    w_actual TcType
    w_expected = do
      {
      -- Note that here we do not call to `submult`, so we check
      -- for strict equality.
      ; coercion <- TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    TypeLevel CtOrigin
    origin TcType
    w_actual TcType
    w_expected
      ; return $ if isReflCo coercion then WpHole else WpMultCoercion coercion }

    {- *********************************************************************
    *                                                                      *
                        Deep subsumption
    *                                                                      *
    ********************************************************************* -}

    {- Note [Deep subsumption]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    The DeepSubsumption extension, documented here

        https://github.com/ghc-proposals/ghc-proposals/pull/511.

    makes a best-efforts attempt implement deep subsumption as it was
    prior to the Simplify Subsumption proposal:

        https://github.com/ghc-proposals/ghc-proposals/pull/287

    The effects are in these main places:

    1. In the subsumption check, tcSubType, we must do deep skolemisation:
       see the call to tcSkolemise Deep in tc_sub_type_deep

    2. In tcPolyExpr we must do deep skolemisation:
       see the call to tcSkolemise in tcSkolemiseExpType

    3. for expression type signatures (e :: ty), and functions with type
       signatures (e.g. f :: ty; f = e), we must deeply skolemise the type;
       see the call to tcDeeplySkolemise in tcSkolemiseScoped.

    4. In GHC.Tc.Gen.App.tcApp we call tcSubTypeDS to match the result
       type. Without deep subsumption, unifyExpectedType would be sufficent.

    In all these cases note that the deep skolemisation must be done /first/.
    Consider (1)
         (forall a. Int -> a -> a)  <=  Int -> (forall b. b -> b)
    We must skolemise the `forall b` before instantiating the `forall a`.
    See also Note [Deep skolemisation].

    Note that we /always/ use shallow subsumption in the ambiguity check.
    See Note [Ambiguity check and deep subsumption].

    Note [Deep skolemisation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    deeplySkolemise decomposes and skolemises a type, returning a type
    with all its arrows visible (ie not buried under foralls)

    Examples:

      deeplySkolemise (Int -> forall a. Ord a => blah)
        =  ( wp, [a], [d:Ord a], Int -> blah )
        where wp = \x:Int. /\a. \(d:Ord a). <hole> x

      deeplySkolemise  (forall a. Ord a => Maybe a -> forall b. Eq b => blah)
        =  ( wp, [a,b], [d1:Ord a,d2:Eq b], Maybe a -> blah )
        where wp = /\a.\(d1:Ord a).\(x:Maybe a)./\b.\(d2:Ord b). <hole> x

    In general,
      if      deeplySkolemise ty = (wrap, tvs, evs, rho)
        and   e :: rho
      then    wrap e :: ty
        and   'wrap' binds tvs, evs

    ToDo: this eta-abstraction plays fast and loose with termination,
          because it can introduce extra lambdas.  Maybe add a `seq` to
          fix this

    Note [Setting the argument context]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider we are doing the ambiguity check for the (bogus)
      f :: (forall a b. C b => a -> a) -> Int

    We'll call
       tcSubType ((forall a b. C b => a->a) -> Int )
                 ((forall a b. C b => a->a) -> Int )

    with a UserTypeCtxt of (FunSigCtxt "f").  Then we'll do the co/contra thing
    on the argument type of the (->) -- and at that point we want to switch
    to a UserTypeCtxt of GenSigCtxt.  Why?

    * Error messages.  If we stick with FunSigCtxt we get errors like
         * Could not deduce: C b
           from the context: C b0
            bound by the type signature for:
                f :: forall a b. C b => a->a
      But of course f does not have that type signature!
      Example tests: T10508, T7220a, Simple14

    * Implications. We may decide to build an implication for the whole
      ambiguity check, but we don't need one for each level within it,
      and TcUnify.alwaysBuildImplication checks the UserTypeCtxt.
      See Note [When to build an implication]

    Note [Multiplicity in deep subsumption]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       t1 ->{mt} t2  <=   s1 ->{ms} s2

    At the moment we /unify/ ms~mt, via tcEqMult.

    Arguably we should use `tcSubMult`. But then if mt=m0 (a unification
    variable) and ms=Many, `tcSubMult` is a no-op (since anything is a
    sub-multiplicty of Many).  But then `m0` may never get unified with
    anything.  It is then skolemised by the zonker; see GHC.HsToCore.Binds
    Note [Free tyvars on rule LHS].  So we in RULE foldr/app in GHC.Base
    we get this

     "foldr/app"     [1] forall ys m1 m2. foldr (\x{m1} \xs{m2}. (:) x xs) ys
                                           = \xs -> xs ++ ys

    where we eta-expanded that (:).  But now foldr expects an argument
    with ->{Many} and gets an argument with ->{m1} or ->{m2}, and Lint
    complains.

    The easiest solution was to use tcEqMult in tc_sub_type_deep, and
    insist on equality. This is only in the DeepSubsumption code anyway.
    -}

    data DeepSubsumptionFlag = Deep | Shallow

    getDeepSubsumptionFlag :: TcM DeepSubsumptionFlag
    getDeepSubsumptionFlag :: TcM DeepSubsumptionFlag
    getDeepSubsumptionFlag = do { ds <- Extension -> TcM Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.DeepSubsumption
                                ; if ds then return Deep else return Shallow }

    tc_sub_type_deep :: (TcType -> TcType -> TcM TcCoercionN)  -- How to unify
                     -> CtOrigin       -- Used when instantiating
                     -> UserTypeCtxt   -- Used when skolemising
                     -> TcSigmaType    -- Actual; a sigma-type
                     -> TcRhoType      -- Expected; deeply skolemised
                     -> TcM HsWrapper

    -- If wrap = tc_sub_type_deep t1 t2
    --    => wrap :: t1 ~> t2
    -- Here is where the work actually happens!
    -- Precondition: ty_expected is deeply skolemised

    tc_sub_type_deep :: (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type_deep TcType -> TcType -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_actual TcType
    ty_expected
      = do { String -> SDoc -> TcM ()
    traceTc String
    "tc_sub_type_deep" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_actual   =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_actual
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ty_expected =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_expected ]
           ; TcType -> TcType -> TcM HsWrapper
    go TcType
    ty_actual TcType
    ty_expected }
      where
        -- NB: 'go' is not recursive, except for doing coreView
        go :: TcType -> TcType -> TcM HsWrapper
    go TcType
    ty_a TcType
    ty_e | Just TcType
    ty_a' <- TcType -> Maybe TcType
    coreView TcType
    ty_a = TcType -> TcType -> TcM HsWrapper
    go TcType
    ty_a' TcType
    ty_e
                     | Just TcType
    ty_e' <- TcType -> Maybe TcType
    coreView TcType
    ty_e = TcType -> TcType -> TcM HsWrapper
    go TcType
    ty_a  TcType
    ty_e'

        go (TyVarTy TcTyVar
    tv_a) TcType
    ty_e
          = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv_a
               ; case lookup_res of
                   Just TcType
    ty_a' ->
                     do { String -> SDoc -> TcM ()
    traceTc String
    "tc_sub_type_deep following filled meta-tyvar:"
                            (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv_a SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "-->" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty_a')
                        ; (TcType -> TcType -> TcM Coercion)
    -> CtOrigin -> UserTypeCtxt -> TcType -> TcType -> TcM HsWrapper
    tc_sub_type_deep TcType -> TcType -> TcM Coercion
    unify CtOrigin
    inst_orig UserTypeCtxt
    ctxt TcType
    ty_a' TcType
    ty_e }
                   Maybe TcType
    Nothing -> TcType -> TcType -> TcM HsWrapper
    just_unify TcType
    ty_actual TcType
    ty_expected }

        go ty_a :: TcType
    ty_a@(FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af1, ft_mult :: TcType -> TcType
    ft_mult = TcType
    act_mult, ft_arg :: TcType -> TcType
    ft_arg = TcType
    act_arg, ft_res :: TcType -> TcType
    ft_res = TcType
    act_res })
           ty_e :: TcType
    ty_e@(FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af2, ft_mult :: TcType -> TcType
    ft_mult = TcType
    exp_mult, ft_arg :: TcType -> TcType
    ft_arg = TcType
    exp_arg, ft_res :: TcType -> TcType
    ft_res = TcType
    exp_res })
          | FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af1, FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af2
          = if (TcType -> Bool
    isTauTy TcType
    ty_a Bool -> Bool -> Bool
    && TcType -> Bool
    isTauTy TcType
    ty_e)       -- Short cut common case to avoid
            then TcType -> TcType -> TcM HsWrapper
    just_unify TcType
    ty_actual TcType
    ty_expected   -- unnecessary eta expansion
            else
            -- This is where we do the co/contra thing, and generate a WpFun, which in turn
            -- causes eta-expansion, which we don't like; hence encouraging NoDeepSubsumption
            do { arg_wrap  <- DeepSubsumptionFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> CtOrigin
    -> UserTypeCtxt
    -> TcType
    -> TcType
    -> TcM HsWrapper
    tc_sub_type_ds DeepSubsumptionFlag
    Deep TcType -> TcType -> TcM Coercion
    unify CtOrigin
    given_orig UserTypeCtxt
    GenSigCtxt TcType
    exp_arg TcType
    act_arg
                              -- GenSigCtxt: See Note [Setting the argument context]
               ; res_wrap  <- tc_sub_type_deep unify inst_orig ctxt act_res exp_res
               ; mult_wrap <- tcEqMult inst_orig act_mult exp_mult
                              -- See Note [Multiplicity in deep subsumption]
               ; return (mult_wrap <.>
                         mkWpFun arg_wrap res_wrap (Scaled exp_mult exp_arg) exp_res) }
                         -- arg_wrap :: exp_arg ~> act_arg
                         -- res_wrap :: act-res ~> exp_res
          where
            given_orig :: CtOrigin
    given_orig = SkolemInfoAnon -> CtOrigin
    GivenOrigin (UserTypeCtxt -> TcType -> [(Name, TcTyVar)] -> SkolemInfoAnon
    SigSkol UserTypeCtxt
    GenSigCtxt TcType
    exp_arg [])

        go TcType
    ty_a TcType
    ty_e
          | let ([TcTyVar]
    tvs, [TcType]
    theta, TcType
    _) = TcType -> ([TcTyVar], [TcType], TcType)
    tcSplitSigmaTy TcType
    ty_a
          , Bool -> Bool
    not ([TcTyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcTyVar]
    tvs Bool -> Bool -> Bool
    && [TcType] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcType]
    theta)
          = do { (in_wrap, in_rho) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    topInstantiate CtOrigin
    inst_orig TcType
    ty_a
               ; body_wrap <- tc_sub_type_deep unify inst_orig ctxt in_rho ty_e
               ; return (body_wrap <.> in_wrap) }

          | Bool
    otherwise   -- Revert to unification
          = do { -- It's still possible that ty_actual has nested foralls. Instantiate
                 -- these, as there's no way unification will succeed with them in.
                 -- See typecheck/should_compile/T11305 for an example of when this
                 -- is important. The problem is that we're checking something like
                 --  a -> forall b. b -> b     <=   alpha beta gamma
                 -- where we end up with alpha := (->)
                 (inst_wrap, rho_a) <- CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    deeplyInstantiate CtOrigin
    inst_orig TcType
    ty_actual
               ; unify_wrap         <- just_unify rho_a ty_expected
               ; return (unify_wrap <.> inst_wrap) }

        just_unify :: TcType -> TcType -> TcM HsWrapper
    just_unify TcType
    ty_a TcType
    ty_e = do { cow <- TcType -> TcType -> TcM Coercion
    unify TcType
    ty_a TcType
    ty_e
                                  ; return (mkWpCastN cow) }

    -----------------------
    deeplySkolemise :: SkolemInfo -> TcSigmaType
                    -> TcM ( HsWrapper
                           , [(Name,TcInvisTVBinder)]     -- All skolemised variables
                           , [EvVar]                      -- All "given"s
                           , TcRhoType )
    -- See Note [Deep skolemisation]
    deeplySkolemise :: SkolemInfo
    -> TcType
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    deeplySkolemise SkolemInfo
    skol_info TcType
    ty
      = Subst
    -> TcType
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    go Subst
    init_subst TcType
    ty
      where
        init_subst :: Subst
    init_subst = InScopeSet -> Subst
    mkEmptySubst (VarSet -> InScopeSet
    mkInScopeSet (TcType -> VarSet
    tyCoVarsOfType TcType
    ty))

        go :: Subst
    -> TcType
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    go Subst
    subst TcType
    ty
          | Just ([Scaled TcType]
    arg_tys, [TcInvisTVBinder]
    bndrs, [TcType]
    theta, TcType
    ty') <- TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    tcDeepSplitSigmaTy_maybe TcType
    ty
          = do { let arg_tys' :: [Scaled TcType]
    arg_tys' = HasDebugCallStack => Subst -> [Scaled TcType] -> [Scaled TcType]
    Subst -> [Scaled TcType] -> [Scaled TcType]
    substScaledTys Subst
    subst [Scaled TcType]
    arg_tys
               ; ids1             <- FastString -> [Scaled TcType] -> TcRnIf TcGblEnv TcLclEnv [TcTyVar]
    forall gbl lcl.
    FastString -> [Scaled TcType] -> TcRnIf gbl lcl [TcTyVar]
    newSysLocalIds (String -> FastString
    fsLit String
    "dk") [Scaled TcType]
    arg_tys'
               ; (subst', bndrs1) <- tcInstSkolTyVarBndrsX skol_info subst bndrs
               ; ev_vars1         <- newEvVars (substTheta subst' theta)
               ; (wrap, tvs_prs2, ev_vars2, rho) <- go subst' ty'
               ; let tvs     = [TcInvisTVBinder] -> [TcTyVar]
    forall tv argf. [VarBndr tv argf] -> [tv]
    binderVars [TcInvisTVBinder]
    bndrs
                     tvs1    = [TcInvisTVBinder] -> [TcTyVar]
    forall tv argf. [VarBndr tv argf] -> [tv]
    binderVars [TcInvisTVBinder]
    bndrs1
                     tv_prs1 = (TcTyVar -> Name) -> [TcTyVar] -> [Name]
    forall a b. (a -> b) -> [a] -> [b]
    map TcTyVar -> Name
    tyVarName [TcTyVar]
    tvs [Name] -> [TcInvisTVBinder] -> [(Name, TcInvisTVBinder)]
    forall a b. [a] -> [b] -> [(a, b)]
    `zip` [TcInvisTVBinder]
    bndrs1
               ; return ( mkWpEta ids1 (mkWpTyLams tvs1
                                        <.> mkWpEvLams ev_vars1
                                        <.> wrap)
                        , tv_prs1  ++ tvs_prs2
                        , ev_vars1 ++ ev_vars2
                        , mkScaledFunTys arg_tys' rho ) }

          | Bool
    otherwise
          = (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    -> IOEnv
         (Env TcGblEnv TcLclEnv)
         (HsWrapper, [(Name, TcInvisTVBinder)], [TcTyVar], TcType)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, [], [], HasDebugCallStack => Subst -> TcType -> TcType
    Subst -> TcType -> TcType
    substTy Subst
    subst TcType
    ty)
            -- substTy is a quick no-op on an empty substitution

    deeplyInstantiate :: CtOrigin -> TcType -> TcM (HsWrapper, Type)
    deeplyInstantiate :: CtOrigin -> TcType -> TcM (HsWrapper, TcType)
    deeplyInstantiate CtOrigin
    orig TcType
    ty
      = Subst -> TcType -> TcM (HsWrapper, TcType)
    go Subst
    init_subst TcType
    ty
      where
        init_subst :: Subst
    init_subst = InScopeSet -> Subst
    mkEmptySubst (VarSet -> InScopeSet
    mkInScopeSet (TcType -> VarSet
    tyCoVarsOfType TcType
    ty))

        go :: Subst -> TcType -> TcM (HsWrapper, TcType)
    go Subst
    subst TcType
    ty
          | Just ([Scaled TcType]
    arg_tys, [TcInvisTVBinder]
    bndrs, [TcType]
    theta, TcType
    rho) <- TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    tcDeepSplitSigmaTy_maybe TcType
    ty
          = do { let tvs :: [TcTyVar]
    tvs = [TcInvisTVBinder] -> [TcTyVar]
    forall tv argf. [VarBndr tv argf] -> [tv]
    binderVars [TcInvisTVBinder]
    bndrs
               ; (subst', tvs') <- Subst -> [TcTyVar] -> TcM (Subst, [TcTyVar])
    newMetaTyVarsX Subst
    subst [TcTyVar]
    tvs
               ; let arg_tys' = HasDebugCallStack => Subst -> [Scaled TcType] -> [Scaled TcType]
    Subst -> [Scaled TcType] -> [Scaled TcType]
    substScaledTys   Subst
    subst' [Scaled TcType]
    arg_tys
                     theta'   = HasDebugCallStack => Subst -> [TcType] -> [TcType]
    Subst -> [TcType] -> [TcType]
    substTheta Subst
    subst' [TcType]
    theta
               ; ids1  <- newSysLocalIds (fsLit "di") arg_tys'
               ; wrap1 <- instCall orig (mkTyVarTys tvs') theta'
               ; (wrap2, rho2) <- go subst' rho
               ; return (mkWpEta ids1 (wrap2 <.> wrap1),
                         mkScaledFunTys arg_tys' rho2) }

          | Bool
    otherwise
          = do { let ty' :: TcType
    ty' = HasDebugCallStack => Subst -> TcType -> TcType
    Subst -> TcType -> TcType
    substTy Subst
    subst TcType
    ty
               ; (HsWrapper, TcType) -> TcM (HsWrapper, TcType)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (HsWrapper
    idHsWrapper, TcType
    ty') }

    tcDeepSplitSigmaTy_maybe
      :: TcSigmaType -> Maybe ([Scaled TcType], [TcInvisTVBinder], ThetaType, TcSigmaType)
    -- Looks for a *non-trivial* quantified type, under zero or more function arrows
    -- By "non-trivial" we mean either tyvars or constraints are non-empty
    tcDeepSplitSigmaTy_maybe :: TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    tcDeepSplitSigmaTy_maybe TcType
    ty
      = TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    go TcType
    ty
      where
      go :: TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    go TcType
    ty | Just (Scaled TcType
    arg_ty, TcType
    res_ty)           <- TcType -> Maybe (Scaled TcType, TcType)
    tcSplitFunTy_maybe TcType
    ty
            , Just ([Scaled TcType]
    arg_tys, [TcInvisTVBinder]
    tvs, [TcType]
    theta, TcType
    rho) <- TcType
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    go TcType
    res_ty
            = ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    forall a. a -> Maybe a
    Just (Scaled TcType
    arg_tyScaled TcType -> [Scaled TcType] -> [Scaled TcType]
    forall a. a -> [a] -> [a]
    :[Scaled TcType]
    arg_tys, [TcInvisTVBinder]
    tvs, [TcType]
    theta, TcType
    rho)

            | ([TcInvisTVBinder]
    tvs, [TcType]
    theta, TcType
    rho) <- TcType -> ([TcInvisTVBinder], [TcType], TcType)
    tcSplitSigmaTyBndrs TcType
    ty
            , Bool -> Bool
    not ([TcInvisTVBinder] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcInvisTVBinder]
    tvs Bool -> Bool -> Bool
    && [TcType] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TcType]
    theta)
            = ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    -> Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    forall a. a -> Maybe a
    Just ([], [TcInvisTVBinder]
    tvs, [TcType]
    theta, TcType
    rho)

            | Bool
    otherwise = Maybe ([Scaled TcType], [TcInvisTVBinder], [TcType], TcType)
    forall a. Maybe a
    Nothing

    isDeepRhoTy :: TcType -> Bool
    -- True if there are no foralls or (=>) at the top, or nested under
    -- arrows to the right.  e.g
    --    forall a. a                  False
    --    Int -> forall a. a           False
    --    (forall a. a) -> Int         True
    -- Returns True iff tcDeepSplitSigmaTy_maybe returns Nothing
    isDeepRhoTy :: TcType -> Bool
    isDeepRhoTy TcType
    ty
      | Bool -> Bool
    not (TcType -> Bool
    isRhoTy TcType
    ty)                       = Bool
    False  -- Foralls or (=>) at top
      | Just (Scaled TcType
    _, TcType
    res) <- TcType -> Maybe (Scaled TcType, TcType)
    tcSplitFunTy_maybe TcType
    ty = TcType -> Bool
    isDeepRhoTy TcType
    res
      | Bool
    otherwise                              = Bool
    True   -- No forall, (=>), or (->) at top

    {-
    ************************************************************************
    *                                                                      *
                    Boxy unification
    *                                                                      *
    ************************************************************************

    The exported functions are all defined as versions of some
    non-exported generic functions.
    -}

    unifyType :: Maybe TypedThing  -- ^ If present, the thing that has type ty1
              -> TcTauType -> TcTauType    -- ty1 (actual), ty2 (expected)
              -> TcM TcCoercionN           -- :: ty1 ~# ty2
    -- Actual and expected types
    -- Returns a coercion : ty1 ~ ty2
    unifyType :: Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyType Maybe TypedThing
    thing TcType
    ty1 TcType
    ty2
      = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    TypeLevel CtOrigin
    origin TcType
    ty1 TcType
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: TcType
    uo_actual   = TcType
    ty1
                              , uo_expected :: TcType
    uo_expected = TcType
    ty2
                              , uo_thing :: Maybe TypedThing
    uo_thing    = Maybe TypedThing
    thing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    unifyInvisibleType :: TcTauType -> TcTauType    -- ty1 (actual), ty2 (expected)
                       -> TcM TcCoercionN           -- :: ty1 ~# ty2
    -- Actual and expected types
    -- Returns a coercion : ty1 ~ ty2
    unifyInvisibleType :: TcType -> TcType -> TcM Coercion
    unifyInvisibleType TcType
    ty1 TcType
    ty2
      = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    TypeLevel CtOrigin
    origin TcType
    ty1 TcType
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: TcType
    uo_actual   = TcType
    ty1
                              , uo_expected :: TcType
    uo_expected = TcType
    ty2
                              , uo_thing :: Maybe TypedThing
    uo_thing    = Maybe TypedThing
    forall a. Maybe a
    Nothing
                              , uo_visible :: Bool
    uo_visible  = Bool
    False }  -- This is the "invisible" bit

    unifyTypeET :: TcTauType -> TcTauType -> TcM CoercionN
    -- Like unifyType, but swap expected and actual in error messages
    -- This is used when typechecking patterns
    unifyTypeET :: TcType -> TcType -> TcM Coercion
    unifyTypeET TcType
    ty1 TcType
    ty2
      = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    TypeLevel CtOrigin
    origin TcType
    ty1 TcType
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: TcType
    uo_actual   = TcType
    ty2   -- NB swapped
                              , uo_expected :: TcType
    uo_expected = TcType
    ty1   -- NB swapped
                              , uo_thing :: Maybe TypedThing
    uo_thing    = Maybe TypedThing
    forall a. Maybe a
    Nothing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    unifyKind :: Maybe TypedThing -> TcKind -> TcKind -> TcM CoercionN
    unifyKind :: Maybe TypedThing -> TcType -> TcType -> TcM Coercion
    unifyKind Maybe TypedThing
    mb_thing TcType
    ty1 TcType
    ty2
      = TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    KindLevel CtOrigin
    origin TcType
    ty1 TcType
    ty2
      where
        origin :: CtOrigin
    origin = TypeEqOrigin { uo_actual :: TcType
    uo_actual   = TcType
    ty1
                              , uo_expected :: TcType
    uo_expected = TcType
    ty2
                              , uo_thing :: Maybe TypedThing
    uo_thing    = Maybe TypedThing
    mb_thing
                              , uo_visible :: Bool
    uo_visible  = Bool
    True }

    unifyTypeAndEmit :: TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM CoercionN
    -- Make a ref-cell, unify, emit the collected constraints
    unifyTypeAndEmit :: TypeOrKind -> CtOrigin -> TcType -> TcType -> TcM Coercion
    unifyTypeAndEmit TypeOrKind
    t_or_k CtOrigin
    orig TcType
    ty1 TcType
    ty2
      = do { ref <- Bag Ct -> IOEnv (Env TcGblEnv TcLclEnv) (TcRef (Bag Ct))
    forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
    newTcRef Bag Ct
    forall a. Bag a
    emptyBag
           ; loc <- getCtLocM orig (Just t_or_k)
           ; let env = UE { u_loc :: CtLoc
    u_loc = CtLoc
    loc, u_role :: Role
    u_role = Role
    Nominal
                          , u_rewriters :: RewriterSet
    u_rewriters = RewriterSet
    emptyRewriterSet  -- ToDo: check this
                          , u_defer :: TcRef (Bag Ct)
    u_defer = TcRef (Bag Ct)
    ref, u_unified :: Maybe (TcRef [TcTyVar])
    u_unified = Maybe (TcRef [TcTyVar])
    forall a. Maybe a
    Nothing }

           -- The hard work happens here
           ; co <- uType env ty1 ty2

           ; cts <- readTcRef ref
           ; unless (null cts) (emitSimples cts)
           ; return co }

    {-
    %************************************************************************
    %*                                                                      *
                     uType and friends
    %*                                                                      *
    %************************************************************************

    Note [The eager unifier]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    The eager unifier, `uType`, is called by

      * The constraint generator (e.g. in GHC.Tc.Gen.Expr),
        via the wrappers `unifyType`, `unifyKind` etc

      * The constraint solver (e.g. in GHC.Tc.Solver.Equality),
        via `GHC.Tc.Solver.Monad.wrapUnifierTcS`.

    `uType` runs in the TcM monad, but it carries a UnifyEnv that tells it
    what to do when unifying a variable or deferring a constraint. Specifically,
      * it collects deferred constraints in `u_defer`, and
      * it records which unification variables it has unified in `u_unified`
    Then it is up to the wrappers (one for the constraint generator, one for
    the constraint solver) to deal with these collected sets.

    Although `uType` runs in the TcM monad for convenience, really it could
    operate just with the ability to
      * write to the accumulators of deferred constraints
        and unification variables in UnifyEnv.
      * read and update existing unification variables
      * zonk types befire unifying (`zonkTcType` in `uUnfilledVar`, and
        `zonkTyCoVarKind` in `uUnfilledVar1`
      * create fresh coercion holes (`newCoercionHole`)
      * emit tracing info for debugging
      * look at the ambient TcLevel: `getTcLevel`
    A job for the future.
    -}

    data UnifyEnv
      = UE { UnifyEnv -> Role
    u_role      :: Role
           , UnifyEnv -> CtLoc
    u_loc       :: CtLoc
           , UnifyEnv -> RewriterSet
    u_rewriters :: RewriterSet

             -- Deferred constraints
           , UnifyEnv -> TcRef (Bag Ct)
    u_defer     :: TcRef (Bag Ct)

             -- Which variables are unified;
             -- if Nothing, we don't care
           , UnifyEnv -> Maybe (TcRef [TcTyVar])
    u_unified :: Maybe (TcRef [TcTyVar])
        }

    setUEnvRole :: UnifyEnv -> Role -> UnifyEnv
    setUEnvRole :: UnifyEnv -> Role -> UnifyEnv
    setUEnvRole UnifyEnv
    uenv Role
    role = UnifyEnv
    uenv { u_role = role }

    updUEnvLoc :: UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
    updUEnvLoc :: UnifyEnv -> (CtLoc -> CtLoc) -> UnifyEnv
    updUEnvLoc uenv :: UnifyEnv
    uenv@(UE { u_loc :: UnifyEnv -> CtLoc
    u_loc = CtLoc
    loc }) CtLoc -> CtLoc
    upd = UnifyEnv
    uenv { u_loc = upd loc }

    mkKindEnv :: UnifyEnv -> TcType -> TcType -> UnifyEnv
    -- Modify the UnifyEnv to be right for unifing
    -- the kinds of these two types
    mkKindEnv :: UnifyEnv -> TcType -> TcType -> UnifyEnv
    mkKindEnv env :: UnifyEnv
    env@(UE { u_loc :: UnifyEnv -> CtLoc
    u_loc = CtLoc
    ctloc }) TcType
    ty1 TcType
    ty2
      = UnifyEnv
    env { u_role = Nominal, u_loc = mkKindEqLoc ty1 ty2 ctloc }

    uType, uType_defer
      :: UnifyEnv
      -> TcType    -- ty1 is the *actual* type
      -> TcType    -- ty2 is the *expected* type
      -> TcM CoercionN

    -- It is always safe to defer unification to the main constraint solver
    -- See Note [Deferred unification]
    uType_defer :: UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType_defer (UE { u_loc :: UnifyEnv -> CtLoc
    u_loc = CtLoc
    loc, u_defer :: UnifyEnv -> TcRef (Bag Ct)
    u_defer = TcRef (Bag Ct)
    ref
                    , u_role :: UnifyEnv -> Role
    u_role = Role
    role, u_rewriters :: UnifyEnv -> RewriterSet
    u_rewriters = RewriterSet
    rewriters })
                TcType
    ty1 TcType
    ty2  -- ty1 is "actual", ty2 is "expected"
      = do { let pred_ty :: TcType
    pred_ty = Role -> TcType -> TcType -> TcType
    mkPrimEqPredRole Role
    role TcType
    ty1 TcType
    ty2
           ; hole <- CtLoc -> TcType -> TcM CoercionHole
    newCoercionHole CtLoc
    loc TcType
    pred_ty
           ; let ct = CtEvidence -> Ct
    mkNonCanonical (CtEvidence -> Ct) -> CtEvidence -> Ct
    forall a b. (a -> b) -> a -> b
    $
                      CtWanted { ctev_pred :: TcType
    ctev_pred      = TcType
    pred_ty
                               , ctev_dest :: TcEvDest
    ctev_dest      = CoercionHole -> TcEvDest
    HoleDest CoercionHole
    hole
                               , ctev_loc :: CtLoc
    ctev_loc       = CtLoc
    loc
                               , ctev_rewriters :: RewriterSet
    ctev_rewriters = RewriterSet
    rewriters }
                 co = CoercionHole -> Coercion
    HoleCo CoercionHole
    hole
           ; updTcRef ref (`snocBag` ct)
             -- snocBag: see Note [Work-list ordering] in GHC.Tc.Solver.Equality

           -- Error trace only
           -- NB. do *not* call mkErrInfo unless tracing is on,
           --     because it is hugely expensive (#5631)
           ; whenDOptM Opt_D_dump_tc_trace $
             do { ctxt <- getErrCtxt
                ; doc  <- mkErrInfo emptyTidyEnv ctxt
                ; traceTc "utype_defer" (vcat [ ppr role
                                              , debugPprType ty1
                                              , debugPprType ty2
                                              , doc])
                ; traceTc "utype_defer2" (ppr co) }

           ; return co }

    --------------
    uType :: UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType env :: UnifyEnv
    env@(UE { u_role :: UnifyEnv -> Role
    u_role = Role
    role }) TcType
    orig_ty1 TcType
    orig_ty2
      | Role
    Phantom <- Role
    role
      = do { kind_co <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType (UnifyEnv -> TcType -> TcType -> UnifyEnv
    mkKindEnv UnifyEnv
    env TcType
    orig_ty1 TcType
    orig_ty2)
                              (HasDebugCallStack => TcType -> TcType
    TcType -> TcType
    typeKind TcType
    orig_ty1) (HasDebugCallStack => TcType -> TcType
    TcType -> TcType
    typeKind TcType
    orig_ty2)
           ; return (mkPhantomCo kind_co orig_ty1 orig_ty2) }

      | Bool
    otherwise
      = do { tclvl <- TcM TcLevel
    getTcLevel
           ; traceTc "u_tys" $ vcat
                  [ text "tclvl" <+> ppr tclvl
                  , sep [ ppr orig_ty1, text "~" <> ppr role, ppr orig_ty2] ]
           ; co <- go orig_ty1 orig_ty2
           ; if isReflCo co
                then traceTc "u_tys yields no coercion" Outputable.empty
                else traceTc "u_tys yields coercion:" (ppr co)
           ; return co }
      where
        go :: TcType -> TcType -> TcM CoercionN
            -- The arguments to 'go' are always semantically identical
            -- to orig_ty{1,2} except for looking through type synonyms

         -- Unwrap casts before looking for variables. This way, we can easily
         -- recognize (t |> co) ~ (t |> co), which is nice. Previously, we
         -- didn't do it this way, and then the unification above was deferred.
        go :: TcType -> TcType -> TcM Coercion
    go (CastTy TcType
    t1 Coercion
    co1) TcType
    t2
          = do { co_tys <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env TcType
    t1 TcType
    t2
               ; return (mkCoherenceLeftCo role t1 co1 co_tys) }

        go TcType
    t1 (CastTy TcType
    t2 Coercion
    co2)
          = do { co_tys <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env TcType
    t1 TcType
    t2
               ; return (mkCoherenceRightCo role t2 co2 co_tys) }

            -- Variables; go for uUnfilledVar
            -- Note that we pass in *original* (before synonym expansion),
            -- so that type variables tend to get filled in with
            -- the most informative version of the type
        go (TyVarTy TcTyVar
    tv1) TcType
    ty2
          = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv1
               ; case lookup_res of
                   Just TcType
    ty1 -> do { String -> SDoc -> TcM ()
    traceTc String
    "found filled tyvar" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    ":->" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1)
                                  ; UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env TcType
    ty1 TcType
    orig_ty2 }
                   Maybe TcType
    Nothing -> UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar UnifyEnv
    env SwapFlag
    NotSwapped TcTyVar
    tv1 TcType
    ty2 }

        go TcType
    ty1 (TyVarTy TcTyVar
    tv2)
          = do { lookup_res <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    tv2
               ; case lookup_res of
                   Just TcType
    ty2 -> do { String -> SDoc -> TcM ()
    traceTc String
    "found filled tyvar" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv2 SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    ":->" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2)
                                  ; UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env TcType
    orig_ty1 TcType
    ty2 }
                   Maybe TcType
    Nothing -> UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar UnifyEnv
    env SwapFlag
    IsSwapped TcTyVar
    tv2 TcType
    ty1 }

          -- See Note [Expanding synonyms during unification]
        go ty1 :: TcType
    ty1@(TyConApp TyCon
    tc1 []) (TyConApp TyCon
    tc2 [])
          | TyCon
    tc1 TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2
          = Coercion -> TcM Coercion
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Role -> TcType -> Coercion
    mkReflCo Role
    role TcType
    ty1

            -- See Note [Expanding synonyms during unification]
            --
            -- Also NB that we recurse to 'go' so that we don't push a
            -- new item on the origin stack. As a result if we have
            --   type Foo = Int
            -- and we try to unify  Foo ~ Bool
            -- we'll end up saying "can't match Foo with Bool"
            -- rather than "can't match "Int with Bool".  See #4535.
        go TcType
    ty1 TcType
    ty2
          | Just TcType
    ty1' <- TcType -> Maybe TcType
    coreView TcType
    ty1 = TcType -> TcType -> TcM Coercion
    go TcType
    ty1' TcType
    ty2
          | Just TcType
    ty2' <- TcType -> Maybe TcType
    coreView TcType
    ty2 = TcType -> TcType -> TcM Coercion
    go TcType
    ty1  TcType
    ty2'

        -- Functions (t1 -> t2) just check the two parts
        go (FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af1, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w1, ft_arg :: TcType -> TcType
    ft_arg = TcType
    arg1, ft_res :: TcType -> TcType
    ft_res = TcType
    res1 })
           (FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af2, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w2, ft_arg :: TcType -> TcType
    ft_arg = TcType
    arg2, ft_res :: TcType -> TcType
    ft_res = TcType
    res2 })
          | FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af1  -- Do not attempt (c => t); just defer
          , FunTyFlag
    af1 FunTyFlag -> FunTyFlag -> Bool
    forall a. Eq a => a -> a -> Bool
    == FunTyFlag
    af2           -- Important!  See #21530
          = do { co_w <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType (UnifyEnv
    env { u_role = funRole role SelMult }) TcType
    w1   TcType
    w2
               ; co_l <- uType (env { u_role = funRole role SelArg })  arg1 arg2
               ; co_r <- uType (env { u_role = funRole role SelRes })  res1 res2
               ; return $ mkNakedFunCo role af1 co_w co_l co_r }

            -- Always defer if a type synonym family (type function)
            -- is involved.  (Data families behave rigidly.)
        go ty1 :: TcType
    ty1@(TyConApp TyCon
    tc1 [TcType]
    _) TcType
    ty2
          | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc1 = TcType -> TcType -> TcM Coercion
    defer TcType
    ty1 TcType
    ty2
        go TcType
    ty1 ty2 :: TcType
    ty2@(TyConApp TyCon
    tc2 [TcType]
    _)
          | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc2 = TcType -> TcType -> TcM Coercion
    defer TcType
    ty1 TcType
    ty2

        go (TyConApp TyCon
    tc1 [TcType]
    tys1) (TyConApp TyCon
    tc2 [TcType]
    tys2)
          -- See Note [Mismatched type lists and application decomposition]
          | TyCon
    tc1 TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    tc2, [TcType] -> [TcType] -> Bool
    forall a b. [a] -> [b] -> Bool
    equalLength [TcType]
    tys1 [TcType]
    tys2
          , TyCon -> Role -> Bool
    isInjectiveTyCon TyCon
    tc1 Role
    role -- don't look under newtypes at Rep equality
          = Bool -> SDoc -> TcM Coercion -> TcM Coercion
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TyCon -> Role -> Bool
    isGenerativeTyCon TyCon
    tc1 Role
    role) (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc1) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $
            do { String -> SDoc -> TcM ()
    traceTc String
    "go-tycon" (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc1 SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ [TcType] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys1 SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ [TcType] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcType]
    tys2 SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ [Role] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (Int -> [Role] -> [Role]
    forall a. Int -> [a] -> [a]
    take Int
    10 (Role -> TyCon -> [Role]
    tyConRoleListX Role
    role TyCon
    tc1)))
               ; cos <- (Bool -> Role -> TcType -> TcType -> TcM Coercion)
    -> [Bool]
    -> [Role]
    -> [TcType]
    -> [TcType]
    -> IOEnv (Env TcGblEnv TcLclEnv) [Coercion]
    forall (m :: * -> *) a b c d e.
    Monad m =>
    (a -> b -> c -> d -> m e) -> [a] -> [b] -> [c] -> [d] -> m [e]
    zipWith4M Bool -> Role -> TcType -> TcType -> TcM Coercion
    u_tc_arg (TyCon -> [Bool]
    tyConVisibilities TyCon
    tc1)   -- Infinite
                                           (Role -> TyCon -> [Role]
    tyConRoleListX Role
    role TyCon
    tc1) -- Infinite
                                           [TcType]
    tys1 [TcType]
    tys2
               ; return $ mkTyConAppCo role tc1 cos }

        go (LitTy TyLit
    m) ty :: TcType
    ty@(LitTy TyLit
    n)
          | TyLit
    m TyLit -> TyLit -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyLit
    n
          = Coercion -> TcM Coercion
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> TcM Coercion) -> Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $ Role -> TcType -> Coercion
    mkReflCo Role
    role TcType
    ty

            -- See Note [Care with type applications]
            -- Do not decompose FunTy against App;
            -- it's often a type error, so leave it for the constraint solver
        go ty1 :: TcType
    ty1@(AppTy TcType
    s1 TcType
    t1) ty2 :: TcType
    ty2@(AppTy TcType
    s2 TcType
    t2)
          = Bool
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcM Coercion
    go_app (TcType -> Bool
    isNextArgVisible TcType
    s1) TcType
    ty1 TcType
    s1 TcType
    t1 TcType
    ty2 TcType
    s2 TcType
    t2

        go ty1 :: TcType
    ty1@(AppTy TcType
    s1 TcType
    t1) ty2 :: TcType
    ty2@(TyConApp TyCon
    tc2 [TcType]
    ts2)
          | Just ([TcType]
    ts2', TcType
    t2') <- [TcType] -> Maybe ([TcType], TcType)
    forall a. [a] -> Maybe ([a], a)
    snocView [TcType]
    ts2
          = Bool -> TcM Coercion -> TcM Coercion
    forall a. HasCallStack => Bool -> a -> a
    assert (Bool -> Bool
    not (TyCon -> Bool
    tyConMustBeSaturated TyCon
    tc2)) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $
            Bool
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcM Coercion
    go_app (TyCon -> [TcType] -> Bool
    isNextTyConArgVisible TyCon
    tc2 [TcType]
    ts2')
                   TcType
    ty1 TcType
    s1 TcType
    t1 TcType
    ty2 (TyCon -> [TcType] -> TcType
    TyConApp TyCon
    tc2 [TcType]
    ts2') TcType
    t2'

        go ty1 :: TcType
    ty1@(TyConApp TyCon
    tc1 [TcType]
    ts1) ty2 :: TcType
    ty2@(AppTy TcType
    s2 TcType
    t2)
          | Just ([TcType]
    ts1', TcType
    t1') <- [TcType] -> Maybe ([TcType], TcType)
    forall a. [a] -> Maybe ([a], a)
    snocView [TcType]
    ts1
          = Bool -> TcM Coercion -> TcM Coercion
    forall a. HasCallStack => Bool -> a -> a
    assert (Bool -> Bool
    not (TyCon -> Bool
    tyConMustBeSaturated TyCon
    tc1)) (TcM Coercion -> TcM Coercion) -> TcM Coercion -> TcM Coercion
    forall a b. (a -> b) -> a -> b
    $
            Bool
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcM Coercion
    go_app (TyCon -> [TcType] -> Bool
    isNextTyConArgVisible TyCon
    tc1 [TcType]
    ts1')
                   TcType
    ty1 (TyCon -> [TcType] -> TcType
    TyConApp TyCon
    tc1 [TcType]
    ts1') TcType
    t1' TcType
    ty2 TcType
    s2 TcType
    t2

        go ty1 :: TcType
    ty1@(CoercionTy Coercion
    co1) ty2 :: TcType
    ty2@(CoercionTy Coercion
    co2)
          = do { kco <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType (UnifyEnv -> TcType -> TcType -> UnifyEnv
    mkKindEnv UnifyEnv
    env TcType
    ty1 TcType
    ty2)
                              (Coercion -> TcType
    coercionType Coercion
    co1) (Coercion -> TcType
    coercionType Coercion
    co2)
               ; return $ mkProofIrrelCo role kco co1 co2 }

            -- Anything else fails
            -- E.g. unifying for-all types, which is relative unusual
        go TcType
    ty1 TcType
    ty2 = TcType -> TcType -> TcM Coercion
    defer TcType
    ty1 TcType
    ty2

        ------------------
        defer :: TcType -> TcType -> TcM Coercion
    defer TcType
    ty1 TcType
    ty2   -- See Note [Check for equality before deferring]
          | TcType
    ty1 HasDebugCallStack => TcType -> TcType -> Bool
    TcType -> TcType -> Bool
    `tcEqType` TcType
    ty2 = Coercion -> TcM Coercion
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Role -> TcType -> Coercion
    mkReflCo Role
    role TcType
    ty1)
          | Bool
    otherwise          = UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType_defer UnifyEnv
    env TcType
    orig_ty1 TcType
    orig_ty2

        ------------------
        u_tc_arg :: Bool -> Role -> TcType -> TcType -> TcM Coercion
    u_tc_arg Bool
    is_vis Role
    role TcType
    ty1 TcType
    ty2
          = do { String -> SDoc -> TcM ()
    traceTc String
    "u_tc_arg" (Role -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Role
    role SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty1 SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2)
               ; UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env_arg TcType
    ty1 TcType
    ty2 }
          where
            env_arg :: UnifyEnv
    env_arg = UnifyEnv
    env { u_loc = adjustCtLoc is_vis False (u_loc env)
                          , u_role = role }

        ------------------
        -- For AppTy, decompose only nominal equalities
        -- See Note [Decomposing AppTy equalities] in GHC.Tc.Solver.Equality
        go_app :: Bool
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcType
    -> TcM Coercion
    go_app Bool
    vis TcType
    ty1 TcType
    s1 TcType
    t1 TcType
    ty2 TcType
    s2 TcType
    t2
          | Role
    Nominal <- Role
    role
          = -- Unify arguments t1/t2 before function s1/s2, because
            -- the former have smaller kinds, and hence simpler error messages
            -- c.f. GHC.Tc.Solver.Equality.can_eq_app
            -- Example: test T8603
            do { let env_arg :: UnifyEnv
    env_arg = UnifyEnv
    env { u_loc = adjustCtLoc vis False (u_loc env) }
               ; co_t <- UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType UnifyEnv
    env_arg TcType
    t1 TcType
    t2
               ; co_s <- uType env s1 s2
               ; return $ mkAppCo co_s co_t }
          | Bool
    otherwise
          = TcType -> TcType -> TcM Coercion
    defer TcType
    ty1 TcType
    ty2

    {- Note [Check for equality before deferring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Particularly in ambiguity checks we can get equalities like (ty ~ ty).
    If ty involves a type function we may defer, which isn't very sensible.
    An egregious example of this was in test T9872a, which has a type signature
           Proxy :: Proxy (Solutions Cubes)
    Doing the ambiguity check on this signature generates the equality
       Solutions Cubes ~ Solutions Cubes
    and currently the constraint solver normalises both sides at vast cost.
    This little short-cut in 'defer' helps quite a bit.

    Note [Care with type applications]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Note: type applications need a bit of care!
    They can match FunTy and TyConApp, so use splitAppTy_maybe
    NB: we've already dealt with type variables and Notes,
    so if one type is an App the other one jolly well better be too

    Note [Mismatched type lists and application decomposition]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we find two TyConApps, you might think that the argument lists
    are guaranteed equal length.  But they aren't. Consider matching
            w (T x) ~ Foo (T x y)
    We do match (w ~ Foo) first, but in some circumstances we simply create
    a deferred constraint; and then go ahead and match (T x ~ T x y).
    This came up in #3950.

    So either
       (a) either we must check for identical argument kinds
           when decomposing applications,

       (b) or we must be prepared for ill-kinded unification sub-problems

    Currently we adopt (b) since it seems more robust -- no need to maintain
    a global invariant.

    Note [Expanding synonyms during unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We expand synonyms during unification, but:
     * We expand *after* the variable case so that we tend to unify
       variables with un-expanded type synonym. This just makes it
       more likely that the inferred types will mention type synonyms
       understandable to the user

     * Similarly, we expand *after* the CastTy case, just in case the
       CastTy wraps a variable.

     * We expand *before* the TyConApp case.  For example, if we have
          type Phantom a = Int
       and are unifying
          Phantom Int ~ Phantom Char
       it is *wrong* to unify Int and Char.

     * The problem case immediately above can happen only with arguments
       to the tycon. So we check for nullary tycons *before* expanding.
       This is particularly helpful when checking (* ~ *), because * is
       now a type synonym.

    Note [Deferred unification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We may encounter a unification ty1 ~ ty2 that cannot be performed syntactically,
    and yet its consistency is undetermined. Previously, there was no way to still
    make it consistent. So a mismatch error was issued.

    Now these unifications are deferred until constraint simplification, where type
    family instances and given equations may (or may not) establish the consistency.
    Deferred unifications are of the form
                    F ... ~ ...
    or              x ~ ...
    where F is a type function and x is a type variable.
    E.g.
            id :: x ~ y => x -> y
            id e = e

    involves the unification x = y. It is deferred until we bring into account the
    context x ~ y to establish that it holds.

    If available, we defer original types (rather than those where closed type
    synonyms have already been expanded via tcCoreView).  This is, as usual, to
    improve error messages.

    ************************************************************************
    *                                                                      *
                     uUnfilledVar and friends
    *                                                                      *
    ************************************************************************

    @uunfilledVar@ is called when at least one of the types being unified is a
    variable.  It does {\em not} assume that the variable is a fixed point
    of the substitution; rather, notice that @uVar@ (defined below) nips
    back into @uTys@ if it turns out that the variable is already bound.
    -}

    ----------
    uUnfilledVar, uUnfilledVar1
        :: UnifyEnv
        -> SwapFlag
        -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                          --    definitely not a /filled/ meta-tyvar
        -> TcTauType      -- Type 2
        -> TcM CoercionN
    -- "Unfilled" means that the variable is definitely not a filled-in meta tyvar
    --            It might be a skolem, or untouchable, or meta
    uUnfilledVar :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar UnifyEnv
    env SwapFlag
    swapped TcTyVar
    tv1 TcType
    ty2
      | Role
    Nominal <- UnifyEnv -> Role
    u_role UnifyEnv
    env
      = do { ty2 <- ZonkM TcType -> TcM TcType
    forall a. ZonkM a -> TcM a
    liftZonkM (ZonkM TcType -> TcM TcType) -> ZonkM TcType -> TcM TcType
    forall a b. (a -> b) -> a -> b
    $ TcType -> ZonkM TcType
    zonkTcType TcType
    ty2
                      -- Zonk to expose things to the occurs check, and so
                      -- that if ty2 looks like a type variable then it
                      -- /is/ a type variable
           ; uUnfilledVar1 env swapped tv1 ty2 }

      | Bool
    otherwise  -- See Note [Do not unify representational equalities]
                   -- in GHC.Tc.Solver.Equality
      = SwapFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> TcType
    -> TcType
    -> TcM Coercion
    forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
    unSwap SwapFlag
    swapped (UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType_defer UnifyEnv
    env) (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv1) TcType
    ty2

    uUnfilledVar1 :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar1 UnifyEnv
    env       -- Precondition: u_role==Nominal
                  SwapFlag
    swapped
                  TcTyVar
    tv1
                  TcType
    ty2       -- ty2 is zonked
      | Just TcTyVar
    tv2 <- TcType -> Maybe TcTyVar
    getTyVar_maybe TcType
    ty2
      = TcTyVar -> TcM Coercion
    go TcTyVar
    tv2

      | Bool
    otherwise
      = UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar2 UnifyEnv
    env SwapFlag
    swapped TcTyVar
    tv1 TcType
    ty2

      where
        -- 'go' handles the case where both are
        -- tyvars so we might want to swap
        -- E.g. maybe tv2 is a meta-tyvar and tv1 is not
        go :: TcTyVar -> TcM Coercion
    go TcTyVar
    tv2 | TcTyVar
    tv1 TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv2  -- Same type variable => no-op
               = Coercion -> TcM Coercion
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType -> Coercion
    mkNomReflCo (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv1))

               | Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars Bool
    False TcTyVar
    tv1 TcTyVar
    tv2   -- Distinct type variables
                   -- Swap meta tyvar to the left if poss
               = do { tv1 <- ZonkM TcTyVar -> TcM TcTyVar
    forall a. ZonkM a -> TcM a
    liftZonkM (ZonkM TcTyVar -> TcM TcTyVar) -> ZonkM TcTyVar -> TcM TcTyVar
    forall a b. (a -> b) -> a -> b
    $ TcTyVar -> ZonkM TcTyVar
    zonkTyCoVarKind TcTyVar
    tv1
                         -- We must zonk tv1's kind because that might
                         -- not have happened yet, and it's an invariant of
                         -- uUnfilledTyVar2 that ty2 is fully zonked
                         -- Omitting this caused #16902
                    ; uUnfilledVar2 env (flipSwap swapped) tv2 (mkTyVarTy tv1) }

               | Bool
    otherwise
               = UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar2 UnifyEnv
    env SwapFlag
    swapped TcTyVar
    tv1 TcType
    ty2

    ----------
    uUnfilledVar2 :: UnifyEnv       -- Precondition: u_role==Nominal
                  -> SwapFlag
                  -> TcTyVar        -- Tyvar 1: not necessarily a meta-tyvar
                                    --    definitely not a /filled/ meta-tyvar
                  -> TcTauType      -- Type 2, zonked
                  -> TcM CoercionN
    uUnfilledVar2 :: UnifyEnv -> SwapFlag -> TcTyVar -> TcType -> TcM Coercion
    uUnfilledVar2 env :: UnifyEnv
    env@(UE { u_defer :: UnifyEnv -> TcRef (Bag Ct)
    u_defer = TcRef (Bag Ct)
    def_eq_ref }) SwapFlag
    swapped TcTyVar
    tv1 TcType
    ty2
      = do { cur_lvl <- TcM TcLevel
    getTcLevel
               -- See Note [Unification preconditions], (UNTOUCHABLE) wrinkles
               -- Here we don't know about given equalities here; so we treat
               -- /any/ level outside this one as untouchable.  Hence cur_lvl.
           ; if not (touchabilityAndShapeTest cur_lvl tv1 ty2
                     && simpleUnifyCheck False tv1 ty2)
             then not_ok_so_defer
             else
        do { def_eqs <- readTcRef def_eq_ref  -- Capture current state of def_eqs

           -- Attempt to unify kinds
           ; co_k <- uType (mkKindEnv env ty1 ty2) (typeKind ty2) (tyVarKind tv1)
           ; traceTc "uUnfilledVar2 ok" $
             vcat [ ppr tv1 <+> dcolon <+> ppr (tyVarKind tv1)
                  , ppr ty2 <+> dcolon <+> ppr (typeKind  ty2)
                  , ppr (isReflCo co_k), ppr co_k ]

           ; if isReflCo co_k
               -- Only proceed if the kinds match
               -- NB: tv1 should still be unfilled, despite the kind unification
               --     because tv1 is not free in ty2' (or, hence, in its kind)
             then do { liftZonkM $ writeMetaTyVar tv1 ty2
                     ; case u_unified env of
                         Maybe (TcRef [TcTyVar])
    Nothing -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
                         Just TcRef [TcTyVar]
    uref -> TcRef [TcTyVar] -> ([TcTyVar] -> [TcTyVar]) -> TcM ()
    forall (m :: * -> *) a. MonadIO m => TcRef a -> (a -> a) -> m ()
    updTcRef TcRef [TcTyVar]
    uref (TcTyVar
    tv1 TcTyVar -> [TcTyVar] -> [TcTyVar]
    forall a. a -> [a] -> [a]
    :)
                     ; return (mkNomReflCo ty2) }  -- Unification is always Nominal

             else -- The kinds don't match yet, so defer instead.
                  do { writeTcRef def_eq_ref def_eqs
                         -- Since we are discarding co_k, also discard any constraints
                         -- emitted by kind unification; they are just useless clutter.
                         -- Do this dicarding by simply restoring the previous state
                         -- of def_eqs; a bit imperative/yukky but works fine.
                     ; defer }
             }}
      where
        ty1 :: TcType
    ty1 = TcTyVar -> TcType
    mkTyVarTy TcTyVar
    tv1
        defer :: TcM Coercion
    defer = SwapFlag
    -> (TcType -> TcType -> TcM Coercion)
    -> TcType
    -> TcType
    -> TcM Coercion
    forall a b. SwapFlag -> (a -> a -> b) -> a -> a -> b
    unSwap SwapFlag
    swapped (UnifyEnv -> TcType -> TcType -> TcM Coercion
    uType_defer UnifyEnv
    env) TcType
    ty1 TcType
    ty2

        not_ok_so_defer :: TcM Coercion
    not_ok_so_defer =
          do { String -> SDoc -> TcM ()
    traceTc String
    "uUnfilledVar2 not ok" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv1 SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ TcType -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcType
    ty2)
                   -- Occurs check or an untouchable: just defer
                   -- NB: occurs check isn't necessarily fatal:
                   --     eg tv1 occurred in type family parameter
              ; TcM Coercion
    defer }

    swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars :: Bool -> TcTyVar -> TcTyVar -> Bool
    swapOverTyVars Bool
    is_given TcTyVar
    tv1 TcTyVar
    tv2
      -- See Note [Unification variables on the left]
      | Bool -> Bool
    not Bool
    is_given, Int
    pri1 Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0, Int
    pri2 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    0 = Bool
    True
      | Bool -> Bool
    not Bool
    is_given, Int
    pri2 Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    == Int
    0, Int
    pri1 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    0 = Bool
    False

      -- Level comparison: see Note [TyVar/TyVar orientation]
      | TcLevel
    lvl1 TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lvl2 = Bool
    False
      | TcLevel
    lvl2 TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lvl1 = Bool
    True

      -- Priority: see Note [TyVar/TyVar orientation]
      | Int
    pri1 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    pri2 = Bool
    False
      | Int
    pri2 Int -> Int -> Bool
    forall a. Ord a => a -> a -> Bool
    > Int
    pri1 = Bool
    True

      -- Names: see Note [TyVar/TyVar orientation]
      | Name -> Bool
    isSystemName Name
    tv2_name, Bool -> Bool
    not (Name -> Bool
    isSystemName Name
    tv1_name) = Bool
    True

      | Bool
    otherwise = Bool
    False

      where
        lvl1 :: TcLevel
    lvl1 = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    tv1
        lvl2 :: TcLevel
    lvl2 = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    tv2
        pri1 :: Int
    pri1 = TcTyVar -> Int
    lhsPriority TcTyVar
    tv1
        pri2 :: Int
    pri2 = TcTyVar -> Int
    lhsPriority TcTyVar
    tv2
        tv1_name :: Name
    tv1_name = TcTyVar -> Name
    Var.varName TcTyVar
    tv1
        tv2_name :: Name
    tv2_name = TcTyVar -> Name
    Var.varName TcTyVar
    tv2

    lhsPriority :: TcTyVar -> Int
    -- Higher => more important to be on the LHS
    --        => more likely to be eliminated
    -- See Note [TyVar/TyVar orientation]
    lhsPriority :: TcTyVar -> Int
    lhsPriority TcTyVar
    tv
      = Bool -> SDoc -> Int -> Int
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TcTyVar -> Bool
    isTyVar TcTyVar
    tv) (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    tv) (Int -> Int) -> Int -> Int
    forall a b. (a -> b) -> a -> b
    $
        case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of
          TcTyVarDetails
    RuntimeUnk  -> Int
    0
          SkolemTv {} -> Int
    0
          MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info } -> case MetaInfo
    info of
                                         MetaInfo
    CycleBreakerTv -> Int
    0
                                         MetaInfo
    TyVarTv        -> Int
    1
                                         ConcreteTv {}  -> Int
    2
                                         MetaInfo
    TauTv          -> Int
    3
                                         MetaInfo
    RuntimeUnkTv   -> Int
    4

    {- Note [Unification preconditions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Question: given a homogeneous equality (alpha ~# ty), when is it OK to
    unify alpha := ty?

    This note only applied to /homogeneous/ equalities, in which both
    sides have the same kind.

    There are five reasons not to unify:

    1. (SKOL-ESC) Skolem-escape
       Consider the constraint
            forall[2] a[2]. alpha[1] ~ Maybe a[2]
       If we unify alpha := Maybe a, the skolem 'a' may escape its scope.
       The level alpha[1] says that alpha may be used outside this constraint,
       where 'a' is not in scope at all.  So we must not unify.

       Bottom line: when looking at a constraint alpha[n] := ty, do not unify
       if any free variable of 'ty' has level deeper (greater) than n

    2. (UNTOUCHABLE) Untouchable unification variables
       Consider the constraint
            forall[2] a[2]. b[1] ~ Int => alpha[1] ~ Int
       There is no (SKOL-ESC) problem with unifying alpha := Int, but it might
       not be the principal solution. Perhaps the "right" solution is alpha := b.
       We simply can't tell.  See "OutsideIn(X): modular type inference with local
       assumptions", section 2.2.  We say that alpha[1] is "untouchable" inside
       this implication.

       Bottom line: at ambient level 'l', when looking at a constraint
       alpha[n] ~ ty, do not unify alpha := ty if there are any given equalities
       between levels 'n' and 'l'.

       Exactly what is a "given equality" for the purpose of (UNTOUCHABLE)?
       Answer: see Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

    3. (TYVAR-TV) Unifying TyVarTvs and CycleBreakerTvs
       This precondition looks at the MetaInfo of the unification variable:

       * TyVarTv: When considering alpha{tyv} ~ ty, if alpha{tyv} is a
         TyVarTv it can only unify with a type variable, not with a
         structured type.  So if 'ty' is a structured type, such as (Maybe x),
         don't unify.

       * CycleBreakerTv: never unified, except by restoreTyVarCycles.

    4. (CONCRETE) A ConcreteTv can only unify with a concrete type,
        by definition.

        That is, if we have `rr[conc] ~ F Int`, we can't unify
        `rr` with `F Int`, so we hold off on unifying.
        Note however that the equality might get rewritten; for instance
        if we can rewrite `F Int` to a concrete type, say `FloatRep`,
        then we will have `rr[conc] ~ FloatRep` and we can unify `rr ~ FloatRep`.

        Note that we can still make progress on unification even if
        we can't fully solve an equality, e.g.

          alpha[conc] ~# TupleRep '[ beta[tau], F gamma[tau] ]

        we can fill beta[tau] := beta[conc]. This is why we call
        'makeTypeConcrete' in startSolvingByUnification.

    5. (COERCION-HOLE) Confusing coercion holes
       Suppose our equality is
         (alpha :: k) ~ (Int |> {co})
       where co :: Type ~ k is an unsolved wanted. Note that this equality
       is homogeneous; both sides have kind k. We refrain from unifying here, because
       of the coercion hole in the RHS -- see Wrinkle (EIK2) in
       Note [Equalities with incompatible kinds] in GHC.Solver.Equality.

    Needless to say, all there are wrinkles:

    * (SKOL-ESC) Promotion.  Given alpha[n] ~ ty, what if beta[k] is free
      in 'ty', where beta is a unification variable, and k>n?  'beta'
      stands for a monotype, and since it is part of a level-n type
      (equal to alpha[n]), we must /promote/ beta to level n.  Just make
      up a fresh gamma[n], and unify beta[k] := gamma[n].

    * (TYVAR-TV) Unification variables.  Suppose alpha[tyv,n] is a level-n
      TyVarTv (see Note [TyVarTv] in GHC.Tc.Types.TcMType)? Now
      consider alpha[tyv,n] ~ Bool.  We don't want to unify because that
      would break the TyVarTv invariant.

      What about alpha[tyv,n] ~ beta[tau,n], where beta is an ordinary
      TauTv?  Again, don't unify, because beta might later be unified
      with, say Bool.  (If levels permit, we reverse the orientation here;
      see Note [TyVar/TyVar orientation].)

    * (UNTOUCHABLE) Untouchability.  When considering (alpha[n] ~ ty), how
      do we know whether there are any given equalities between level n
      and the ambient level?  We answer in two ways:

      * In the eager unifier, we only unify if l=n.  If not, alpha may be
        untouchable, and defer to the constraint solver.  This check is
        made in GHC.Tc.Utils.uUnifilledVar2, in the guard
        isTouchableMetaTyVar.

      * In the constraint solver, we track where Given equalities occur
        and use that to guard unification in
        GHC.Tc.Utils.Unify.touchabilityAndShapeTest. More details in
        Note [Tracking Given equalities] in GHC.Tc.Solver.InertSet

        Historical note: in the olden days (pre 2021) the constraint solver
        also used to unify only if l=n.  Equalities were "floated" out of the
        implication in a separate step, so that they would become touchable.
        But the float/don't-float question turned out to be very delicate,
        as you can see if you look at the long series of Notes associated with
        GHC.Tc.Solver.floatEqualities, around Nov 2020.  It's much easier
        to unify in-place, with no floating.

    Note [TyVar/TyVar orientation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    See also Note [Fundeps with instances, and equality orientation]
    where the kind equality orientation is important

    Given (a ~ b), should we orient the equality as (a~b) or (b~a)?
    This is a surprisingly tricky question!

    The question is answered by swapOverTyVars, which is used
      - in the eager unifier, in GHC.Tc.Utils.Unify.uUnfilledVar1
      - in the constraint solver, in GHC.Tc.Solver.Equality.canEqCanLHS2

    First note: only swap if you have to!
       See Note [Avoid unnecessary swaps]

    So we look for a positive reason to swap, using a three-step test:

    * Level comparison. If 'a' has deeper level than 'b',
      put 'a' on the left.  See Note [Deeper level on the left]

    * Priority.  If the levels are the same, look at what kind of
      type variable it is, using 'lhsPriority'.

      Generally speaking we always try to put a MetaTv on the left in
      preference to SkolemTv or RuntimeUnkTv, because the MetaTv may be
      touchable and can be unified.

      Tie-breaking rules for MetaTvs:
      - CycleBreakerTv: This is essentially a stand-in for another type;
           it's untouchable and should have the same priority as a skolem: 0.

      - TyVarTv: These can unify only with another tyvar, but we can't unify
           a TyVarTv with a TauTv, because then the TyVarTv could (transitively)
           get a non-tyvar type. So give these a low priority: 1.

      - ConcreteTv: These are like TauTv, except they can only unify with
        a concrete type. So we want to be able to write to them, but not quite
        as much as TauTvs: 2.

      - TauTv: This is the common case; we want these on the left so that they
           can be written to: 3.

      - RuntimeUnkTv: These aren't really meta-variables used in type inference,
           but just a convenience in the implementation of the GHCi debugger.
           Eagerly write to these: 4. See Note [RuntimeUnkTv] in
           GHC.Runtime.Heap.Inspect.

    * Names. If the level and priority comparisons are all
      equal, try to eliminate a TyVar with a System Name in
      favour of ones with a Name derived from a user type signature

    * Age.  At one point in the past we tried to break any remaining
      ties by eliminating the younger type variable, based on their
      Uniques.  See Note [Eliminate younger unification variables]
      (which also explains why we don't do this any more)

    Note [Unification variables on the left]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    For wanteds, but not givens, swap (skolem ~ meta-tv) regardless of
    level, so that the unification variable is on the left.

    * We /don't/ want this for Givens because if we ave
        [G] a[2] ~ alpha[1]
        [W] Bool ~ a[2]
      we want to rewrite the wanted to Bool ~ alpha[1],
      so we can float the constraint and solve it.

    * But for Wanteds putting the unification variable on
      the left means an easier job when floating, and when
      reporting errors -- just fewer cases to consider.

      In particular, we get better skolem-escape messages:
      see #18114

    Note [Deeper level on the left]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The most important thing is that we want to put tyvars with
    the deepest level on the left.  The reason to do so differs for
    Wanteds and Givens, but either way, deepest wins!  Simple.

    * Wanteds.  Putting the deepest variable on the left maximise the
      chances that it's a touchable meta-tyvar which can be solved.

    * Givens. Suppose we have something like
         forall a[2]. b[1] ~ a[2] => beta[1] ~ a[2]

      If we orient the Given a[2] on the left, we'll rewrite the Wanted to
      (beta[1] ~ b[1]), and that can float out of the implication.
      Otherwise it can't.  By putting the deepest variable on the left
      we maximise our changes of eliminating skolem capture.

      See also GHC.Tc.Solver.InertSet Note [Let-bound skolems] for another reason
      to orient with the deepest skolem on the left.

      IMPORTANT NOTE: this test does a level-number comparison on
      skolems, so it's important that skolems have (accurate) level
      numbers.

    See #15009 for an further analysis of why "deepest on the left"
    is a good plan.

    Note [Avoid unnecessary swaps]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we swap without actually improving matters, we can get an infinite loop.
    Consider
        work item:  a ~ b
       inert item:  b ~ c
    We canonicalise the work-item to (a ~ c).  If we then swap it before
    adding to the inert set, we'll add (c ~ a), and therefore kick out the
    inert guy, so we get
       new work item:  b ~ c
       inert item:     c ~ a
    And now the cycle just repeats

    Historical Note [Eliminate younger unification variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given a choice of unifying
         alpha := beta   or   beta := alpha
    we try, if possible, to eliminate the "younger" one, as determined
    by `ltUnique`.  Reason: the younger one is less likely to appear free in
    an existing inert constraint, and hence we are less likely to be forced
    into kicking out and rewriting inert constraints.

    This is a performance optimisation only.  It turns out to fix
    #14723 all by itself, but clearly not reliably so!

    It's simple to implement (see nicer_to_update_tv2 in swapOverTyVars).
    But, to my surprise, it didn't seem to make any significant difference
    to the compiler's performance, so I didn't take it any further.  Still
    it seemed too nice to discard altogether, so I'm leaving these
    notes.  SLPJ Jan 18.

    Note [Prevent unification with type families]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We prevent unification with type families because of an uneasy compromise.
    It's perfectly sound to unify with type families, and it even improves the
    error messages in the testsuite. It also modestly improves performance, at
    least in some cases. But it's disastrous for test case perf/compiler/T3064.
    Here is the problem: Suppose we have (F ty) where we also have [G] F ty ~ a.
    What do we do? Do we reduce F? Or do we use the given? Hard to know what's
    best. GHC reduces. This is a disaster for T3064, where the type's size
    spirals out of control during reduction. If we prevent
    unification with type families, then the solver happens to use the equality
    before expanding the type family.

    It would be lovely in the future to revisit this problem and remove this
    extra, unnecessary check. But we retain it for now as it seems to work
    better in practice.

    Revisited in Nov '20, along with removing flattening variables. Problem
    is still present, and the solution is still the same.

    Note [Non-TcTyVars in GHC.Tc.Utils.Unify]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Because the same code is now shared between unifying types and unifying
    kinds, we sometimes will see proper TyVars floating around the unifier.
    Example (from test case polykinds/PolyKinds12):

        type family Apply (f :: k1 -> k2) (x :: k1) :: k2
        type instance Apply g y = g y

    When checking the instance declaration, we first *kind-check* the LHS
    and RHS, discovering that the instance really should be

        type instance Apply k3 k4 (g :: k3 -> k4) (y :: k3) = g y

    During this kind-checking, all the tyvars will be TcTyVars. Then, however,
    as a second pass, we desugar the RHS (which is done in functions prefixed
    with "tc" in GHC.Tc.TyCl"). By this time, all the kind-vars are proper
    TyVars, not TcTyVars, get some kind unification must happen.

    Thus, we always check if a TyVar is a TcTyVar before asking if it's a
    meta-tyvar.

    This used to not be necessary for type-checking (that is, before * :: *)
    because expressions get desugared via an algorithm separate from
    type-checking (with wrappers, etc.). Types get desugared very differently,
    causing this wibble in behavior seen here.
    -}

    -- | Breaks apart a function kind into its pieces.
    matchExpectedFunKind
      :: TypedThing     -- ^ type, only for errors
      -> Arity           -- ^ n: number of desired arrows
      -> TcKind          -- ^ fun_kind
      -> TcM Coercion    -- ^ co :: fun_kind ~ (arg1 -> ... -> argn -> res)

    matchExpectedFunKind :: TypedThing -> Int -> TcType -> TcM Coercion
    matchExpectedFunKind TypedThing
    hs_ty Int
    n TcType
    k = Int -> TcType -> TcM Coercion
    go Int
    n TcType
    k
      where
        go :: Int -> TcType -> TcM Coercion
    go Int
    0 TcType
    k = Coercion -> TcM Coercion
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (TcType -> Coercion
    mkNomReflCo TcType
    k)

        go Int
    n TcType
    k | Just TcType
    k' <- TcType -> Maybe TcType
    coreView TcType
    k = Int -> TcType -> TcM Coercion
    go Int
    n TcType
    k'

        go Int
    n k :: TcType
    k@(TyVarTy TcTyVar
    kvar)
          | TcTyVar -> Bool
    isMetaTyVar TcTyVar
    kvar
          = do { maybe_kind <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) MetaDetails
    forall (m :: * -> *). MonadIO m => TcTyVar -> m MetaDetails
    readMetaTyVar TcTyVar
    kvar
               ; case maybe_kind of
                    Indirect TcType
    fun_kind -> Int -> TcType -> TcM Coercion
    go Int
    n TcType
    fun_kind
                    MetaDetails
    Flexi ->             Int -> TcType -> TcM Coercion
    defer Int
    n TcType
    k }

        go Int
    n (FunTy { ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w, ft_arg :: TcType -> TcType
    ft_arg = TcType
    arg, ft_res :: TcType -> TcType
    ft_res = TcType
    res })
          | FunTyFlag -> Bool
    isVisibleFunArg FunTyFlag
    af
          = do { co <- Int -> TcType -> TcM Coercion
    go (Int
    nInt -> Int -> Int
    forall a. Num a => a -> a -> a
    -Int
    1) TcType
    res
               ; return (mkNakedFunCo Nominal af (mkNomReflCo w) (mkNomReflCo arg) co) }

        go Int
    n TcType
    other
         = Int -> TcType -> TcM Coercion
    defer Int
    n TcType
    other

        defer :: Int -> TcType -> TcM Coercion
    defer Int
    n TcType
    k
          = do { arg_kinds <- Int -> TcM [TcType]
    newMetaKindVars Int
    n
               ; res_kind  <- newMetaKindVar
               ; let new_fun = [TcType] -> TcType -> TcType
    mkVisFunTysMany [TcType]
    arg_kinds TcType
    res_kind
                     origin  = TypeEqOrigin { uo_actual :: TcType
    uo_actual   = TcType
    k
                                            , uo_expected :: TcType
    uo_expected = TcType
    new_fun
                                            , uo_thing :: Maybe TypedThing
    uo_thing    = TypedThing -> Maybe TypedThing
    forall a. a -> Maybe a
    Just TypedThing
    hs_ty
                                            , uo_visible :: Bool
    uo_visible  = Bool
    True
                                            }
               ; unifyTypeAndEmit KindLevel origin k new_fun }

    {- *********************************************************************
    *                                                                      *
                     Checking alpha ~ ty
                  for the on-the-fly unifier
    *                                                                      *
    ********************************************************************* -}

    simpleUnifyCheck :: Bool -> TcTyVar -> TcType -> Bool
    -- A fast check: True <=> unification is OK
    -- If it says 'False' then unification might still be OK, but
    -- it'll take more work to do -- use the full checkTypeEq
    --
    -- * Always rejects foralls unless lhs_tv is RuntimeUnk
    --   (used by GHCi debugger)
    -- * Rejects a non-concrete type if lhs_tv is concrete
    -- * Rejects type families unless fam_ok=True
    -- * Does a level-check for type variables
    --
    -- This function is pretty heavily used, so it's optimised not to allocate
    simpleUnifyCheck :: Bool -> TcTyVar -> TcType -> Bool
    simpleUnifyCheck Bool
    fam_ok TcTyVar
    lhs_tv TcType
    rhs
      = TcType -> Bool
    go TcType
    rhs
      where
        !(TcType -> Bool
    occ_in_ty, Coercion -> Bool
    occ_in_co) = TcTyVar -> (TcType -> Bool, Coercion -> Bool)
    mkOccFolders TcTyVar
    lhs_tv

        lhs_tv_lvl :: TcLevel
    lhs_tv_lvl         = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    lhs_tv
        lhs_tv_is_concrete :: Bool
    lhs_tv_is_concrete = TcTyVar -> Bool
    isConcreteTyVar TcTyVar
    lhs_tv
        forall_ok :: Bool
    forall_ok          = case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    lhs_tv of
                                MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    RuntimeUnkTv } -> Bool
    True
                                TcTyVarDetails
    _                                  -> Bool
    False

        go :: TcType -> Bool
    go (TyVarTy TcTyVar
    tv)
          | TcTyVar
    lhs_tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    tv                                 = Bool
    False
          | TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    tv TcLevel -> TcLevel -> Bool
    forall a. Ord a => a -> a -> Bool
    > TcLevel
    lhs_tv_lvl                 = Bool
    False
          | Bool
    lhs_tv_is_concrete, Bool -> Bool
    not (TcTyVar -> Bool
    isConcreteTyVar TcTyVar
    tv) = Bool
    False
          | TcType -> Bool
    occ_in_ty (TcType -> Bool) -> TcType -> Bool
    forall a b. (a -> b) -> a -> b
    $! (TcTyVar -> TcType
    tyVarKind TcTyVar
    tv)                  = Bool
    False
          | Bool
    otherwise                                    = Bool
    True

        go (FunTy {ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w, ft_arg :: TcType -> TcType
    ft_arg = TcType
    a, ft_res :: TcType -> TcType
    ft_res = TcType
    r})
          | FunTyFlag -> Bool
    isInvisibleFunArg FunTyFlag
    af, Bool -> Bool
    not Bool
    forall_ok = Bool
    False
          | Bool
    otherwise                           = TcType -> Bool
    go TcType
    w Bool -> Bool -> Bool
    && TcType -> Bool
    go TcType
    a Bool -> Bool -> Bool
    && TcType -> Bool
    go TcType
    r

        go (TyConApp TyCon
    tc [TcType]
    tys)
          | Bool
    lhs_tv_is_concrete, Bool -> Bool
    not (TyCon -> Bool
    isConcreteTyCon TyCon
    tc) = Bool
    False
          | Bool -> Bool
    not (TyCon -> Bool
    isTauTyCon TyCon
    tc)                          = Bool
    False
          | Bool -> Bool
    not Bool
    fam_ok, Bool -> Bool
    not (TyCon -> Bool
    isFamFreeTyCon TyCon
    tc)          = Bool
    False
          | Bool
    otherwise                                    = (TcType -> Bool) -> [TcType] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all TcType -> Bool
    go [TcType]
    tys

        go (AppTy TcType
    t1 TcType
    t2)    = TcType -> Bool
    go TcType
    t1 Bool -> Bool -> Bool
    && TcType -> Bool
    go TcType
    t2
        go (ForAllTy (Bndr TcTyVar
    tv ForAllTyFlag
    _) TcType
    ty)
          | Bool
    forall_ok = TcType -> Bool
    go (TcTyVar -> TcType
    tyVarKind TcTyVar
    tv) Bool -> Bool -> Bool
    && (TcTyVar
    tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    lhs_tv Bool -> Bool -> Bool
    || TcType -> Bool
    go TcType
    ty)
          | Bool
    otherwise = Bool
    False

        go (CastTy TcType
    ty Coercion
    co)   = Bool -> Bool
    not (Coercion -> Bool
    occ_in_co Coercion
    co) Bool -> Bool -> Bool
    && TcType -> Bool
    go TcType
    ty
        go (CoercionTy Coercion
    co)  = Bool -> Bool
    not (Coercion -> Bool
    occ_in_co Coercion
    co)
        go (LitTy {})       = Bool
    True

    mkOccFolders :: TcTyVar -> (TcType -> Bool, TcCoercion -> Bool)
    -- These functions return True
    --   * if lhs_tv occurs (incl deeply, in the kind of variable)
    --   * if there is a coercion hole
    -- No expansion of type synonyms
    mkOccFolders :: TcTyVar -> (TcType -> Bool, Coercion -> Bool)
    mkOccFolders TcTyVar
    lhs_tv = (Any -> Bool
    getAny (Any -> Bool) -> (TcType -> Any) -> TcType -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcType -> Any
    check_ty, Any -> Bool
    getAny (Any -> Bool) -> (Coercion -> Any) -> Coercion -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Coercion -> Any
    check_co)
      where
        !(TcType -> Any
    check_ty, [TcType] -> Any
    _, Coercion -> Any
    check_co, [Coercion] -> Any
    _) = TyCoFolder VarSet Any
    -> VarSet
    -> (TcType -> Any, [TcType] -> Any, Coercion -> Any,
        [Coercion] -> Any)
    forall a env.
    Monoid a =>
    TyCoFolder env a
    -> env
    -> (TcType -> a, [TcType] -> a, Coercion -> a, [Coercion] -> a)
    foldTyCo TyCoFolder VarSet Any
    occ_folder VarSet
    emptyVarSet
        occ_folder :: TyCoFolder VarSet Any
    occ_folder = TyCoFolder { tcf_view :: TcType -> Maybe TcType
    tcf_view  = TcType -> Maybe TcType
    noView  -- Don't expand synonyms
                                , tcf_tyvar :: VarSet -> TcTyVar -> Any
    tcf_tyvar = VarSet -> TcTyVar -> Any
    do_tcv, tcf_covar :: VarSet -> TcTyVar -> Any
    tcf_covar = VarSet -> TcTyVar -> Any
    do_tcv
                                , tcf_hole :: VarSet -> CoercionHole -> Any
    tcf_hole  = VarSet -> CoercionHole -> Any
    forall {p} {p}. p -> p -> Any
    do_hole
                                , tcf_tycobinder :: VarSet -> TcTyVar -> ForAllTyFlag -> VarSet
    tcf_tycobinder = VarSet -> TcTyVar -> ForAllTyFlag -> VarSet
    forall {p}. VarSet -> TcTyVar -> p -> VarSet
    do_bndr }

        do_tcv :: VarSet -> TcTyVar -> Any
    do_tcv VarSet
    is TcTyVar
    v = Bool -> Any
    Any (Bool -> Bool
    not (TcTyVar
    v TcTyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    is) Bool -> Bool -> Bool
    && TcTyVar
    v TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    lhs_tv)
                      Any -> Any -> Any
    forall a. Monoid a => a -> a -> a
    `mappend` TcType -> Any
    check_ty (TcTyVar -> TcType
    varType TcTyVar
    v)

        do_bndr :: VarSet -> TcTyVar -> p -> VarSet
    do_bndr VarSet
    is TcTyVar
    tcv p
    _faf = VarSet -> TcTyVar -> VarSet
    extendVarSet VarSet
    is TcTyVar
    tcv
        do_hole :: p -> p -> Any
    do_hole p
    _is p
    _hole = Bool -> Any
    DM.Any Bool
    True  -- Reject coercion holes

    {- *********************************************************************
    *                                                                      *
                     Equality invariant checking
    *                                                                      *
    ********************************************************************* -}

    {-  Note [Checking for foralls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We never want to unify
        alpha ~ (forall a. a->a) -> Int
    So we look for foralls hidden inside the type, and it's convenient
    to do that at the same time as the occurs check (which looks for
    occurrences of alpha).

    However, it's not just a question of looking for foralls /anywhere/!
    Consider
       (alpha :: forall k. k->*)  ~  (beta :: forall k. k->*)
    This is legal; e.g. dependent/should_compile/T11635.

    We don't want to reject it because of the forall in beta's kind, but
    (see Note [Occurrence checking: look inside kinds] in GHC.Core.Type)
    we do need to look in beta's kind.  So we carry a flag saying if a
    'forall' is OK, and switch the flag on when stepping inside a kind.

    Why is it OK?  Why does it not count as impredicative polymorphism?
    The reason foralls are bad is because we reply on "seeing" foralls
    when doing implicit instantiation.  But the forall inside the kind is
    fine.  We'll generate a kind equality constraint
      (forall k. k->*) ~ (forall k. k->*)
    to check that the kinds of lhs and rhs are compatible.  If alpha's
    kind had instead been
      (alpha :: kappa)
    then this kind equality would rightly complain about unifying kappa
    with (forall k. k->*)

    Note [Forgetful synonyms in checkTyConApp]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       type S a b = b   -- Forgets 'a'

       [W] alpha[2] ~ Maybe (S beta[4] gamma[2])

    We don't want to promote beta to level 2; rather, we should
    expand the synonym. (Currently, in checkTypeEqRhs promotion
    is irrevocable, by side effect.)

    To avoid this risk we eagerly expand forgetful synonyms.
    This also means we won't get an occurs check in
       a ~ Maybe (S a b)

    The annoyance is that we might expand the synonym unnecessarily,
    something we generally try to avoid.  But for now, this seems
    simple.

    In a forgetful case like a ~ Maybe (S a b), `checkTyEqRhs` returns
    a Reduction that looks
        Reduction { reductionCoercion    = Refl
                  , reductionReducedType = Maybe b }
    We must jolly well use that reductionReduced type, even though the
    reductionCoercion is Refl.  See `canEqCanLHSFinish_no_unification`.
    -}

    data PuResult a b = PuFail CheckTyEqResult
                      | PuOK (Bag a) b

    instance Functor (PuResult a) where
      fmap :: forall a b. (a -> b) -> PuResult a a -> PuResult a b
    fmap a -> b
    _ (PuFail CheckTyEqResult
    prob) = CheckTyEqResult -> PuResult a b
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    prob
      fmap a -> b
    f (PuOK Bag a
    cts a
    x)  = Bag a -> b -> PuResult a b
    forall a b. Bag a -> b -> PuResult a b
    PuOK Bag a
    cts (a -> b
    f a
    x)

    instance Applicative (PuResult a) where
      pure :: forall a. a -> PuResult a a
    pure a
    x = Bag a -> a -> PuResult a a
    forall a b. Bag a -> b -> PuResult a b
    PuOK Bag a
    forall a. Bag a
    emptyBag a
    x
      PuFail CheckTyEqResult
    p1 <*> :: forall a b. PuResult a (a -> b) -> PuResult a a -> PuResult a b
    <*> PuFail CheckTyEqResult
    p2 = CheckTyEqResult -> PuResult a b
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail (CheckTyEqResult
    p1 CheckTyEqResult -> CheckTyEqResult -> CheckTyEqResult
    forall a. Semigroup a => a -> a -> a
    S.<> CheckTyEqResult
    p2)
      PuFail CheckTyEqResult
    p1 <*> PuOK {}   = CheckTyEqResult -> PuResult a b
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    p1
      PuOK {}   <*> PuFail CheckTyEqResult
    p2 = CheckTyEqResult -> PuResult a b
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    p2
      PuOK Bag a
    c1 a -> b
    f <*> PuOK Bag a
    c2 a
    x = Bag a -> b -> PuResult a b
    forall a b. Bag a -> b -> PuResult a b
    PuOK (Bag a
    c1 Bag a -> Bag a -> Bag a
    forall a. Bag a -> Bag a -> Bag a
    `unionBags` Bag a
    c2) (a -> b
    f a
    x)

    instance (Outputable a, Outputable b) => Outputable (PuResult a b) where
      ppr :: PuResult a b -> SDoc
    ppr (PuFail CheckTyEqResult
    prob) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "PuFail" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> (CheckTyEqResult -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CheckTyEqResult
    prob)
      ppr (PuOK Bag a
    cts b
    x)  = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "PuOK" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    braces
                            ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "redn:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> b -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr b
    x
                                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "cts:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Bag a -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Bag a
    cts ])

    pprPur :: PuResult a b -> SDoc
    -- For debugging
    pprPur :: forall a b. PuResult a b -> SDoc
    pprPur (PuFail CheckTyEqResult
    prob) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "PuFail:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> CheckTyEqResult -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CheckTyEqResult
    prob
    pprPur (PuOK {})     = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "PuOK"

    okCheckRefl :: TcType -> TcM (PuResult a Reduction)
    okCheckRefl :: forall a. TcType -> TcM (PuResult a Reduction)
    okCheckRefl TcType
    ty = PuResult a Reduction
    -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Reduction)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Bag a -> Reduction -> PuResult a Reduction
    forall a b. Bag a -> b -> PuResult a b
    PuOK Bag a
    forall a. Bag a
    emptyBag (Role -> TcType -> Reduction
    mkReflRedn Role
    Nominal TcType
    ty))

    failCheckWith :: CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith :: forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith CheckTyEqResult
    p = PuResult a b -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a b)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CheckTyEqResult -> PuResult a b
    forall a b. CheckTyEqResult -> PuResult a b
    PuFail CheckTyEqResult
    p)

    mapCheck :: (x -> TcM (PuResult a Reduction))
             -> [x]
             -> TcM (PuResult a Reductions)
    mapCheck :: forall x a.
    (x -> TcM (PuResult a Reduction))
    -> [x] -> TcM (PuResult a Reductions)
    mapCheck x -> TcM (PuResult a Reduction)
    f [x]
    xs
      = do { (ress :: [PuResult a Reduction]) <- (x -> TcM (PuResult a Reduction))
    -> [x] -> IOEnv (Env TcGblEnv TcLclEnv) [PuResult a Reduction]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM x -> TcM (PuResult a Reduction)
    f [x]
    xs
           ; return (unzipRedns <$> sequenceA ress) }
             -- sequenceA :: [PuResult a Reduction] -> PuResult a [Reduction]
             -- unzipRedns :: [Reduction] -> Reductions

    -----------------------------
    -- | Options describing how to deal with a type equality
    -- in the pure unifier. See 'checkTyEqRhs'
    data TyEqFlags a
      = TEF { forall a. TyEqFlags a -> Bool
    tef_foralls  :: Bool         -- Allow foralls
            , forall a. TyEqFlags a -> CanEqLHS
    tef_lhs      :: CanEqLHS     -- LHS of the constraint
            , forall a. TyEqFlags a -> AreUnifying
    tef_unifying :: AreUnifying  -- Always NotUnifying if tef_lhs is TyFamLHS
            , forall a. TyEqFlags a -> TyEqFamApp a
    tef_fam_app  :: TyEqFamApp a
            , forall a. TyEqFlags a -> CheckTyEqProblem
    tef_occurs   :: CheckTyEqProblem }  -- Soluble or insoluble occurs check

    -- | What to do when encountering a type-family application while processing
    -- a type equality in the pure unifier.
    --
    -- See Note [Family applications in canonical constraints]
    data TyEqFamApp a
      = TEFA_Fail                    -- Always fail
      | TEFA_Recurse                 -- Just recurse
      | TEFA_Break (FamAppBreaker a) -- Recurse, but replace with cycle breaker if fails,
                                     -- using the FamAppBreaker

    data AreUnifying
      = Unifying
           MetaInfo         -- MetaInfo of the LHS tyvar (which is a meta-tyvar)
           TcLevel          -- Level of the LHS tyvar
           LevelCheck

      | NotUnifying         -- Not attempting to unify

    data LevelCheck
      = LC_None       -- Level check not needed: we should never encounter
                      -- a tyvar at deeper level than the LHS

      | LC_Check      -- Do a level check between the LHS tyvar and the occurrence tyvar
                      -- Fail if the level check fails

      | LC_Promote    -- Do a level check between the LHS tyvar and the occurrence tyvar
                      -- If the level check fails, and the occurrence is a unification
                      -- variable, promote it

    instance Outputable (TyEqFlags a) where
      ppr :: TyEqFlags a -> SDoc
    ppr (TEF { Bool
    CheckTyEqProblem
    CanEqLHS
    AreUnifying
    TyEqFamApp a
    tef_foralls :: forall a. TyEqFlags a -> Bool
    tef_lhs :: forall a. TyEqFlags a -> CanEqLHS
    tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_fam_app :: forall a. TyEqFlags a -> TyEqFamApp a
    tef_occurs :: forall a. TyEqFlags a -> CheckTyEqProblem
    tef_foralls :: Bool
    tef_lhs :: CanEqLHS
    tef_unifying :: AreUnifying
    tef_fam_app :: TyEqFamApp a
    tef_occurs :: CheckTyEqProblem
    .. }) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "TEF" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    braces (
                            [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "tef_foralls =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Bool -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Bool
    tef_foralls
                                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "tef_lhs =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> CanEqLHS -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CanEqLHS
    tef_lhs
                                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "tef_unifying =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> AreUnifying -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr AreUnifying
    tef_unifying
                                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "tef_fam_app =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TyEqFamApp a -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyEqFamApp a
    tef_fam_app
                                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "tef_occurs =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> CheckTyEqProblem -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr CheckTyEqProblem
    tef_occurs ])

    instance Outputable (TyEqFamApp a) where
      ppr :: TyEqFamApp a -> SDoc
    ppr TyEqFamApp a
    TEFA_Fail       = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "TEFA_Fail"
      ppr TyEqFamApp a
    TEFA_Recurse    = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "TEFA_Fail"
      ppr (TEFA_Break {}) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "TEFA_Break"

    instance Outputable AreUnifying where
      ppr :: AreUnifying -> SDoc
    ppr AreUnifying
    NotUnifying = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "NotUnifying"
      ppr (Unifying MetaInfo
    mi TcLevel
    lvl LevelCheck
    lc) = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Unifying" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+>
             SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    braces (MetaInfo -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr MetaInfo
    mi SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    comma SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcLevel -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcLevel
    lvl SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    comma SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> LevelCheck -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr LevelCheck
    lc)

    instance Outputable LevelCheck where
      ppr :: LevelCheck -> SDoc
    ppr LevelCheck
    LC_None    = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "LC_None"
      ppr LevelCheck
    LC_Check   = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "LC_Check"
      ppr LevelCheck
    LC_Promote = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "LC_Promote"

    famAppArgFlags :: TyEqFlags a -> TyEqFlags a
    -- Adjust the flags when going undter a type family
    -- Only the outer family application gets the loop-breaker treatment
    -- Ditto tyvar promotion.  E.g.
    --        [W] alpha[2] ~ Maybe (F beta[3])
    -- Do not promote beta[3]; instead promote (F beta[3])
    famAppArgFlags :: forall a. TyEqFlags a -> TyEqFlags a
    famAppArgFlags flags :: TyEqFlags a
    flags@(TEF { tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_unifying = AreUnifying
    unifying })
      = TyEqFlags a
    flags { tef_fam_app  = TEFA_Recurse
              , tef_unifying = zap_promotion unifying
              , tef_occurs   = cteSolubleOccurs }
                -- tef_occurs: under a type family, an occurs check is not definitely-insoluble
      where
        zap_promotion :: AreUnifying -> AreUnifying
    zap_promotion (Unifying MetaInfo
    info TcLevel
    lvl LevelCheck
    LC_Promote) = MetaInfo -> TcLevel -> LevelCheck -> AreUnifying
    Unifying MetaInfo
    info TcLevel
    lvl LevelCheck
    LC_Check
        zap_promotion AreUnifying
    unifying                       = AreUnifying
    unifying

    type FamAppBreaker a = TcType -> TcM (PuResult a Reduction)
         -- Given a family-application ty, return a Reduction :: ty ~ cvb
         -- where 'cbv' is a fresh loop-breaker tyvar (for Given), or
         -- just a fresh TauTv (for Wanted)

    checkTyEqRhs :: forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs :: forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags TcType
    ty
      = case TcType
    ty of
          LitTy {}        -> TcType -> TcM (PuResult a Reduction)
    forall a. TcType -> TcM (PuResult a Reduction)
    okCheckRefl TcType
    ty
          TyConApp TyCon
    tc [TcType]
    tys -> TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    forall a.
    TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    checkTyConApp TyEqFlags a
    flags TcType
    ty TyCon
    tc [TcType]
    tys
          TyVarTy TcTyVar
    tv      -> TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
    checkTyVar TyEqFlags a
    flags TcTyVar
    tv
            -- Don't worry about foralls inside the kind; see Note [Checking for foralls]
            -- Nor can we expand synonyms; see Note [Occurrence checking: look inside kinds]
            --                             in GHC.Core.FVs

          FunTy {ft_af :: TcType -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_mult :: TcType -> TcType
    ft_mult = TcType
    w, ft_arg :: TcType -> TcType
    ft_arg = TcType
    a, ft_res :: TcType -> TcType
    ft_res = TcType
    r}
           | FunTyFlag -> Bool
    isInvisibleFunArg FunTyFlag
    af  -- e.g.  Num a => blah
           , Bool -> Bool
    not (TyEqFlags a -> Bool
    forall a. TyEqFlags a -> Bool
    tef_foralls TyEqFlags a
    flags)
           -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith CheckTyEqResult
    impredicativeProblem -- Not allowed (TyEq:F)
           | Bool
    otherwise
           -> do { w_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags TcType
    w
                 ; a_res <- checkTyEqRhs flags a
                 ; r_res <- checkTyEqRhs flags r
                 ; return (mkFunRedn Nominal af <$> w_res <*> a_res <*> r_res) }

          AppTy TcType
    fun TcType
    arg -> do { fun_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags TcType
    fun
                              ; arg_res <- checkTyEqRhs flags arg
                              ; return (mkAppRedn <$> fun_res <*> arg_res) }

          CastTy TcType
    ty Coercion
    co  -> do { ty_res <- TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags TcType
    ty
                              ; co_res <- checkCo flags co
                              ; return (mkCastRedn1 Nominal ty <$> co_res <*> ty_res) }

          CoercionTy Coercion
    co -> do { co_res <- TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
    forall a. TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
    checkCo TyEqFlags a
    flags Coercion
    co
                              ; return (mkReflCoRedn Nominal <$> co_res) }

          ForAllTy {}
             | TyEqFlags a -> Bool
    forall a. TyEqFlags a -> Bool
    tef_foralls TyEqFlags a
    flags -> TcType -> TcM (PuResult a Reduction)
    forall a. TcType -> TcM (PuResult a Reduction)
    okCheckRefl TcType
    ty
             | Bool
    otherwise         -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith CheckTyEqResult
    impredicativeProblem  -- Not allowed (TyEq:F)

    -------------------
    checkCo :: TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
    -- See Note [checkCo]
    checkCo :: forall a. TyEqFlags a -> Coercion -> TcM (PuResult a Coercion)
    checkCo (TEF { tef_lhs :: forall a. TyEqFlags a -> CanEqLHS
    tef_lhs = TyFamLHS {} }) Coercion
    co
      = PuResult a Coercion
    -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Coercion)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> PuResult a Coercion
    forall a. a -> PuResult a a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure Coercion
    co)

    checkCo (TEF { tef_lhs :: forall a. TyEqFlags a -> CanEqLHS
    tef_lhs = TyVarLHS TcTyVar
    lhs_tv
                 , tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_unifying = AreUnifying
    unifying
                 , tef_occurs :: forall a. TyEqFlags a -> CheckTyEqProblem
    tef_occurs = CheckTyEqProblem
    occ_prob }) Coercion
    co
      -- Check for coercion holes, if unifying
      -- See (COERCION-HOLE) in Note [Unification preconditions]
      | Coercion -> Bool
    hasCoercionHoleCo Coercion
    co
      = CheckTyEqResult
    -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Coercion)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteCoercionHole)

      -- Occurs check (can promote)
      | Unifying MetaInfo
    _ TcLevel
    lhs_tv_lvl LevelCheck
    LC_Promote <- AreUnifying
    unifying
      = do { reason <- CheckTyEqProblem
    -> TcTyVar -> TcLevel -> VarSet -> TcM CheckTyEqResult
    checkPromoteFreeVars CheckTyEqProblem
    occ_prob TcTyVar
    lhs_tv TcLevel
    lhs_tv_lvl (Coercion -> VarSet
    tyCoVarsOfCo Coercion
    co)
           ; if cterHasNoProblem reason
             then return (pure co)
             else failCheckWith reason }

      -- Occurs check (no promotion)
      | TcTyVar
    lhs_tv TcTyVar -> VarSet -> Bool
    `elemVarSet` Coercion -> VarSet
    tyCoVarsOfCo Coercion
    co
      = CheckTyEqResult
    -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Coercion)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    occ_prob)

      | Bool
    otherwise
      = PuResult a Coercion
    -> IOEnv (Env TcGblEnv TcLclEnv) (PuResult a Coercion)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Coercion -> PuResult a Coercion
    forall a. a -> PuResult a a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure Coercion
    co)

    {- Note [checkCo]
    ~~~~~~~~~~~~~~~~~
    We don't often care about the contents of coercions, so checking
    coercions before making an equality constraint may be surprising.
    But there are several cases we need to be wary of:

    (1) When we're unifying a variable, we must make sure that the variable
        appears nowhere on the RHS -- even in a coercion. Otherwise, we'll
        create a loop.

    (2) We must still make sure that no variable in a coercion is at too
        high a level. But, when unifying, we can promote any variables we encounter.

    (3) We do not unify variables with a type with a free coercion hole.
        See (COERCION-HOLE) in Note [Unification preconditions].

    Note [Promotion and level-checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    "Promotion" happens when we have this:

      [W] w1: alpha[2] ~ Maybe beta[4]

    Here we must NOT unify alpha := Maybe beta, because beta may turn out
    to stand for a type involving some inner skolem.  Yikes!
    Skolem-escape.  So instead we /promote/ beta, like this:

      beta[4] := beta'[2]
      [W] w1: alpha[2] ~ Maybe beta'[2]

    Now we can unify alpha := Maybe beta', which might unlock other
    constraints.  But if some other constraint wants to unify beta with a
    nested skolem, it'll get stuck with a skolem-escape error.

    Now consider `w2` where a type family is involved (#22194):

      [W] w2: alpha[2] ~ Maybe (F gamma beta[4])

    In `w2`, it may or may not be the case that `beta` is level 2; suppose
    we later discover gamma := Int, and type instance F Int _ = Int.
    So, instead, we promote the entire funcion call:

      [W] w2': alpha[2] ~ Maybe gamma[2]
      [W] w3:  gamma[2] ~ F gamma beta[4]

    Now we can unify alpha := Maybe gamma, which is a Good Thng.

    Wrinkle (W1)

    There is an important wrinkle: /all this only applies when unifying/.
    For example, suppose we have
     [G] a[2] ~ Maybe b[4]
    where 'a' is a skolem.  This Given might arise from a GADT match, and
    we can absolutely use it to rewrite locally. In fact we must do so:
    that is how we exploit local knowledge about the outer skolem a[2].
    This applies equally for a Wanted [W] a[2] ~ Maybe b[4]. Using it for
    local rewriting is fine. (It's not clear to me that it is /useful/,
    but it's fine anyway.)

    So we only do the level-check in checkTyVar when /unifying/ not for
    skolems (or untouchable unification variables).

    Note [Family applications in canonical constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A constraint with a type family application in the RHS needs special care.

    * First, occurs checks.  If we have
         [G] a ~ Maybe (F (Maybe a))
         [W] alpha ~ Maybe (F (Maybe alpha))
      it looks as if we have an occurs check.  But go read
      Note [Type equality cycles] in GHC.Tc.Solver.Equality

      The same considerations apply when the LHS is a type family:
         [G] G a ~ Maybe (F (Maybe (G a)))
         [W] G alpha ~ Maybe (F (Maybe (G alpha)))

    * Second, promotion. If we have (#22194)
         [W] alpha[2] ~ Maybe (F beta[4])
      it is wrong to promote beta.  Instead we want to split to
         [W] alpha[2] ~ Maybe gamma[2]
         [W] gamma[2] ~ F beta[4]
      See Note [Promotion and level-checking] above.

    * Third, concrete type variables.  If we have
         [W] alpha[conc] ~ Maybe (F tys)
      we want to add an extra variable thus:
         [W] alpha[conc] ~ Maybe gamma[conc]
         [W] gamma[conc] ~ F tys
      Now we can unify alpha, and that might unlock something else.

    In all these cases we want to create a fresh type variable, and
    emit a new equality connecting it to the type family application.

    The `tef_fam_app` field of `TypeEqFlags` says what to do at a type
    family application in the RHS of the constraint.  `TEFA_Fail` and
    `TEFA_Recurse` are straightforward.  `TEFA_Break` is the clever
    one. As you can see in `checkFamApp`, it
      * Checks the arguments, but using `famAppArgFlags` to record that
        we are now "under" a type-family application. It `tef_fam_app` to
        `TEFA_Recurse`.
      * If any of the arguments fail (level-check error, occurs check)
        use the `FamAppBreaker` to create the extra binding.

    Note that this always cycle-breaks the /outermost/ family application.
    If we have  [W] alpha ~ Maybe (F (G alpha))
    * We'll use checkFamApp on `(F (G alpha))`
    * It will recurse into `(G alpha)` with TEFA_Recurse, but not cycle-break it
    * The occurs check will fire when we hit `alpha`
    * `checkFamApp` on `(F (G alpha))` will see the failure and invoke
      the `FamAppBreaker`.
    -}

    -------------------
    checkTyConApp :: TyEqFlags a
                  -> TcType -> TyCon -> [TcType]
                  -> TcM (PuResult a Reduction)
    checkTyConApp :: forall a.
    TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    checkTyConApp flags :: TyEqFlags a
    flags@(TEF { tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_unifying = AreUnifying
    unifying, tef_foralls :: forall a. TyEqFlags a -> Bool
    tef_foralls = Bool
    foralls_ok })
                  TcType
    tc_app TyCon
    tc [TcType]
    tys
      | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc
      , let arity :: Int
    arity = TyCon -> Int
    tyConArity TyCon
    tc
      = if [TcType]
    tys [TcType] -> Int -> Bool
    forall a. [a] -> Int -> Bool
    `lengthIs` Int
    arity
        then TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    forall a.
    TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    checkFamApp TyEqFlags a
    flags TcType
    tc_app TyCon
    tc [TcType]
    tys  -- Common case
        else do { let ([TcType]
    fun_args, [TcType]
    extra_args) = Int -> [TcType] -> ([TcType], [TcType])
    forall a. Int -> [a] -> ([a], [a])
    splitAt (TyCon -> Int
    tyConArity TyCon
    tc) [TcType]
    tys
                      fun_app :: TcType
    fun_app                = TyCon -> [TcType] -> TcType
    mkTyConApp TyCon
    tc [TcType]
    fun_args
                ; fun_res   <- TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    forall a.
    TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    checkFamApp TyEqFlags a
    flags TcType
    fun_app TyCon
    tc [TcType]
    fun_args
                ; extra_res <- mapCheck (checkTyEqRhs flags) extra_args
                ; traceTc "Over-sat" (ppr tc <+> ppr tys $$ ppr arity $$ pprPur fun_res $$ pprPur extra_res)
                ; return (mkAppRedns <$> fun_res <*> extra_res) }

      | Just TcType
    ty' <- TcType -> Maybe TcType
    rewriterView TcType
    tc_app
           -- e.g. S a  where  type S a = F [a]
           --             or   type S a = Int
           -- See Note [Forgetful synonyms in checkTyConApp]
      = TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags TcType
    ty'

      | Bool -> Bool
    not (TyCon -> Bool
    isTauTyCon TyCon
    tc Bool -> Bool -> Bool
    || Bool
    foralls_ok)
      = CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith CheckTyEqResult
    impredicativeProblem

      | Unifying MetaInfo
    info TcLevel
    _ LevelCheck
    _ <- AreUnifying
    unifying
      , MetaInfo -> Bool
    isConcreteInfo MetaInfo
    info
      , Bool -> Bool
    not (TyCon -> Bool
    isConcreteTyCon TyCon
    tc)
      = CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteConcrete)

      | Bool
    otherwise  -- Recurse on arguments
      = TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    forall a.
    TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    recurseIntoTyConApp TyEqFlags a
    flags TyCon
    tc [TcType]
    tys

    recurseIntoTyConApp :: TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    recurseIntoTyConApp :: forall a.
    TyEqFlags a -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    recurseIntoTyConApp TyEqFlags a
    flags TyCon
    tc [TcType]
    tys
      = do { tys_res <- (TcType -> TcM (PuResult a Reduction))
    -> [TcType] -> TcM (PuResult a Reductions)
    forall x a.
    (x -> TcM (PuResult a Reduction))
    -> [x] -> TcM (PuResult a Reductions)
    mapCheck (TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    flags) [TcType]
    tys
           ; return (mkTyConAppRedn Nominal tc <$> tys_res) }

    -------------------
    checkFamApp :: TyEqFlags a
                -> TcType -> TyCon -> [TcType]  -- Saturated family application
                -> TcM (PuResult a Reduction)
    -- See Note [Family applications in canonical constraints]
    checkFamApp :: forall a.
    TyEqFlags a
    -> TcType -> TyCon -> [TcType] -> TcM (PuResult a Reduction)
    checkFamApp flags :: TyEqFlags a
    flags@(TEF { tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_unifying = AreUnifying
    unifying, tef_occurs :: forall a. TyEqFlags a -> CheckTyEqProblem
    tef_occurs = CheckTyEqProblem
    occ_prob
                           , tef_fam_app :: forall a. TyEqFlags a -> TyEqFamApp a
    tef_fam_app = TyEqFamApp a
    fam_app_flag, tef_lhs :: forall a. TyEqFlags a -> CanEqLHS
    tef_lhs = CanEqLHS
    lhs })
                TcType
    fam_app TyCon
    tc [TcType]
    tys
      = case TyEqFamApp a
    fam_app_flag of
          TyEqFamApp a
    TEFA_Fail -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteTypeFamily)

          TyEqFamApp a
    _ | TyFamLHS TyCon
    lhs_tc [TcType]
    lhs_tys <- CanEqLHS
    lhs
            , TyCon -> [TcType] -> TyCon -> [TcType] -> Bool
    tcEqTyConApps TyCon
    lhs_tc [TcType]
    lhs_tys TyCon
    tc [TcType]
    tys   -- F ty ~ ...(F ty)...
            -> case TyEqFamApp a
    fam_app_flag of
                 TyEqFamApp a
    TEFA_Recurse       -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    occ_prob)
                 TEFA_Break FamAppBreaker a
    breaker -> FamAppBreaker a
    breaker TcType
    fam_app

          TyEqFamApp a
    _ | Unifying MetaInfo
    lhs_info TcLevel
    _ LevelCheck
    _ <- AreUnifying
    unifying
            , MetaInfo -> Bool
    isConcreteInfo MetaInfo
    lhs_info
            -> case TyEqFamApp a
    fam_app_flag of
                 TyEqFamApp a
    TEFA_Recurse       -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteConcrete)
                 TEFA_Break FamAppBreaker a
    breaker -> FamAppBreaker a
    breaker TcType
    fam_app

          TyEqFamApp a
    TEFA_Recurse
            -> do { tys_res <- FamAppBreaker a -> [TcType] -> TcM (PuResult a Reductions)
    forall x a.
    (x -> TcM (PuResult a Reduction))
    -> [x] -> TcM (PuResult a Reductions)
    mapCheck (TyEqFlags a -> FamAppBreaker a
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    arg_flags) [TcType]
    tys
                  ; traceTc "under" (ppr tc $$ pprPur tys_res $$ ppr flags)
                  ; return (mkTyConAppRedn Nominal tc <$> tys_res) }

          TEFA_Break FamAppBreaker a
    breaker    -- Recurse; and break if there is a problem
            -> do { tys_res <- FamAppBreaker a -> [TcType] -> TcM (PuResult a Reductions)
    forall x a.
    (x -> TcM (PuResult a Reduction))
    -> [x] -> TcM (PuResult a Reductions)
    mapCheck (TyEqFlags a -> FamAppBreaker a
    forall a. TyEqFlags a -> TcType -> TcM (PuResult a Reduction)
    checkTyEqRhs TyEqFlags a
    arg_flags) [TcType]
    tys
                  ; case tys_res of
                      PuOK Bag a
    cts Reductions
    redns -> PuResult a Reduction -> TcM (PuResult a Reduction)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Bag a -> Reduction -> PuResult a Reduction
    forall a b. Bag a -> b -> PuResult a b
    PuOK Bag a
    cts (Role -> TyCon -> Reductions -> Reduction
    mkTyConAppRedn Role
    Nominal TyCon
    tc Reductions
    redns))
                      PuFail {}      -> FamAppBreaker a
    breaker TcType
    fam_app }
      where
        arg_flags :: TyEqFlags a
    arg_flags = TyEqFlags a -> TyEqFlags a
    forall a. TyEqFlags a -> TyEqFlags a
    famAppArgFlags TyEqFlags a
    flags

    -------------------
    checkTyVar :: forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
    checkTyVar :: forall a. TyEqFlags a -> TcTyVar -> TcM (PuResult a Reduction)
    checkTyVar (TEF { tef_lhs :: forall a. TyEqFlags a -> CanEqLHS
    tef_lhs = CanEqLHS
    lhs, tef_unifying :: forall a. TyEqFlags a -> AreUnifying
    tef_unifying = AreUnifying
    unifying, tef_occurs :: forall a. TyEqFlags a -> CheckTyEqProblem
    tef_occurs = CheckTyEqProblem
    occ_prob }) TcTyVar
    occ_tv
      = case CanEqLHS
    lhs of
          TyFamLHS {}     -> TcM (PuResult a Reduction)
    success   -- Nothing to do if the LHS is a type-family
          TyVarLHS TcTyVar
    lhs_tv -> AreUnifying -> TcTyVar -> TcM (PuResult a Reduction)
    check_tv AreUnifying
    unifying TcTyVar
    lhs_tv
      where
        lvl_occ :: TcLevel
    lvl_occ = TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    occ_tv
        success :: TcM (PuResult a Reduction)
    success = TcType -> TcM (PuResult a Reduction)
    forall a. TcType -> TcM (PuResult a Reduction)
    okCheckRefl (TcTyVar -> TcType
    mkTyVarTy TcTyVar
    occ_tv)

        ---------------------
        check_tv :: AreUnifying -> TcTyVar -> TcM (PuResult a Reduction)
    check_tv AreUnifying
    NotUnifying TcTyVar
    lhs_tv
          = TcTyVar -> TcM (PuResult a Reduction)
    simple_occurs_check TcTyVar
    lhs_tv
          -- We need an occurs-check here, but no level check
          -- See Note [Promotion and level-checking] wrinkle (W1)

        check_tv (Unifying MetaInfo
    info TcLevel
    lvl LevelCheck
    prom) TcTyVar
    lhs_tv
          = do { mb_done <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    occ_tv
               ; case mb_done of
                   Just {} -> TcM (PuResult a Reduction)
    success
                   -- Already promoted; job done
                   -- Example alpha[2] ~ Maybe (beta[4], beta[4])
                   -- We promote the first occurrence, and then encounter it
                   -- a second time; we don't want to re-promote it!
                   -- Remember, the entire process started with a fully zonked type

                   Maybe TcType
    Nothing -> MetaInfo
    -> TcLevel -> LevelCheck -> TcTyVar -> TcM (PuResult a Reduction)
    check_unif MetaInfo
    info TcLevel
    lvl LevelCheck
    prom TcTyVar
    lhs_tv }

        ---------------------
        -- We are in the Unifying branch of AreUnifing
        check_unif :: MetaInfo -> TcLevel -> LevelCheck
                   -> TcTyVar -> TcM (PuResult a Reduction)
        check_unif :: MetaInfo
    -> TcLevel -> LevelCheck -> TcTyVar -> TcM (PuResult a Reduction)
    check_unif MetaInfo
    lhs_tv_info TcLevel
    lhs_tv_lvl LevelCheck
    prom TcTyVar
    lhs_tv
          | MetaInfo -> Bool
    isConcreteInfo MetaInfo
    lhs_tv_info
          , Bool -> Bool
    not (TcTyVar -> Bool
    isConcreteTyVar TcTyVar
    occ_tv)
          = if TcTyVar -> Bool
    can_make_concrete TcTyVar
    occ_tv
            then TcTyVar -> MetaInfo -> TcLevel -> TcM (PuResult a Reduction)
    promote TcTyVar
    lhs_tv MetaInfo
    lhs_tv_info TcLevel
    lhs_tv_lvl
            else CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteConcrete)

          | TcLevel
    lvl_occ TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lhs_tv_lvl
          = case LevelCheck
    prom of
               LevelCheck
    LC_None    -> String -> SDoc -> TcM (PuResult a Reduction)
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "check_unif" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    lhs_tv SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    occ_tv)
               LevelCheck
    LC_Check   -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteSkolemEscape)
               LevelCheck
    LC_Promote
                 | TcTyVar -> Bool
    isSkolemTyVar TcTyVar
    occ_tv  -> CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteSkolemEscape)
                 | Bool
    otherwise             -> TcTyVar -> MetaInfo -> TcLevel -> TcM (PuResult a Reduction)
    promote TcTyVar
    lhs_tv MetaInfo
    lhs_tv_info TcLevel
    lhs_tv_lvl

          | Bool
    otherwise
          = TcTyVar -> TcM (PuResult a Reduction)
    simple_occurs_check TcTyVar
    lhs_tv

        ---------------------
        simple_occurs_check :: TcTyVar -> TcM (PuResult a Reduction)
    simple_occurs_check TcTyVar
    lhs_tv
          | TcTyVar
    lhs_tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    occ_tv Bool -> Bool -> Bool
    || TcType -> Bool
    check_kind (TcTyVar -> TcType
    tyVarKind TcTyVar
    occ_tv)
          = CheckTyEqResult -> TcM (PuResult a Reduction)
    forall a b. CheckTyEqResult -> TcM (PuResult a b)
    failCheckWith (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    occ_prob)
          | Bool
    otherwise
          = TcM (PuResult a Reduction)
    success
          where
            (TcType -> Bool
    check_kind, Coercion -> Bool
    _) = TcTyVar -> (TcType -> Bool, Coercion -> Bool)
    mkOccFolders TcTyVar
    lhs_tv

        ---------------------
        can_make_concrete :: TcTyVar -> Bool
    can_make_concrete TcTyVar
    occ_tv = case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    occ_tv of
          MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info } -> case MetaInfo
    info of
                                          ConcreteTv {} -> Bool
    True
                                          TauTv {}      -> Bool
    True
                                          MetaInfo
    _             -> Bool
    False
          TcTyVarDetails
    _ -> Bool
    False  -- Don't attempt to make other type variables concrete
                      -- (e.g. SkolemTv, TyVarTv, CycleBreakerTv, RuntimeUnkTv).

        ---------------------
        -- occ_tv is definitely a MetaTyVar
        promote :: TcTyVar -> MetaInfo -> TcLevel -> TcM (PuResult a Reduction)
    promote TcTyVar
    lhs_tv MetaInfo
    lhs_tv_info TcLevel
    lhs_tv_lvl
          | MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info_occ, mtv_tclvl :: TcTyVarDetails -> TcLevel
    mtv_tclvl = TcLevel
    lvl_occ } <- TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    occ_tv
          = do { let new_info :: MetaInfo
    new_info | MetaInfo -> Bool
    isConcreteInfo MetaInfo
    lhs_tv_info = MetaInfo
    lhs_tv_info
                              | Bool
    otherwise                  = MetaInfo
    info_occ
                     new_lvl :: TcLevel
    new_lvl = TcLevel
    lhs_tv_lvl TcLevel -> TcLevel -> TcLevel
    `minTcLevel` TcLevel
    lvl_occ
                               -- c[conc,3] ~ p[tau,2]: want to clone p:=p'[conc,2]
                               -- c[tau,2]  ~ p[tau,3]: want to clone p:=p'[tau,2]

               -- Check the kind of occ_tv
               ; reason <- CheckTyEqProblem
    -> TcTyVar -> TcLevel -> VarSet -> TcM CheckTyEqResult
    checkPromoteFreeVars CheckTyEqProblem
    occ_prob TcTyVar
    lhs_tv TcLevel
    lhs_tv_lvl (TcType -> VarSet
    tyCoVarsOfType (TcTyVar -> TcType
    tyVarKind TcTyVar
    occ_tv))

               ; if cterHasNoProblem reason  -- Successfully promoted
                 then do { new_tv_ty <- promote_meta_tyvar new_info new_lvl occ_tv
                         ; okCheckRefl new_tv_ty }
                 else failCheckWith reason }

          | Bool
    otherwise = String -> SDoc -> TcM (PuResult a Reduction)
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "promote" (TcTyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcTyVar
    occ_tv)

    -------------------------
    checkPromoteFreeVars :: CheckTyEqProblem    -- What occurs check problem to report
                         -> TcTyVar -> TcLevel
                         -> TyCoVarSet -> TcM CheckTyEqResult
    -- Check this set of TyCoVars for
    --   (a) occurs check
    --   (b) promote if necessary, or report skolem escape
    checkPromoteFreeVars :: CheckTyEqProblem
    -> TcTyVar -> TcLevel -> VarSet -> TcM CheckTyEqResult
    checkPromoteFreeVars CheckTyEqProblem
    occ_prob TcTyVar
    lhs_tv TcLevel
    lhs_tv_lvl VarSet
    vs
      = do { oks <- (TcTyVar -> TcM CheckTyEqResult)
    -> [TcTyVar] -> IOEnv (Env TcGblEnv TcLclEnv) [CheckTyEqResult]
    forall (t :: * -> *) (m :: * -> *) a b.
    (Traversable t, Monad m) =>
    (a -> m b) -> t a -> m (t b)
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> [a] -> m [b]
    mapM TcTyVar -> TcM CheckTyEqResult
    do_one (VarSet -> [TcTyVar]
    forall elt. UniqSet elt -> [elt]
    nonDetEltsUniqSet VarSet
    vs)
           ; return (mconcat oks) }
      where
        do_one :: TyCoVar -> TcM CheckTyEqResult
        do_one :: TcTyVar -> TcM CheckTyEqResult
    do_one TcTyVar
    v | TcTyVar -> Bool
    isCoVar TcTyVar
    v           = CheckTyEqResult -> TcM CheckTyEqResult
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CheckTyEqResult
    cteOK
                 | TcTyVar
    lhs_tv TcTyVar -> TcTyVar -> Bool
    forall a. Eq a => a -> a -> Bool
    == TcTyVar
    v         = CheckTyEqResult -> TcM CheckTyEqResult
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    occ_prob)
                 | Bool
    no_promotion        = CheckTyEqResult -> TcM CheckTyEqResult
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return CheckTyEqResult
    cteOK
                 | Bool -> Bool
    not (TcTyVar -> Bool
    isMetaTyVar TcTyVar
    v) = CheckTyEqResult -> TcM CheckTyEqResult
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CheckTyEqProblem -> CheckTyEqResult
    cteProblem CheckTyEqProblem
    cteSkolemEscape)
                 | Bool
    otherwise           = TcTyVar -> TcM CheckTyEqResult
    promote_one TcTyVar
    v
          where
            no_promotion :: Bool
    no_promotion = Bool -> Bool
    not (TcTyVar -> TcLevel
    tcTyVarLevel TcTyVar
    v TcLevel -> TcLevel -> Bool
    `strictlyDeeperThan` TcLevel
    lhs_tv_lvl)

        -- isCoVar case: coercion variables are not an escape risk
        -- If an implication binds a coercion variable, it'll have equalities,
        -- so the "intervening given equalities" test above will catch it
        -- Coercion holes get filled with coercions, so again no problem.

        promote_one :: TcTyVar -> TcM CheckTyEqResult
    promote_one TcTyVar
    tv = do { _ <- MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
    promote_meta_tyvar MetaInfo
    TauTv TcLevel
    lhs_tv_lvl TcTyVar
    tv
                            ; return cteOK }

    promote_meta_tyvar :: MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
    promote_meta_tyvar :: MetaInfo -> TcLevel -> TcTyVar -> TcM TcType
    promote_meta_tyvar MetaInfo
    info TcLevel
    dest_lvl TcTyVar
    occ_tv
      = do { -- Check whether occ_tv is already unified. The rhs-type
             -- started zonked, but we may have promoted one of its type
             -- variables, and we then encounter it for the second time.
             -- But if so, it'll definitely be another already-checked TyVar
             mb_filled <- TcTyVar -> IOEnv (Env TcGblEnv TcLclEnv) (Maybe TcType)
    isFilledMetaTyVar_maybe TcTyVar
    occ_tv
           ; case mb_filled of {
               Just TcType
    ty -> TcType -> TcM TcType
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return TcType
    ty ;
               Maybe TcType
    Nothing ->

        -- OK, not done already, so clone/promote it
        do { new_tv <- MetaInfo -> TcLevel -> TcTyVar -> TcM TcTyVar
    cloneMetaTyVarWithInfo MetaInfo
    info TcLevel
    dest_lvl TcTyVar
    occ_tv
           ; liftZonkM $ writeMetaTyVar occ_tv (mkTyVarTy new_tv)
           ; traceTc "promoteTyVar" (ppr occ_tv <+> text "-->" <+> ppr new_tv)
           ; return (mkTyVarTy new_tv) } } }

    -------------------------
    touchabilityAndShapeTest :: TcLevel -> TcTyVar -> TcType -> Bool
    -- This is the key test for untouchability:
    -- See Note [Unification preconditions] in GHC.Tc.Utils.Unify
    -- and Note [Solve by unification] in GHC.Tc.Solver.Equality
    -- True <=> touchability and shape are OK
    touchabilityAndShapeTest :: TcLevel -> TcTyVar -> TcType -> Bool
    touchabilityAndShapeTest TcLevel
    given_eq_lvl TcTyVar
    tv TcType
    rhs
      | MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    info, mtv_tclvl :: TcTyVarDetails -> TcLevel
    mtv_tclvl = TcLevel
    tv_lvl } <- TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv
      , MetaInfo -> TcType -> Bool
    checkTopShape MetaInfo
    info TcType
    rhs
      = TcLevel
    tv_lvl TcLevel -> TcLevel -> Bool
    `deeperThanOrSame` TcLevel
    given_eq_lvl
      | Bool
    otherwise
      = Bool
    False

    -------------------------
    -- | checkTopShape checks (TYVAR-TV)
    -- Note [Unification preconditions]; returns True if these conditions
    -- are satisfied. But see the Note for other preconditions, too.
    checkTopShape :: MetaInfo -> TcType -> Bool
    checkTopShape :: MetaInfo -> TcType -> Bool
    checkTopShape MetaInfo
    info TcType
    xi
      = case MetaInfo
    info of
          MetaInfo
    TyVarTv ->
            case TcType -> Maybe TcTyVar
    getTyVar_maybe TcType
    xi of   -- Looks through type synonyms
               Maybe TcTyVar
    Nothing -> Bool
    False
               Just TcTyVar
    tv -> case TcTyVar -> TcTyVarDetails
    tcTyVarDetails TcTyVar
    tv of -- (TYVAR-TV) wrinkle
                            SkolemTv {} -> Bool
    True
                            TcTyVarDetails
    RuntimeUnk  -> Bool
    True
                            MetaTv { mtv_info :: TcTyVarDetails -> MetaInfo
    mtv_info = MetaInfo
    TyVarTv } -> Bool
    True
                            TcTyVarDetails
    _                             -> Bool
    False
          MetaInfo
    CycleBreakerTv -> Bool
    False  -- We never unify these
          MetaInfo
    _ -> Bool
    True