    {-# LANGUAGE MultiWayIf, RecursiveDo, TupleSections #-}

    module GHC.Tc.Solver(
           InferMode(..), simplifyInfer, findInferredDiff,
           growThetaTyVars,
           simplifyAmbiguityCheck,
           simplifyDefault,
           simplifyTop, simplifyTopImplic,
           simplifyInteractive,
           solveEqualities,
           pushLevelAndSolveEqualities, pushLevelAndSolveEqualitiesX,
           reportUnsolvedEqualities,
           simplifyWantedsTcM,
           tcCheckGivens,
           tcCheckWanteds,
           tcNormalise,

           captureTopConstraints,

           simplifyTopWanteds,

           promoteTyVarSet, simplifyAndEmitFlatConstraints,

           -- For Rules we need these
           solveWanteds,
           approximateWC

      ) where

    import GHC.Prelude

    import GHC.Data.Bag
    import GHC.Core.Class
    import GHC.Core
    import GHC.Core.DataCon
    import GHC.Core.Make
    import GHC.Driver.DynFlags
    import GHC.Data.FastString
    import GHC.Data.List.SetOps
    import GHC.Types.Name
    import GHC.Types.Unique.Set
    import GHC.Types.Id
    import GHC.Utils.Outputable
    import GHC.Builtin.Utils
    import GHC.Builtin.Names
    import GHC.Tc.Errors
    import GHC.Tc.Errors.Types
    import GHC.Tc.Types.Evidence
    import GHC.Tc.Solver.Solve   ( solveSimpleGivens, solveSimpleWanteds )
    import GHC.Tc.Solver.Dict    ( makeSuperClasses, solveCallStack )
    import GHC.Tc.Solver.Rewrite ( rewriteType )
    import GHC.Tc.Utils.Unify    ( buildTvImplication )
    import GHC.Tc.Utils.TcMType as TcM
    import GHC.Tc.Utils.Monad   as TcM
    import GHC.Tc.Zonk.TcType     as TcM
    import GHC.Tc.Solver.InertSet
    import GHC.Tc.Solver.Monad  as TcS
    import GHC.Tc.Types.Constraint
    import GHC.Tc.Instance.FunDeps
    import GHC.Core.Predicate
    import GHC.Tc.Types.Origin
    import GHC.Tc.Utils.TcType
    import GHC.Core.Type
    import GHC.Core.Ppr
    import GHC.Core.TyCon    ( TyConBinder, isTypeFamilyTyCon )
    import GHC.Builtin.Types
    import GHC.Core.Unify    ( tcMatchTyKis )
    import GHC.Unit.Module ( getModule )
    import GHC.Utils.Misc
    import GHC.Utils.Panic
    import GHC.Types.TyThing ( MonadThings(lookupId) )
    import GHC.Types.Var
    import GHC.Types.Var.Env
    import GHC.Types.Var.Set
    import GHC.Types.Basic
    import GHC.Types.Id.Make  ( unboxedUnitExpr )
    import GHC.Types.Error
    import qualified GHC.LanguageExtensions as LangExt

    import Control.Monad
    import Control.Monad.Trans.Class        ( lift )
    import Control.Monad.Trans.State.Strict ( StateT(runStateT), put )
    import Data.Foldable      ( toList, traverse_ )
    import Data.List          ( partition )
    import Data.List.NonEmpty ( NonEmpty(..), nonEmpty )
    import qualified Data.List.NonEmpty as NE
    import GHC.Data.Maybe     ( mapMaybe, runMaybeT, MaybeT )

    {-
    *********************************************************************************
    *                                                                               *
    *                           External interface                                  *
    *                                                                               *
    *********************************************************************************
    -}

    captureTopConstraints :: TcM a -> TcM (a, WantedConstraints)
    -- (captureTopConstraints m) runs m, and returns the type constraints it
    -- generates plus the constraints produced by static forms inside.
    -- If it fails with an exception, it reports any insolubles
    -- (out of scope variables) before doing so
    --
    -- captureTopConstraints is used exclusively by GHC.Tc.Module at the top
    -- level of a module.
    --
    -- Importantly, if captureTopConstraints propagates an exception, it
    -- reports any insoluble constraints first, lest they be lost
    -- altogether.  This is important, because solveEqualities (maybe
    -- other things too) throws an exception without adding any error
    -- messages; it just puts the unsolved constraints back into the
    -- monad. See GHC.Tc.Utils.Monad Note [Constraints and errors]
    -- #16376 is an example of what goes wrong if you don't do this.
    --
    -- NB: the caller should bring any environments into scope before
    -- calling this, so that the reportUnsolved has access to the most
    -- complete GlobalRdrEnv
    captureTopConstraints :: forall a. TcM a -> TcM (a, WantedConstraints)
    captureTopConstraints TcM a
    thing_inside
      = do { static_wc_var <- WantedConstraints
    -> IOEnv (Env TcGblEnv TcLclEnv) (TcRef WantedConstraints)
    forall (m :: * -> *) a. MonadIO m => a -> m (TcRef a)
    TcM.newTcRef WantedConstraints
    emptyWC ;
           ; (mb_res, lie) <- TcM.updGblEnv (\TcGblEnv
    env -> TcGblEnv
    env { tcg_static_wc = static_wc_var } ) $
                              TcM.tryCaptureConstraints thing_inside
           ; stWC <- TcM.readTcRef static_wc_var

           -- See GHC.Tc.Utils.Monad Note [Constraints and errors]
           -- If the thing_inside threw an exception, but generated some insoluble
           -- constraints, report the latter before propagating the exception
           -- Otherwise they will be lost altogether
           ; case mb_res of
               Just a
    res -> (a, WantedConstraints)
    -> IOEnv (Env TcGblEnv TcLclEnv) (a, WantedConstraints)
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a
    res, WantedConstraints
    lie WantedConstraints -> WantedConstraints -> WantedConstraints
    `andWC` WantedConstraints
    stWC)
               Maybe a
    Nothing  -> do { _ <- WantedConstraints -> TcM (Bag EvBind)
    simplifyTop WantedConstraints
    lie; failM } }
                    -- This call to simplifyTop is the reason
                    -- this function is here instead of GHC.Tc.Utils.Monad
                    -- We call simplifyTop so that it does defaulting
                    -- (esp of runtime-reps) before reporting errors

    simplifyTopImplic :: Bag Implication -> TcM ()
    simplifyTopImplic :: Bag Implication -> TcM ()
    simplifyTopImplic Bag Implication
    implics
      = do { empty_binds <- WantedConstraints -> TcM (Bag EvBind)
    simplifyTop (Bag Implication -> WantedConstraints
    mkImplicWC Bag Implication
    implics)

           -- Since all the inputs are implications the returned bindings will be empty
           ; massertPpr (isEmptyBag empty_binds) (ppr empty_binds)

           ; return () }

    simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
    -- Simplify top-level constraints
    -- Usually these will be implications,
    -- but when there is nothing to quantify we don't wrap
    -- in a degenerate implication, so we do that here instead
    simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
    simplifyTop WantedConstraints
    wanteds
      = do { String -> SDoc -> TcM ()
    traceTc String
    "simplifyTop {" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "wanted = " SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> WantedConstraints -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wanteds
           ; ((final_wc, unsafe_ol), binds1) <- TcS (WantedConstraints, Bag DictCt)
    -> TcM ((WantedConstraints, Bag DictCt), EvBindMap)
    forall a. TcS a -> TcM (a, EvBindMap)
    runTcS (TcS (WantedConstraints, Bag DictCt)
     -> TcM ((WantedConstraints, Bag DictCt), EvBindMap))
    -> TcS (WantedConstraints, Bag DictCt)
    -> TcM ((WantedConstraints, Bag DictCt), EvBindMap)
    forall a b. (a -> b) -> a -> b
    $
                do { final_wc <- WantedConstraints -> TcS WantedConstraints
    simplifyTopWanteds WantedConstraints
    wanteds
                   ; unsafe_ol <- getSafeOverlapFailures
                   ; return (final_wc, unsafe_ol) }
           ; traceTc "End simplifyTop }" empty

           ; binds2 <- reportUnsolved final_wc

           ; traceTc "reportUnsolved (unsafe overlapping) {" empty
           ; unless (isEmptyBag unsafe_ol) $ do {
               -- grab current error messages and clear, warnAllUnsolved will
               -- update error messages which we'll grab and then restore saved
               -- messages.
               ; errs_var  <- getErrsVar
               ; saved_msg <- TcM.readTcRef errs_var
               ; TcM.writeTcRef errs_var emptyMessages

               ; warnAllUnsolved $ emptyWC { wc_simple = fmap CDictCan unsafe_ol }

               ; whyUnsafe <- getWarningMessages <$> TcM.readTcRef errs_var
               ; TcM.writeTcRef errs_var saved_msg
               ; recordUnsafeInfer (mkMessages whyUnsafe)
               }
           ; traceTc "reportUnsolved (unsafe overlapping) }" empty

           ; return (evBindMapBinds binds1 `unionBags` binds2) }

    pushLevelAndSolveEqualities :: SkolemInfoAnon -> [TyConBinder] -> TcM a -> TcM a
    -- Push level, and solve all resulting equalities
    -- If there are any unsolved equalities, report them
    -- and fail (in the monad)
    --
    -- Panics if we solve any non-equality constraints.  (In runTCSEqualities
    -- we use an error thunk for the evidence bindings.)
    pushLevelAndSolveEqualities :: forall a. SkolemInfoAnon -> [TyConBinder] -> TcM a -> TcM a
    pushLevelAndSolveEqualities SkolemInfoAnon
    skol_info_anon [TyConBinder]
    tcbs TcM a
    thing_inside
      = do { (tclvl, wanted, res) <- String -> TcM a -> TcM (TcLevel, WantedConstraints, a)
    forall a. String -> TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndSolveEqualitiesX
                                          String
    "pushLevelAndSolveEqualities" TcM a
    thing_inside
           ; report_unsolved_equalities skol_info_anon (binderVars tcbs) tclvl wanted
           ; return res }

    pushLevelAndSolveEqualitiesX :: String -> TcM a
                                 -> TcM (TcLevel, WantedConstraints, a)
    -- Push the level, gather equality constraints, and then solve them.
    -- Returns any remaining unsolved equalities.
    -- Does not report errors.
    --
    -- Panics if we solve any non-equality constraints.  (In runTCSEqualities
    -- we use an error thunk for the evidence bindings.)
    pushLevelAndSolveEqualitiesX :: forall a. String -> TcM a -> TcM (TcLevel, WantedConstraints, a)
    pushLevelAndSolveEqualitiesX String
    callsite TcM a
    thing_inside
      = do { String -> SDoc -> TcM ()
    traceTc String
    "pushLevelAndSolveEqualitiesX {" (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Called from" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    callsite)
           ; (tclvl, (wanted, res))
                <- TcM (WantedConstraints, a) -> TcM (TcLevel, (WantedConstraints, a))
    forall a. TcM a -> TcM (TcLevel, a)
    pushTcLevelM (TcM (WantedConstraints, a)
     -> TcM (TcLevel, (WantedConstraints, a)))
    -> TcM (WantedConstraints, a)
    -> TcM (TcLevel, (WantedConstraints, a))
    forall a b. (a -> b) -> a -> b
    $
                   do { (res, wanted) <- TcM a -> TcM (a, WantedConstraints)
    forall a. TcM a -> TcM (a, WantedConstraints)
    captureConstraints TcM a
    thing_inside
                      ; wanted <- runTcSEqualities (simplifyTopWanteds wanted)
                      ; return (wanted,res) }
           ; traceTc "pushLevelAndSolveEqualities }" (vcat [ text "Residual:" <+> ppr wanted
                                                           , text "Level:" <+> ppr tclvl ])
           ; return (tclvl, wanted, res) }

    -- | Type-check a thing that emits only equality constraints, solving any
    -- constraints we can and re-emitting constraints that we can't.
    -- Use this variant only when we'll get another crack at it later
    -- See Note [Failure in local type signatures]
    --
    -- Panics if we solve any non-equality constraints.  (In runTCSEqualities
    -- we use an error thunk for the evidence bindings.)
    solveEqualities :: String -> TcM a -> TcM a
    solveEqualities :: forall a. String -> TcM a -> TcM a
    solveEqualities String
    callsite TcM a
    thing_inside
      = do { String -> SDoc -> TcM ()
    traceTc String
    "solveEqualities {" (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "Called from" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    callsite)
           ; (res, wanted)   <- TcM a -> TcM (a, WantedConstraints)
    forall a. TcM a -> TcM (a, WantedConstraints)
    captureConstraints TcM a
    thing_inside
           ; simplifyAndEmitFlatConstraints wanted
                -- simplifyAndEmitFlatConstraints fails outright unless
                --  the only unsolved constraints are soluble-looking
                --  equalities that can float out
           ; traceTc "solveEqualities }" empty
           ; return res }

    simplifyAndEmitFlatConstraints :: WantedConstraints -> TcM ()
    -- See Note [Failure in local type signatures]
    simplifyAndEmitFlatConstraints :: WantedConstraints -> TcM ()
    simplifyAndEmitFlatConstraints WantedConstraints
    wanted
      = do { -- Solve and zonk to establish the
             -- preconditions for floatKindEqualities
             wanted <- TcS WantedConstraints
    -> IOEnv (Env TcGblEnv TcLclEnv) WantedConstraints
    forall a. TcS a -> TcM a
    runTcSEqualities (WantedConstraints -> TcS WantedConstraints
    solveWanteds WantedConstraints
    wanted)
           ; wanted <- TcM.liftZonkM $ TcM.zonkWC wanted

           ; traceTc "emitFlatConstraints {" (ppr wanted)
           ; case floatKindEqualities wanted of
               Maybe (Cts, Bag DelayedError)
    Nothing -> do { String -> SDoc -> TcM ()
    traceTc String
    "emitFlatConstraints } failing" (WantedConstraints -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wanted)
                             -- Emit the bad constraints, wrapped in an implication
                             -- See Note [Wrapping failing kind equalities]
                             ; tclvl  <- TcM TcLevel
    TcM.getTcLevel
                             ; implic <- buildTvImplication unkSkolAnon [] (pushTcLevel tclvl) wanted
                                            --                  ^^^^^^   |  ^^^^^^^^^^^^^^^^^
                                            -- it's OK to use unkSkol    |  we must increase the TcLevel,
                                            -- because we don't bind     |  as explained in
                                            -- any skolem variables here |  Note [Wrapping failing kind equalities]
                             ; emitImplication implic
                             ; failM }
               Just (Cts
    simples, Bag DelayedError
    errs)
                  -> do { _ <- HasDebugCallStack => VarSet -> TcM Bool
    VarSet -> TcM Bool
    promoteTyVarSet (Cts -> VarSet
    tyCoVarsOfCts Cts
    simples)
                        ; traceTc "emitFlatConstraints }" $
                          vcat [ text "simples:" <+> ppr simples
                               , text "errs:   " <+> ppr errs ]
                          -- Holes and other delayed errors don't need promotion
                        ; emitDelayedErrors errs
                        ; emitSimples simples } }

    floatKindEqualities :: WantedConstraints -> Maybe (Bag Ct, Bag DelayedError)
    -- Float out all the constraints from the WantedConstraints,
    -- Return Nothing if any constraints can't be floated (captured
    -- by skolems), or if there is an insoluble constraint, or
    -- IC_Telescope telescope error
    -- Precondition 1: we have tried to solve the 'wanteds', both so that
    --    the ic_status field is set, and because solving can make constraints
    --    more floatable.
    -- Precondition 2: the 'wanteds' are zonked, since floatKindEqualities
    --    is not monadic
    -- See Note [floatKindEqualities vs approximateWC]
    floatKindEqualities :: WantedConstraints -> Maybe (Cts, Bag DelayedError)
    floatKindEqualities WantedConstraints
    wc = VarSet -> WantedConstraints -> Maybe (Cts, Bag DelayedError)
    float_wc VarSet
    emptyVarSet WantedConstraints
    wc
      where
        float_wc :: TcTyCoVarSet -> WantedConstraints -> Maybe (Bag Ct, Bag DelayedError)
        float_wc :: VarSet -> WantedConstraints -> Maybe (Cts, Bag DelayedError)
    float_wc VarSet
    trapping_tvs (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples
                                  , wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics
                                  , wc_errors :: WantedConstraints -> Bag DelayedError
    wc_errors = Bag DelayedError
    errs })
          | (Ct -> Bool) -> Cts -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all Ct -> Bool
    is_floatable Cts
    simples
          = do { (inner_simples, inner_errs)
                    <- (Implication -> Maybe (Cts, Bag DelayedError))
    -> Bag Implication -> Maybe (Cts, Bag DelayedError)
    forall (m :: * -> *) a b c.
    Monad m =>
    (a -> m (Bag b, Bag c)) -> Bag a -> m (Bag b, Bag c)
    flatMapBagPairM (VarSet -> Implication -> Maybe (Cts, Bag DelayedError)
    float_implic VarSet
    trapping_tvs) Bag Implication
    implics
               ; return ( simples `unionBags` inner_simples
                        , errs `unionBags` inner_errs) }
          | Bool
    otherwise
          = Maybe (Cts, Bag DelayedError)
    forall a. Maybe a
    Nothing
          where
            is_floatable :: Ct -> Bool
    is_floatable Ct
    ct
               | Ct -> Bool
    insolubleCt Ct
    ct = Bool
    False
               | Bool
    otherwise      = Ct -> VarSet
    tyCoVarsOfCt Ct
    ct VarSet -> VarSet -> Bool
    `disjointVarSet` VarSet
    trapping_tvs

        float_implic :: TcTyCoVarSet -> Implication -> Maybe (Bag Ct, Bag DelayedError)
        float_implic :: VarSet -> Implication -> Maybe (Cts, Bag DelayedError)
    float_implic VarSet
    trapping_tvs (Implic { ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WantedConstraints
    wanted, ic_given_eqs :: Implication -> HasGivenEqs
    ic_given_eqs = HasGivenEqs
    given_eqs
                                          , ic_skols :: Implication -> [TyVar]
    ic_skols = [TyVar]
    skols, ic_status :: Implication -> ImplicStatus
    ic_status = ImplicStatus
    status })
          | ImplicStatus -> Bool
    isInsolubleStatus ImplicStatus
    status
          = Maybe (Cts, Bag DelayedError)
    forall a. Maybe a
    Nothing   -- A short cut /plus/ we must keep track of IC_BadTelescope
          | Bool
    otherwise
          = do { (simples, holes) <- VarSet -> WantedConstraints -> Maybe (Cts, Bag DelayedError)
    float_wc VarSet
    new_trapping_tvs WantedConstraints
    wanted
               ; when (not (isEmptyBag simples) && given_eqs == MaybeGivenEqs) $
                 Nothing
                     -- If there are some constraints to float out, but we can't
                     -- because we don't float out past local equalities
                     -- (c.f GHC.Tc.Solver.approximateWC), then fail
               ; return (simples, holes) }
          where
            new_trapping_tvs :: VarSet
    new_trapping_tvs = VarSet
    trapping_tvs VarSet -> [TyVar] -> VarSet
    `extendVarSetList` [TyVar]
    skols

    {- Note [Failure in local type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When kind checking a type signature, we like to fail fast if we can't
    solve all the kind equality constraints, for two reasons:

      * A kind-bogus type signature may cause a cascade of knock-on
        errors if we let it pass

      * More seriously, we don't have a convenient term-level place to add
        deferred bindings for unsolved kind-equality constraints.  In
        earlier GHCs this led to un-filled-in coercion holes, which caused
        GHC to crash with "fvProv falls into a hole" See #11563, #11520,
        #11516, #11399

    But what about /local/ type signatures, mentioning in-scope type
    variables for which there might be 'given' equalities?  For these we
    might not be able to solve all the equalities locally. Here's an
    example (T15076b):

      class (a ~ b) => C a b
      data SameKind :: k -> k -> Type where { SK :: SameKind a b }

      bar :: forall (a :: Type) (b :: Type).
             C a b => Proxy a -> Proxy b -> ()
      bar _ _ = const () (undefined :: forall (x :: a) (y :: b). SameKind x y)

    Consider the type signature on 'undefined'. It's ill-kinded unless
    a~b.  But the superclass of (C a b) means that indeed (a~b). So all
    should be well. BUT it's hard to see that when kind-checking the signature
    for undefined.  We want to emit a residual (a~b) constraint, to solve
    later.

    Another possibility is that we might have something like
       F alpha ~ [Int]
    where alpha is bound further out, which might become soluble
    "later" when we learn more about alpha.  So we want to emit
    those residual constraints.

    BUT it's no good simply wrapping all unsolved constraints from
    a type signature in an implication constraint to solve later. The
    problem is that we are going to /use/ that signature, including
    instantiate it.  Say we have
         f :: forall a.  (forall b. blah) -> blah2
         f x = <body>
    To typecheck the definition of f, we have to instantiate those
    foralls.  Moreover, any unsolved kind equalities will be coercion
    holes in the type.  If we naively wrap them in an implication like
         forall a. (co1:k1~k2,  forall b.  co2:k3~k4)
    hoping to solve it later, we might end up filling in the holes
    co1 and co2 with coercions involving 'a' and 'b' -- but by now
    we've instantiated the type.  Chaos!

    Moreover, the unsolved constraints might be skolem-escape things, and
    if we proceed with f bound to a nonsensical type, we get a cascade of
    follow-up errors. For example polykinds/T12593, T15577, and many others.

    So here's the plan (see tcHsSigType):

    * pushLevelAndSolveEqualitiesX: try to solve the constraints

    * kindGeneraliseSome: do kind generalisation

    * buildTvImplication: build an implication for the residual, unsolved
      constraint

    * simplifyAndEmitFlatConstraints: try to float out every unsolved equality
      inside that implication, in the hope that it constrains only global
      type variables, not the locally-quantified ones.

      * If we fail, or find an insoluble constraint, emit the implication,
        so that the errors will be reported, and fail.

      * If we succeed in floating all the equalities, promote them and
        re-emit them as flat constraint, not wrapped at all (since they
        don't mention any of the quantified variables.

    * Note that this float-and-promote step means that anonymous
      wildcards get floated to top level, as we want; see
      Note [Checking partial type signatures] in GHC.Tc.Gen.HsType.

    All this is done:

    * In GHC.Tc.Gen.HsType.tcHsSigType, as above

    * solveEqualities. Use this when there no kind-generalisation
      step to complicate matters; then we don't need to push levels,
      and can solve the equalities immediately without needing to
      wrap it in an implication constraint.  (You'll generally see
      a kindGeneraliseNone nearby.)

    * In GHC.Tc.TyCl and GHC.Tc.TyCl.Instance; see calls to
      pushLevelAndSolveEqualitiesX, followed by quantification, and
      then reportUnsolvedEqualities.

      NB: we call reportUnsolvedEqualities before zonkTcTypeToType
      because the latter does not expect to see any un-filled-in
      coercions, which will happen if we have unsolved equalities.
      By calling reportUnsolvedEqualities first, which fails after
      reporting errors, we avoid that happening.

    See also #18062, #11506

    Note [Wrapping failing kind equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In simplifyAndEmitFlatConstraints, if we fail to get down to simple
    flat constraints we will
    * re-emit the constraints so that they are reported
    * fail in the monad
    But there is a Terrible Danger that, if -fdefer-type-errors is on, and
    we just re-emit an insoluble constraint like (* ~ (*->*)), that we'll
    report only a warning and proceed with compilation.  But if we ever fail
    in the monad it should be fatal; we should report an error and stop after
    the type checker.  If not, chaos results: #19142.

    Our solution is this:
    * Even with -fdefer-type-errors, inside an implication with no place for
      value bindings (ic_binds = CoEvBindsVar), report failing equalities as
      errors.  We have to do this anyway; see GHC.Tc.Errors
      Note [Failing equalities with no evidence bindings].

    * Right here in simplifyAndEmitFlatConstraints, use buildTvImplication
      to wrap the failing constraint in a degenerate implication (no
      skolems, no theta), with ic_binds = CoEvBindsVar.  This setting of
      `ic_binds` means that any failing equalities will lead to an
      error not a warning, irrespective of -fdefer-type-errors: see
      Note [Failing equalities with no evidence bindings] in GHC.Tc.Errors,
      and `maybeSwitchOffDefer` in that module.

      We still take care to bump the TcLevel of the implication.  Partly,
      that ensures that nested implications have increasing level numbers
      which seems nice.  But more specifically, suppose the outer level
      has a Given `(C ty)`, which has pending (not-yet-expanded)
      superclasses. Consider what happens when we process this implication
      constraint (which we have re-emitted) in that context:
        - in the inner implication we'll call `getPendingGivenScs`,
        - we /do not/ want to get the `(C ty)` from the outer level,
        lest we try to add an evidence term for the superclass,
        which we can't do because we have specifically set
        `ic_binds` = `CoEvBindsVar`.
        - as `getPendingGivenSCcs is careful to only get Givens from
        the /current/ level, and we bumped the `TcLevel` of the implication,
        we're OK.

      TL;DR: bump the `TcLevel` when creating the nested implication.
      If we don't we get a panic in `GHC.Tc.Utils.Monad.addTcEvBind` (#20043).

    We re-emit the implication rather than reporting the errors right now,
    so that the error messages are improved by other solving and defaulting.
    e.g. we prefer
        Cannot match 'Type->Type' with 'Type'
    to  Cannot match 'Type->Type' with 'TYPE r0'

    Note [floatKindEqualities vs approximateWC]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    floatKindEqualities and approximateWC are strikingly similar to each
    other, but

    * floatKindEqualites tries to float /all/ equalities, and fails if
      it can't, or if any implication is insoluble.
    * approximateWC just floats out any constraints
      (not just equalities) that can float; it never fails.
    -}

    reportUnsolvedEqualities :: SkolemInfo -> [TcTyVar] -> TcLevel
                             -> WantedConstraints -> TcM ()
    -- Reports all unsolved wanteds provided; fails in the monad if there are any.
    --
    -- The provided SkolemInfo and [TcTyVar] arguments are used in an implication to
    -- provide skolem info for any errors.
    reportUnsolvedEqualities :: SkolemInfo -> [TyVar] -> TcLevel -> WantedConstraints -> TcM ()
    reportUnsolvedEqualities SkolemInfo
    skol_info [TyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      = SkolemInfoAnon -> [TyVar] -> TcLevel -> WantedConstraints -> TcM ()
    report_unsolved_equalities (SkolemInfo -> SkolemInfoAnon
    getSkolemInfo SkolemInfo
    skol_info) [TyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted

    report_unsolved_equalities :: SkolemInfoAnon -> [TcTyVar] -> TcLevel
                               -> WantedConstraints -> TcM ()
    report_unsolved_equalities :: SkolemInfoAnon -> [TyVar] -> TcLevel -> WantedConstraints -> TcM ()
    report_unsolved_equalities SkolemInfoAnon
    skol_info_anon [TyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanted
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

      | Bool
    otherwise  -- NB: we build an implication /even if skol_tvs is empty/,
                   -- just to ensure that our level invariants hold, specifically
                   -- (WantedInv).  See Note [TcLevel invariants].
      = TcM () -> TcM ()
    forall r. TcM r -> TcM r
    checkNoErrs (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $   -- Fail
        do { implic <- SkolemInfoAnon
    -> [TyVar] -> TcLevel -> WantedConstraints -> TcM Implication
    buildTvImplication SkolemInfoAnon
    skol_info_anon [TyVar]
    skol_tvs TcLevel
    tclvl WantedConstraints
    wanted
           ; reportAllUnsolved (mkImplicWC (unitBag implic)) }

    -- | Simplify top-level constraints, but without reporting any unsolved
    -- constraints nor unsafe overlapping.
    simplifyTopWanteds :: WantedConstraints -> TcS WantedConstraints
        -- See Note [Top-level Defaulting Plan]
    simplifyTopWanteds :: WantedConstraints -> TcS WantedConstraints
    simplifyTopWanteds WantedConstraints
    wanteds
      = do { wc_first_go <- TcS WantedConstraints -> TcS WantedConstraints
    forall a. TcS a -> TcS a
    nestTcS (WantedConstraints -> TcS WantedConstraints
    solveWanteds WantedConstraints
    wanteds)
                                -- This is where the main work happens
           ; dflags <- getDynFlags
           ; wc_defaulted <- try_tyvar_defaulting dflags wc_first_go

           -- See Note [Implementation of Unsatisfiable constraints] in GHC.Tc.Errors,
           -- point (C).
           ; useUnsatisfiableGivens wc_defaulted }
      where
        try_tyvar_defaulting :: DynFlags -> WantedConstraints -> TcS WantedConstraints
        try_tyvar_defaulting :: DynFlags -> WantedConstraints -> TcS WantedConstraints
    try_tyvar_defaulting DynFlags
    dflags WantedConstraints
    wc
          | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wc
          = WantedConstraints -> TcS WantedConstraints
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return WantedConstraints
    wc
          | WantedConstraints -> Bool
    insolubleWC WantedConstraints
    wc
          , GeneralFlag -> DynFlags -> Bool
    gopt GeneralFlag
    Opt_PrintExplicitRuntimeReps DynFlags
    dflags -- See Note [Defaulting insolubles]
          = WantedConstraints -> TcS WantedConstraints
    try_class_defaulting WantedConstraints
    wc
          | Bool
    otherwise
          = do { -- Need to zonk first, as the WantedConstraints are not yet zonked.
               ; free_tvs <- [TyVar] -> TcS [TyVar]
    TcS.zonkTyCoVarsAndFVList (WantedConstraints -> [TyVar]
    tyCoVarsOfWCList WantedConstraints
    wc)
               ; let defaultable_tvs = (TyVar -> Bool) -> [TyVar] -> [TyVar]
    forall a. (a -> Bool) -> [a] -> [a]
    filter TyVar -> Bool
    can_default [TyVar]
    free_tvs
                     can_default TyVar
    tv
                       =   TyVar -> Bool
    isTyVar TyVar
    tv
                           -- Weed out coercion variables.

                        Bool -> Bool -> Bool
    && TyVar -> Bool
    isMetaTyVar TyVar
    tv
                           -- Weed out runtime-skolems in GHCi, which we definitely
                           -- shouldn't try to default.

                        Bool -> Bool -> Bool
    && Bool -> Bool
    not (TyVar
    tv TyVar -> VarSet -> Bool
    `elemVarSet` WantedConstraints -> VarSet
    nonDefaultableTyVarsOfWC WantedConstraints
    wc)
                           -- Weed out variables for which defaulting would be unhelpful,
                           -- e.g. alpha appearing in [W] alpha[conc] ~# rr[sk].

               ; defaulted <- mapM defaultTyVarTcS defaultable_tvs -- Has unification side effects
               ; if or defaulted
                 then do { wc_residual <- nestTcS (solveWanteds wc)
                                -- See Note [Must simplify after defaulting]
                         ; try_class_defaulting wc_residual }
                 else try_class_defaulting wc }     -- No defaulting took place

        try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
        try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_class_defaulting WantedConstraints
    wc
          | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wc Bool -> Bool -> Bool
    || WantedConstraints -> Bool
    insolubleWC WantedConstraints
    wc -- See Note [Defaulting insolubles]
          = WantedConstraints -> TcS WantedConstraints
    try_callstack_defaulting WantedConstraints
    wc
          | Bool
    otherwise  -- See Note [When to do type-class defaulting]
          = do { something_happened <- WantedConstraints -> TcS Bool
    applyDefaultingRules WantedConstraints
    wc
                                       -- See Note [Top-level Defaulting Plan]
               ; if something_happened
                 then do { wc_residual <- nestTcS (solveWanteds wc)
                         ; try_class_defaulting wc_residual }
                      -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
                 else try_callstack_defaulting wc }

        try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints
        try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_callstack_defaulting WantedConstraints
    wc
          = [CtDefaultingStrategy]
    -> WantedConstraints -> TcS WantedConstraints
    defaultConstraints [CtDefaultingStrategy
    defaultCallStack, CtDefaultingStrategy
    defaultExceptionContext] WantedConstraints
    wc

    -- | If an implication contains a Given of the form @Unsatisfiable msg@, use
    -- it to solve all Wanteds within the implication.
    --
    -- This does a complete walk over the implication tree.
    --
    -- See point (C) in Note [Implementation of Unsatisfiable constraints] in GHC.Tc.Errors.
    useUnsatisfiableGivens :: WantedConstraints -> TcS WantedConstraints
    useUnsatisfiableGivens :: WantedConstraints -> TcS WantedConstraints
    useUnsatisfiableGivens WantedConstraints
    wc =
      do { (final_wc, did_work) <- (StateT Bool TcS WantedConstraints
    -> Bool -> TcS (WantedConstraints, Bool)
    forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
    `runStateT` Bool
    False) (StateT Bool TcS WantedConstraints
     -> TcS (WantedConstraints, Bool))
    -> StateT Bool TcS WantedConstraints
    -> TcS (WantedConstraints, Bool)
    forall a b. (a -> b) -> a -> b
    $ WantedConstraints -> StateT Bool TcS WantedConstraints
    go_wc WantedConstraints
    wc
         ; if did_work
           then nestTcS (solveWanteds final_wc)
           else return final_wc }
      where
        go_wc :: WantedConstraints -> StateT Bool TcS WantedConstraints
    go_wc (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    wtds, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    impls, wc_errors :: WantedConstraints -> Bag DelayedError
    wc_errors = Bag DelayedError
    errs })
          = do impls' <- (Implication -> StateT Bool TcS (Maybe Implication))
    -> Bag Implication -> StateT Bool TcS (Bag Implication)
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m (Maybe b)) -> Bag a -> m (Bag b)
    mapMaybeBagM Implication -> StateT Bool TcS (Maybe Implication)
    go_impl Bag Implication
    impls
               return $ WC { wc_simple = wtds, wc_impl = impls', wc_errors = errs }
        go_impl :: Implication -> StateT Bool TcS (Maybe Implication)
    go_impl Implication
    impl
          | ImplicStatus -> Bool
    isSolvedStatus (Implication -> ImplicStatus
    ic_status Implication
    impl)
          = Maybe Implication -> StateT Bool TcS (Maybe Implication)
    forall a. a -> StateT Bool TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Maybe Implication -> StateT Bool TcS (Maybe Implication))
    -> Maybe Implication -> StateT Bool TcS (Maybe Implication)
    forall a b. (a -> b) -> a -> b
    $ Implication -> Maybe Implication
    forall a. a -> Maybe a
    Just Implication
    impl
          -- Is there a Given with type "Unsatisfiable msg"?
          -- If so, use it to solve all other Wanteds.
          | (TyVar, Type)
    unsat_given:[(TyVar, Type)]
    _ <- (TyVar -> Maybe (TyVar, Type)) -> [TyVar] -> [(TyVar, Type)]
    forall a b. (a -> Maybe b) -> [a] -> [b]
    mapMaybe TyVar -> Maybe (TyVar, Type)
    unsatisfiableEv_maybe (Implication -> [TyVar]
    ic_given Implication
    impl)
          = do { Bool -> StateT Bool TcS ()
    forall (m :: * -> *) s. Monad m => s -> StateT s m ()
    put Bool
    True
               ; TcS (Maybe Implication) -> StateT Bool TcS (Maybe Implication)
    forall (m :: * -> *) a. Monad m => m a -> StateT Bool m a
    forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
    (MonadTrans t, Monad m) =>
    m a -> t m a
    lift (TcS (Maybe Implication) -> StateT Bool TcS (Maybe Implication))
    -> TcS (Maybe Implication) -> StateT Bool TcS (Maybe Implication)
    forall a b. (a -> b) -> a -> b
    $ (TyVar, Type) -> Implication -> TcS (Maybe Implication)
    solveImplicationUsingUnsatGiven (TyVar, Type)
    unsat_given Implication
    impl }
          -- Otherwise, recurse.
          | Bool
    otherwise
          = do { wcs' <- WantedConstraints -> StateT Bool TcS WantedConstraints
    go_wc (Implication -> WantedConstraints
    ic_wanted Implication
    impl)
               ; lift $ setImplicationStatus $ impl { ic_wanted = wcs' } }

    -- | Is this evidence variable the evidence for an 'Unsatisfiable' constraint?
    --
    -- If so, return the variable itself together with the error message type.
    unsatisfiableEv_maybe :: EvVar -> Maybe (EvVar, Type)
    unsatisfiableEv_maybe :: TyVar -> Maybe (TyVar, Type)
    unsatisfiableEv_maybe TyVar
    v = (TyVar
    v,) (Type -> (TyVar, Type)) -> Maybe Type -> Maybe (TyVar, Type)
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Type -> Maybe Type
    isUnsatisfiableCt_maybe (TyVar -> Type
    idType TyVar
    v)

    -- | We have an implication with an 'Unsatisfiable' Given; use that Given to
    -- solve all the other Wanted constraints, including those nested within
    -- deeper implications.
    solveImplicationUsingUnsatGiven :: (EvVar, Type) -> Implication -> TcS (Maybe Implication)
    solveImplicationUsingUnsatGiven :: (TyVar, Type) -> Implication -> TcS (Maybe Implication)
    solveImplicationUsingUnsatGiven
      unsat_given :: (TyVar, Type)
    unsat_given@(TyVar
    given_ev,Type
    _)
      impl :: Implication
    impl@(Implic { ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WantedConstraints
    wtd, ic_tclvl :: Implication -> TcLevel
    ic_tclvl = TcLevel
    tclvl, ic_binds :: Implication -> EvBindsVar
    ic_binds = EvBindsVar
    ev_binds_var, ic_need_inner :: Implication -> VarSet
    ic_need_inner = VarSet
    inner })
      | EvBindsVar -> Bool
    isCoEvBindsVar EvBindsVar
    ev_binds_var
      -- We can't use Unsatisfiable evidence in kinds.
      -- See Note [Coercion evidence only] in GHC.Tc.Types.Evidence.
      = Maybe Implication -> TcS (Maybe Implication)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Maybe Implication -> TcS (Maybe Implication))
    -> Maybe Implication -> TcS (Maybe Implication)
    forall a b. (a -> b) -> a -> b
    $ Implication -> Maybe Implication
    forall a. a -> Maybe a
    Just Implication
    impl
      | Bool
    otherwise
      = do { wcs <- EvBindsVar
    -> TcLevel -> TcS WantedConstraints -> TcS WantedConstraints
    forall a. EvBindsVar -> TcLevel -> TcS a -> TcS a
    nestImplicTcS EvBindsVar
    ev_binds_var TcLevel
    tclvl (TcS WantedConstraints -> TcS WantedConstraints)
    -> TcS WantedConstraints -> TcS WantedConstraints
    forall a b. (a -> b) -> a -> b
    $ WantedConstraints -> TcS WantedConstraints
    go_wc WantedConstraints
    wtd
           ; setImplicationStatus $
             impl { ic_wanted = wcs
                  , ic_need_inner = inner `extendVarSet` given_ev } }
      where
        go_wc :: WantedConstraints -> TcS WantedConstraints
        go_wc :: WantedConstraints -> TcS WantedConstraints
    go_wc wc :: WantedConstraints
    wc@(WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    wtds, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    impls })
          = do { (Ct -> TcS ()) -> Cts -> TcS ()
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> Bag a -> m ()
    mapBagM_ Ct -> TcS ()
    go_simple Cts
    wtds
               ; impls <- (Implication -> TcS (Maybe Implication))
    -> Bag Implication -> TcS (Bag Implication)
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m (Maybe b)) -> Bag a -> m (Bag b)
    mapMaybeBagM ((TyVar, Type) -> Implication -> TcS (Maybe Implication)
    solveImplicationUsingUnsatGiven (TyVar, Type)
    unsat_given) Bag Implication
    impls
               ; return $ wc { wc_simple = emptyBag, wc_impl = impls } }
        go_simple :: Ct -> TcS ()
        go_simple :: Ct -> TcS ()
    go_simple Ct
    ct = case Ct -> CtEvidence
    ctEvidence Ct
    ct of
          CtWanted { ctev_pred :: CtEvidence -> Type
    ctev_pred = Type
    pty, ctev_dest :: CtEvidence -> TcEvDest
    ctev_dest = TcEvDest
    dst }
            -> do { ev_expr <- (TyVar, Type) -> Type -> TcS EvExpr
    unsatisfiableEvExpr (TyVar, Type)
    unsat_given Type
    pty
                  ; setWantedEvTerm dst True $ EvExpr ev_expr }
          CtEvidence
    _ -> () -> TcS ()
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    -- | Create an evidence expression for an arbitrary constraint using
    -- evidence for an "Unsatisfiable" Given.
    --
    -- See Note [Evidence terms from Unsatisfiable Givens]
    unsatisfiableEvExpr :: (EvVar, ErrorMsgType) -> PredType -> TcS EvExpr
    unsatisfiableEvExpr :: (TyVar, Type) -> Type -> TcS EvExpr
    unsatisfiableEvExpr (TyVar
    unsat_ev, Type
    given_msg) Type
    wtd_ty
      = do { mod <- TcS Module
    forall (m :: * -> *). HasModule m => m Module
    getModule
             -- If we're typechecking GHC.TypeError, return a bogus expression;
             -- it's only used for the ambiguity check, which throws the evidence away anyway.
             -- This avoids problems with circularity; where we are trying to look
             -- up the "unsatisfiable" Id while we are in the middle of typechecking it.
           ; if mod == gHC_INTERNAL_TYPEERROR then return (Var unsat_ev) else
        do { unsatisfiable_id <- tcLookupId unsatisfiableIdName

             -- See Note [Evidence terms from Unsatisfiable Givens]
             -- for a description of what evidence term we are constructing here.

           ; let -- (##) -=> wtd_ty
                 fun_ty = HasDebugCallStack => FunTyFlag -> Type -> Type -> Type -> Type
    FunTyFlag -> Type -> Type -> Type -> Type
    mkFunTy FunTyFlag
    visArgConstraintLike Type
    ManyTy Type
    unboxedUnitTy Type
    wtd_ty
                 mkDictBox = case Type -> BoxingInfo Any
    forall b. Type -> BoxingInfo b
    boxingDataCon Type
    fun_ty of
                   BI_Box { bi_data_con :: forall b. BoxingInfo b -> DataCon
    bi_data_con = DataCon
    mkDictBox } -> DataCon
    mkDictBox
                   BoxingInfo Any
    _ -> String -> SDoc -> DataCon
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "unsatisfiableEvExpr: no DictBox!" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    wtd_ty)
                 dictBox = DataCon -> TyCon
    dataConTyCon DataCon
    mkDictBox
           ; ev_bndr <- mkSysLocalM (fsLit "ct") ManyTy fun_ty
                 -- Dict ((##) -=> wtd_ty)
           ; let scrut_ty = TyCon -> [Type] -> Type
    mkTyConApp TyCon
    dictBox [Type
    fun_ty]
                 -- unsatisfiable @{LiftedRep} @given_msg @(Dict ((##) -=> wtd_ty)) unsat_ev
                 scrut =
                   EvExpr -> [EvExpr] -> EvExpr
    mkCoreApps (TyVar -> EvExpr
    forall b. TyVar -> Expr b
    Var TyVar
    unsatisfiable_id)
                     [ Type -> EvExpr
    forall b. Type -> Expr b
    Type Type
    liftedRepTy
                     , Type -> EvExpr
    forall b. Type -> Expr b
    Type Type
    given_msg
                     , Type -> EvExpr
    forall b. Type -> Expr b
    Type Type
    scrut_ty
                     , TyVar -> EvExpr
    forall b. TyVar -> Expr b
    Var TyVar
    unsat_ev ]
                 -- case scrut of { MkDictBox @((##) -=> wtd_ty)) ct -> ct (# #) }
                 ev_expr =
                   EvExpr -> Scaled Type -> Type -> [CoreAlt] -> EvExpr
    mkWildCase EvExpr
    scrut (Type -> Scaled Type
    forall a. a -> Scaled a
    unrestricted (Type -> Scaled Type) -> Type -> Scaled Type
    forall a b. (a -> b) -> a -> b
    $ Type
    scrut_ty) Type
    wtd_ty
                   [ AltCon -> [TyVar] -> EvExpr -> CoreAlt
    forall b. AltCon -> [b] -> Expr b -> Alt b
    Alt (DataCon -> AltCon
    DataAlt DataCon
    mkDictBox) [TyVar
    ev_bndr] (EvExpr -> CoreAlt) -> EvExpr -> CoreAlt
    forall a b. (a -> b) -> a -> b
    $
                       EvExpr -> [EvExpr] -> EvExpr
    mkCoreApps (TyVar -> EvExpr
    forall b. TyVar -> Expr b
    Var TyVar
    ev_bndr) [EvExpr
    unboxedUnitExpr]
                   ]
            ; return ev_expr } }

    {- Note [Evidence terms from Unsatisfiable Givens]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    An Unsatisfiable Given constraint, of the form [G] Unsatisfiable msg, should be
    able to solve ANY Wanted constraint whatsoever.

    Recall that we have

      unsatisfiable :: forall {rep} (msg :: ErrorMessage) (a :: TYPE rep)
                    .  Unsatisfiable msg => a

    We want to use this function, together with the evidence
    [G] unsat_ev :: Unsatisfiable msg, to solve any other constraint [W] wtd_ty.

    We could naively think that a valid evidence term for the Wanted might be:

      wanted_ev = unsatisfiable @{rep} @msg @wtd_ty unsat_ev

    Unfortunately, this is a kind error: "wtd_ty :: CONSTRAINT rep", but
    "unsatisfiable" expects the third type argument to be of kind "TYPE rep".

    Instead, we use a boxing data constructor to box the constraint into a type.
    In the end, we construct the following evidence for the implication:

      [G] unsat_ev :: Unsatisfiable msg
        ==>
          [W] wtd_ev :: wtd_ty

      wtd_ev =
        case unsatisfiable @{LiftedRep} @msg @(Dict ((##) -=> wtd_ty)) unsat_ev of
          MkDictBox ct -> ct (# #)

    Note that we play the same trick with the function arrow -=> that we did
    in order to define "unsatisfiable" in terms of "unsatisfiableLifted", as described
    in Note [The Unsatisfiable representation-polymorphism trick] in base:GHC.TypeError.
    This allows us to indirectly box constraints with different representations
    (such as primitive equality constraints).
    -}

    -- | A 'TcS' action which can may default a 'Ct'.
    type CtDefaultingStrategy = Ct -> MaybeT TcS ()

    -- | Default @ExceptionContext@ constraints to @emptyExceptionContext@.
    defaultExceptionContext :: CtDefaultingStrategy
    defaultExceptionContext :: CtDefaultingStrategy
    defaultExceptionContext Ct
    ct
      = do { ClassPred cls tys <- Pred -> MaybeT TcS Pred
    forall a. a -> MaybeT TcS a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure (Pred -> MaybeT TcS Pred) -> Pred -> MaybeT TcS Pred
    forall a b. (a -> b) -> a -> b
    $ Type -> Pred
    classifyPredType (Ct -> Type
    ctPred Ct
    ct)
           ; Just {} <- pure $ isExceptionContextPred cls tys
           ; emptyEC <- Var <$> lift (lookupId emptyExceptionContextName)
           ; let ev = Ct -> CtEvidence
    ctEvidence Ct
    ct
           ; let ev_tm = EvExpr -> TcCoercion -> EvTerm
    mkEvCast EvExpr
    emptyEC (Type -> TcCoercion
    wrapIP (CtEvidence -> Type
    ctEvPred CtEvidence
    ev))
           ; lift $ warnTcS $ TcRnDefaultedExceptionContext (ctLoc ct)
           ; lift $ setEvBindIfWanted ev False ev_tm
           }

    -- | Default any remaining @CallStack@ constraints to empty @CallStack@s.
    -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
    defaultCallStack :: CtDefaultingStrategy
    defaultCallStack :: CtDefaultingStrategy
    defaultCallStack Ct
    ct
      = do { ClassPred cls tys <- Pred -> MaybeT TcS Pred
    forall a. a -> MaybeT TcS a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure (Pred -> MaybeT TcS Pred) -> Pred -> MaybeT TcS Pred
    forall a b. (a -> b) -> a -> b
    $ Type -> Pred
    classifyPredType (Ct -> Type
    ctPred Ct
    ct)
           ; Just {} <- pure $ isCallStackPred cls tys
           ; lift $ solveCallStack (ctEvidence ct) EvCsEmpty
           }

    defaultConstraints :: [CtDefaultingStrategy]
                       -> WantedConstraints
                       -> TcS WantedConstraints
    -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
    defaultConstraints :: [CtDefaultingStrategy]
    -> WantedConstraints -> TcS WantedConstraints
    defaultConstraints [CtDefaultingStrategy]
    defaulting_strategies WantedConstraints
    wanteds
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanteds = WantedConstraints -> TcS WantedConstraints
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return WantedConstraints
    wanteds
      | Bool
    otherwise
      = do simples <- Cts -> TcS Cts
    handle_simples (WantedConstraints -> Cts
    wc_simple WantedConstraints
    wanteds)
           mb_implics <- mapBagM handle_implic (wc_impl wanteds)
           return (wanteds { wc_simple = simples
                           , wc_impl = catBagMaybes mb_implics })

      where
      handle_simples :: Bag Ct -> TcS (Bag Ct)
      handle_simples :: Cts -> TcS Cts
    handle_simples Cts
    simples
        = Bag (Maybe Ct) -> Cts
    forall a. Bag (Maybe a) -> Bag a
    catBagMaybes (Bag (Maybe Ct) -> Cts) -> TcS (Bag (Maybe Ct)) -> TcS Cts
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> (Ct -> TcS (Maybe Ct)) -> Cts -> TcS (Bag (Maybe Ct))
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> Bag a -> m (Bag b)
    mapBagM Ct -> TcS (Maybe Ct)
    handle_simple Cts
    simples
        where
          handle_simple :: Ct -> TcS (Maybe Ct)
          handle_simple :: Ct -> TcS (Maybe Ct)
    handle_simple Ct
    ct = [CtDefaultingStrategy] -> TcS (Maybe Ct)
    go [CtDefaultingStrategy]
    defaulting_strategies
            where
              go :: [CtDefaultingStrategy] -> TcS (Maybe Ct)
    go [] = Maybe Ct -> TcS (Maybe Ct)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Ct -> Maybe Ct
    forall a. a -> Maybe a
    Just Ct
    ct)
              go (CtDefaultingStrategy
    f:[CtDefaultingStrategy]
    fs) = do
                  mb <- MaybeT TcS () -> TcS (Maybe ())
    forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
    runMaybeT (CtDefaultingStrategy
    f Ct
    ct)
                  case mb of
                    Just () -> Maybe Ct -> TcS (Maybe Ct)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe Ct
    forall a. Maybe a
    Nothing
                    Maybe ()
    Nothing -> [CtDefaultingStrategy] -> TcS (Maybe Ct)
    go [CtDefaultingStrategy]
    fs

      handle_implic :: Implication -> TcS (Maybe Implication)
      -- The Maybe is because solving the CallStack constraint
      -- may well allow us to discard the implication entirely
      handle_implic :: Implication -> TcS (Maybe Implication)
    handle_implic Implication
    implic
        | ImplicStatus -> Bool
    isSolvedStatus (Implication -> ImplicStatus
    ic_status Implication
    implic)
        = Maybe Implication -> TcS (Maybe Implication)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Implication -> Maybe Implication
    forall a. a -> Maybe a
    Just Implication
    implic)
        | Bool
    otherwise
        = do { wanteds <- EvBindsVar -> TcS WantedConstraints -> TcS WantedConstraints
    forall a. EvBindsVar -> TcS a -> TcS a
    setEvBindsTcS (Implication -> EvBindsVar
    ic_binds Implication
    implic) (TcS WantedConstraints -> TcS WantedConstraints)
    -> TcS WantedConstraints -> TcS WantedConstraints
    forall a b. (a -> b) -> a -> b
    $
                          -- defaultCallStack sets a binding, so
                          -- we must set the correct binding group
                          [CtDefaultingStrategy]
    -> WantedConstraints -> TcS WantedConstraints
    defaultConstraints [CtDefaultingStrategy]
    defaulting_strategies (Implication -> WantedConstraints
    ic_wanted Implication
    implic)
             ; setImplicationStatus (implic { ic_wanted = wanteds }) }

    {- Note [When to do type-class defaulting]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In GHC 7.6 and 7.8.2, we did type-class defaulting only if insolubleWC
    was false, on the grounds that defaulting can't help solve insoluble
    constraints.  But if we *don't* do defaulting we may report a whole
    lot of errors that would be solved by defaulting; these errors are
    quite spurious because fixing the single insoluble error means that
    defaulting happens again, which makes all the other errors go away.
    This is jolly confusing: #9033.

    So it seems better to always do type-class defaulting.

    However, always doing defaulting does mean that we'll do it in
    situations like this (#5934):
       run :: (forall s. GenST s) -> Int
       run = fromInteger 0
    We don't unify the return type of fromInteger with the given function
    type, because the latter involves foralls.  So we're left with
        (Num alpha, alpha ~ (forall s. GenST s) -> Int)
    Now we do defaulting, get alpha := Integer, and report that we can't
    match Integer with (forall s. GenST s) -> Int.  That's not totally
    stupid, but perhaps a little strange.

    Another potential alternative would be to suppress *all* non-insoluble
    errors if there are *any* insoluble errors, anywhere, but that seems
    too drastic.

    Note [Don't default in syntactic equalities]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When there are unsolved syntactic equalities such as

      rr[sk] ~S# alpha[conc]

    we should not default alpha, lest we obtain a poor error message such as

      Couldn't match kind `rr' with `LiftedRep'

    We would rather preserve the original syntactic equality to be
    reported to the user, especially as the concrete metavariable alpha
    might store an informative origin for the user.

    Note [Must simplify after defaulting]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We may have a deeply buried constraint
        (t:*) ~ (a:Open)
    which we couldn't solve because of the kind incompatibility, and 'a' is free.
    Then when we default 'a' we can solve the constraint.  And we want to do
    that before starting in on type classes.  We MUST do it before reporting
    errors, because it isn't an error!  #7967 was due to this.

    Note [Top-level Defaulting Plan]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We have considered two design choices for where/when to apply defaulting.
       (i) Do it in SimplCheck mode only /whenever/ you try to solve some
           simple constraints, maybe deep inside the context of implications.
           This used to be the case in GHC 7.4.1.
       (ii) Do it in a tight loop at simplifyTop, once all other constraints have
            finished. This is the current story.

    Option (i) had many disadvantages:
       a) Firstly, it was deep inside the actual solver.
       b) Secondly, it was dependent on the context (Infer a type signature,
          or Check a type signature, or Interactive) since we did not want
          to always start defaulting when inferring (though there is an exception to
          this, see Note [Default while Inferring]).
       c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs:
              f :: Int -> Bool
              f x = const True (\y -> let w :: a -> a
                                          w a = const a (y+1)
                                      in w y)
          We will get an implication constraint (for beta the type of y):
                   [untch=beta] forall a. 0 => Num beta
          which we really cannot default /while solving/ the implication, since beta is
          untouchable.

    Instead our new defaulting story is to pull defaulting out of the solver loop and
    go with option (ii), implemented at SimplifyTop. Namely:
         - First, have a go at solving the residual constraint of the whole
           program
         - Try to approximate it with a simple constraint
         - Figure out derived defaulting equations for that simple constraint
         - Go round the loop again if you did manage to get some equations

    Now, that has to do with class defaulting. However there exists type variable /kind/
    defaulting. Again this is done at the top-level and the plan is:
         - At the top-level, once you had a go at solving the constraint, do
           figure out /all/ the touchable unification variables of the wanted constraints.
         - Apply defaulting to their kinds

    More details in Note [DefaultTyVar].

    Note [Safe Haskell Overlapping Instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In Safe Haskell, we apply an extra restriction to overlapping instances. The
    motive is to prevent untrusted code provided by a third-party, changing the
    behavior of trusted code through type-classes. This is due to the global and
    implicit nature of type-classes that can hide the source of the dictionary.

    Another way to state this is: if a module M compiles without importing another
    module N, changing M to import N shouldn't change the behavior of M.

    Overlapping instances with type-classes can violate this principle. However,
    overlapping instances aren't always unsafe. They are just unsafe when the most
    selected dictionary comes from untrusted code (code compiled with -XSafe) and
    overlaps instances provided by other modules.

    In particular, in Safe Haskell at a call site with overlapping instances, we
    apply the following rule to determine if it is a 'unsafe' overlap:

     1) Most specific instance, I1, defined in an `-XSafe` compiled module.
     2) I1 is an orphan instance or a MPTC.
     3) At least one overlapped instance, Ix, is both:
        A) from a different module than I1
        B) Ix is not marked `OVERLAPPABLE`

    This is a slightly involved heuristic, but captures the situation of an
    imported module N changing the behavior of existing code. For example, if
    condition (2) isn't violated, then the module author M must depend either on a
    type-class or type defined in N.

    Secondly, when should these heuristics be enforced? We enforced them when the
    type-class method call site is in a module marked `-XSafe` or `-XTrustworthy`.
    This allows `-XUnsafe` modules to operate without restriction, and for Safe
    Haskell inference to infer modules with unsafe overlaps as unsafe.

    One alternative design would be to also consider if an instance was imported as
    a `safe` import or not and only apply the restriction to instances imported
    safely. However, since instances are global and can be imported through more
    than one path, this alternative doesn't work.

    Note [Safe Haskell Overlapping Instances Implementation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    How is this implemented? It's complicated! So we'll step through it all:

     1) `InstEnv.lookupInstEnv` -- Performs instance resolution, so this is where
        we check if a particular type-class method call is safe or unsafe. We do this
        through the return type, `ClsInstLookupResult`, where the last parameter is a
        list of instances that are unsafe to overlap. When the method call is safe,
        the list is null.

     2) `GHC.Tc.Solver.Dict.matchClassInst` -- This module drives the instance resolution
        / dictionary generation. The return type is `ClsInstResult`, which either
        says no instance matched, or one found, and if it was a safe or unsafe
        overlap.

     3) `GHC.Tc.Solver.Dict.tryInstances` -- Takes a dictionary / class constraint and
         tries to resolve it by calling (in part) `matchClassInst`. The resolving
         mechanism has a work list (of constraints) that it process one at a time. If
         the constraint can't be resolved, it's added to an inert set. When compiling
         an `-XSafe` or `-XTrustworthy` module, we follow this approach as we know
         compilation should fail. These are handled as normal constraint resolution
         failures from here-on (see step 6).

         Otherwise, we may be inferring safety (or using `-Wunsafe`), and
         compilation should succeed, but print warnings and/or mark the compiled module
         as `-XUnsafe`. In this case, we call `insertSafeOverlapFailureTcS` which adds
         the unsafe (but resolved!) constraint to the `inert_safehask` field of
         `InertCans`.

     4) `GHC.Tc.Solver.simplifyTop`:
           * Call simplifyTopWanteds, the top-level function for driving the simplifier for
             constraint resolution.

           * Once finished, call `getSafeOverlapFailures` to retrieve the
             list of overlapping instances that were successfully resolved,
             but unsafe. Remember, this is only applicable for generating warnings
             (`-Wunsafe`) or inferring a module unsafe. `-XSafe` and `-XTrustworthy`
             cause compilation failure by not resolving the unsafe constraint at all.

           * For unresolved constraints (all types), call `GHC.Tc.Errors.reportUnsolved`,
             while for resolved but unsafe overlapping dictionary constraints, call
             `GHC.Tc.Errors.warnAllUnsolved`. Both functions convert constraints into a
             warning message for the user.

           * In the case of `warnAllUnsolved` for resolved, but unsafe
             dictionary constraints, we collect the generated warning
             message (pop it) and call `GHC.Tc.Utils.Monad.recordUnsafeInfer` to
             mark the module we are compiling as unsafe, passing the
             warning message along as the reason.

     5) `GHC.Tc.Errors.*Unsolved` -- Generates error messages for constraints by
        actually calling `InstEnv.lookupInstEnv` again! Yes, confusing, but all we
        know is the constraint that is unresolved or unsafe. For dictionary, all we
        know is that we need a dictionary of type C, but not what instances are
        available and how they overlap. So we once again call `lookupInstEnv` to
        figure that out so we can generate a helpful error message.

     6) `GHC.Tc.Utils.Monad.recordUnsafeInfer` -- Save the unsafe result and reason in
          IORefs called `tcg_safe_infer` and `tcg_safe_infer_reason`.

     7) `GHC.Driver.Main.tcRnModule'` -- Reads `tcg_safe_infer` after type-checking, calling
        `GHC.Driver.Main.markUnsafeInfer` (passing the reason along) when safe-inference
        failed.

    Note [No defaulting in the ambiguity check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When simplifying constraints for the ambiguity check, we use
    solveWanteds, not simplifyTopWanteds, so that we do no defaulting.
    #11947 was an example:
       f :: Num a => Int -> Int
    This is ambiguous of course, but we don't want to default the
    (Num alpha) constraint to (Num Int)!  Doing so gives a defaulting
    warning, but no error.

    Note [Defaulting insolubles]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If a set of wanteds is insoluble, we have no hope of accepting the
    program. Yet we do not stop constraint solving, etc., because we may
    simplify the wanteds to produce better error messages. So, once
    we have an insoluble constraint, everything we do is just about producing
    helpful error messages.

    Should we default in this case or not? Let's look at an example (tcfail004):

      (f,g) = (1,2,3)

    With defaulting, we get a conflict between (a0,b0) and (Integer,Integer,Integer).
    Without defaulting, we get a conflict between (a0,b0) and (a1,b1,c1). I (Richard)
    find the latter more helpful. Several other test cases (e.g. tcfail005) suggest
    similarly. So: we should not do class defaulting with insolubles.

    On the other hand, RuntimeRep-defaulting is different. Witness tcfail078:

      f :: Integer i => i
      f =               0

    Without RuntimeRep-defaulting, we GHC suggests that Integer should have kind
    TYPE r0 -> Constraint and then complains that r0 is actually untouchable
    (presumably, because it can't be sure if `Integer i` entails an equality).
    If we default, we are told of a clash between (* -> Constraint) and Constraint.
    The latter seems far better, suggesting we *should* do RuntimeRep-defaulting
    even on insolubles.

    But, evidently, not always. Witness UnliftedNewtypesInfinite:

      newtype Foo = FooC (# Int#, Foo #)

    This should fail with an occurs-check error on the kind of Foo (with -XUnliftedNewtypes).
    If we default RuntimeRep-vars, we get

      Expecting a lifted type, but ‘(# Int#, Foo #)’ is unlifted

    which is just plain wrong.

    Another situation in which we don't want to default involves concrete metavariables.

    In equalities such as   alpha[conc] ~# rr[sk]  ,  alpha[conc] ~# RR beta[tau]
    for a type family RR (all at kind RuntimeRep), we would prefer to report a
    representation-polymorphism error rather than default alpha and get error:

      Could not unify `rr` with `Lifted` / Could not unify `RR b0` with `Lifted`

    which is very confusing. For this reason, we weed out the concrete
    metavariables participating in such equalities in nonDefaultableTyVarsOfWC.
    Just looking at insolublity is not enough, as `alpha[conc] ~# RR beta[tau]` could
    become soluble after defaulting beta (see also #21430).

    Conclusion: we should do RuntimeRep-defaulting on insolubles only when the
    user does not want to hear about RuntimeRep stuff -- that is, when
    -fprint-explicit-runtime-reps is not set.
    However, we must still take care not to default concrete type variables
    participating in an equality with a non-concrete type, as seen in the
    last example above.
    -}

    ------------------
    simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM ()
    simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM ()
    simplifyAmbiguityCheck Type
    ty WantedConstraints
    wanteds
      = do { String -> SDoc -> TcM ()
    traceTc String
    "simplifyAmbiguityCheck {" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "type = " SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "wanted = " SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> WantedConstraints -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wanteds

           ; (final_wc, _) <- TcS WantedConstraints -> TcM (WantedConstraints, EvBindMap)
    forall a. TcS a -> TcM (a, EvBindMap)
    runTcS (TcS WantedConstraints -> TcM (WantedConstraints, EvBindMap))
    -> TcS WantedConstraints -> TcM (WantedConstraints, EvBindMap)
    forall a b. (a -> b) -> a -> b
    $ WantedConstraints -> TcS WantedConstraints
    useUnsatisfiableGivens (WantedConstraints -> TcS WantedConstraints)
    -> TcS WantedConstraints -> TcS WantedConstraints
    forall (m :: * -> *) a b. Monad m => (a -> m b) -> m a -> m b
    =<< WantedConstraints -> TcS WantedConstraints
    solveWanteds WantedConstraints
    wanteds
                 -- NB: no defaulting!  See Note [No defaulting in the ambiguity check]
                 -- Note: we do still use Unsatisfiable Givens to solve Wanteds,
                 -- see Wrinkle [Ambiguity] under point (C) of
                 -- Note [Implementation of Unsatisfiable constraints] in GHC.Tc.Errors.

           ; discardResult (reportUnsolved final_wc)

           ; traceTc "End simplifyAmbiguityCheck }" empty }

    ------------------
    simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
    simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
    simplifyInteractive WantedConstraints
    wanteds
      = String -> SDoc -> TcM ()
    traceTc String
    "simplifyInteractive" SDoc
    forall doc. IsOutput doc => doc
    empty TcM () -> TcM (Bag EvBind) -> TcM (Bag EvBind)
    forall a b.
    IOEnv (Env TcGblEnv TcLclEnv) a
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
    >>
        WantedConstraints -> TcM (Bag EvBind)
    simplifyTop WantedConstraints
    wanteds

    ------------------
    simplifyDefault :: ThetaType    -- Wanted; has no type variables in it
                    -> TcM Bool     -- Return if the constraint is soluble
    simplifyDefault :: [Type] -> TcM Bool
    simplifyDefault [Type]
    theta
      = do { String -> SDoc -> TcM ()
    traceTc String
    "simplifyDefault" SDoc
    forall doc. IsOutput doc => doc
    empty
           ; wanteds  <- CtOrigin -> [Type] -> TcM [CtEvidence]
    newWanteds CtOrigin
    DefaultOrigin [Type]
    theta
           ; (unsolved, _) <- runTcS (solveWanteds (mkSimpleWC wanteds))
           ; return (isEmptyWC unsolved) }

    ------------------
    {- Note [Pattern match warnings with insoluble Givens]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A pattern match on a GADT can introduce new type-level information, which needs
    to be analysed in order to get the expected pattern match warnings.

    For example:

    > type IsBool :: Type -> Constraint
    > type family IsBool a where
    >   IsBool Bool = ()
    >   IsBool b    = b ~ Bool
    >
    > data T a where
    >   MkTInt  :: Int -> T Int
    >   MkTBool :: IsBool b => b -> T b
    >
    > f :: T Int -> Int
    > f (MkTInt i) = i

    The pattern matching performed by `f` is complete: we can't ever call
    `f (MkTBool b)`, as type-checking that application would require producing
    evidence for `Int ~ Bool`, which can't be done.

    The pattern match checker uses `tcCheckGivens` to accumulate all the Given
    constraints, and relies on `tcCheckGivens` to return Nothing if the
    Givens become insoluble.   `tcCheckGivens` in turn relies on `insolubleCt`
    to identify these insoluble constraints.  So the precise definition of
    `insolubleCt` has a big effect on pattern match overlap warnings.

    To detect this situation, we check whether there are any insoluble Given
    constraints. In the example above, the insoluble constraint was an
    equality constraint, but it is also important to detect custom type errors:

    > type NotInt :: Type -> Constraint
    > type family NotInt a where
    >   NotInt Int = TypeError (Text "That's Int, silly.")
    >   NotInt _   = ()
    >
    > data R a where
    >   MkT1 :: a -> R a
    >   MkT2 :: NotInt a => R a
    >
    > foo :: R Int -> Int
    > foo (MkT1 x) = x

    To see that we can't call `foo (MkT2)`, we must detect that `NotInt Int` is insoluble
    because it is a custom type error.
    Failing to do so proved quite inconvenient for users, as evidence by the
    tickets #11503 #14141 #16377 #20180.
    Test cases: T11503, T14141.

    Examples of constraints that tcCheckGivens considers insoluble:
      - Int ~ Bool,
      - Coercible Float Word,
      - TypeError msg.

    Non-examples:
      - constraints which we know aren't satisfied,
        e.g. Show (Int -> Int) when no such instance is in scope,
      - Eq (TypeError msg),
      - C (Int ~ Bool), with @class C (c :: Constraint)@.
    -}

    tcCheckGivens :: InertSet -> Bag EvVar -> TcM (Maybe InertSet)
    -- ^ Return (Just new_inerts) if the Givens are satisfiable, Nothing if definitely
    -- contradictory.
    --
    -- See Note [Pattern match warnings with insoluble Givens] above.
    tcCheckGivens :: InertSet -> Bag TyVar -> TcM (Maybe InertSet)
    tcCheckGivens InertSet
    inerts Bag TyVar
    given_ids = do
      (sat, new_inerts) <- InertSet -> TcS Bool -> TcM (Bool, InertSet)
    forall a. InertSet -> TcS a -> TcM (a, InertSet)
    runTcSInerts InertSet
    inerts (TcS Bool -> TcM (Bool, InertSet))
    -> TcS Bool -> TcM (Bool, InertSet)
    forall a b. (a -> b) -> a -> b
    $ do
        String -> SDoc -> TcS ()
    traceTcS String
    "checkGivens {" (InertSet -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr InertSet
    inerts SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Bag TyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Bag TyVar
    given_ids)
        lcl_env <- TcS TcLclEnv
    TcS.getLclEnv
        let given_loc = TcLevel -> SkolemInfoAnon -> CtLocEnv -> CtLoc
    mkGivenLoc TcLevel
    topTcLevel (SkolemInfo -> SkolemInfoAnon
    getSkolemInfo SkolemInfo
    HasCallStack => SkolemInfo
    unkSkol) (TcLclEnv -> CtLocEnv
    mkCtLocEnv TcLclEnv
    lcl_env)
        let given_cts = CtLoc -> [TyVar] -> [Ct]
    mkGivens CtLoc
    given_loc (Bag TyVar -> [TyVar]
    forall a. Bag a -> [a]
    bagToList Bag TyVar
    given_ids)
        -- See Note [Superclasses and satisfiability]
        solveSimpleGivens given_cts
        insols <- getInertInsols
        insols <- try_harder insols
        traceTcS "checkGivens }" (ppr insols)
        return (isEmptyBag insols)
      return $ if sat then Just new_inerts else Nothing
      where
        try_harder :: Cts -> TcS Cts
        -- Maybe we have to search up the superclass chain to find
        -- an unsatisfiable constraint.  Example: pmcheck/T3927b.
        -- At the moment we try just once
        try_harder :: Cts -> TcS Cts
    try_harder Cts
    insols
          | Bool -> Bool
    not (Cts -> Bool
    forall a. Bag a -> Bool
    isEmptyBag Cts
    insols)   -- We've found that it's definitely unsatisfiable
          = Cts -> TcS Cts
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Cts
    insols             -- Hurrah -- stop now.
          | Bool
    otherwise
          = do { pending_given <- TcS [Ct]
    getPendingGivenScs
               ; new_given <- makeSuperClasses pending_given
               ; solveSimpleGivens new_given
               ; getInertInsols }

    tcCheckWanteds :: InertSet -> ThetaType -> TcM Bool
    -- ^ Return True if the Wanteds are soluble, False if not
    tcCheckWanteds :: InertSet -> [Type] -> TcM Bool
    tcCheckWanteds InertSet
    inerts [Type]
    wanteds = do
      cts <- CtOrigin -> [Type] -> TcM [CtEvidence]
    newWanteds CtOrigin
    PatCheckOrigin [Type]
    wanteds
      (sat, _new_inerts) <- runTcSInerts inerts $ do
        traceTcS "checkWanteds {" (ppr inerts <+> ppr wanteds)
        -- See Note [Superclasses and satisfiability]
        wcs <- solveWanteds (mkSimpleWC cts)
        traceTcS "checkWanteds }" (ppr wcs)
        return (isSolvedWC wcs)
      return sat

    -- | Normalise a type as much as possible using the given constraints.
    -- See @Note [tcNormalise]@.
    tcNormalise :: InertSet -> Type -> TcM Type
    tcNormalise :: InertSet -> Type -> TcM Type
    tcNormalise InertSet
    inerts Type
    ty
      = do { norm_loc <- CtOrigin -> Maybe TypeOrKind -> TcM CtLoc
    getCtLocM CtOrigin
    PatCheckOrigin Maybe TypeOrKind
    forall a. Maybe a
    Nothing
           ; (res, _new_inerts) <- runTcSInerts inerts $
                 do { traceTcS "tcNormalise {" (ppr inerts)
                    ; ty' <- rewriteType norm_loc ty
                    ; traceTcS "tcNormalise }" (ppr ty')
                    ; pure ty' }
           ; return res }

    {- Note [Superclasses and satisfiability]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Expand superclasses before starting, because (Int ~ Bool), has
    (Int ~~ Bool) as a superclass, which in turn has (Int ~N# Bool)
    as a superclass, and it's the latter that is insoluble.  See
    Note [The equality types story] in GHC.Builtin.Types.Prim.

    If we fail to prove unsatisfiability we (arbitrarily) try just once to
    find superclasses, using try_harder.  Reason: we might have a type
    signature
       f :: F op (Implements push) => ..
    where F is a type function.  This happened in #3972.

    We could do more than once but we'd have to have /some/ limit: in the
    the recursive case, we would go on forever in the common case where
    the constraints /are/ satisfiable (#10592 comment:12!).

    For straightforward situations without type functions the try_harder
    step does nothing.

    Note [tcNormalise]
    ~~~~~~~~~~~~~~~~~~
    tcNormalise is a rather atypical entrypoint to the constraint solver. Whereas
    most invocations of the constraint solver are intended to simplify a set of
    constraints or to decide if a particular set of constraints is satisfiable,
    the purpose of tcNormalise is to take a type, plus some locally solved
    constraints in the form of an InertSet, and normalise the type as much as
    possible with respect to those constraints.

    It does *not* reduce type or data family applications or look through newtypes.

    Why is this useful? As one example, when coverage-checking an EmptyCase
    expression, it's possible that the type of the scrutinee will only reduce
    if some local equalities are solved for. See "Wrinkle: Local equalities"
    in Note [Type normalisation] in "GHC.HsToCore.Pmc".

    To accomplish its stated goal, tcNormalise first initialises the solver monad
    with the given InertCans, then uses rewriteType to simplify the desired type
    with respect to the Givens in the InertCans.

    ***********************************************************************************
    *                                                                                 *
    *                            Inference
    *                                                                                 *
    ***********************************************************************************

    Note [Inferring the type of a let-bound variable]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       f x = rhs

    To infer f's type we do the following:
     * Gather the constraints for the RHS with ambient level *one more than*
       the current one.  This is done by the call
            pushLevelAndCaptureConstraints (tcMonoBinds...)
       in GHC.Tc.Gen.Bind.tcPolyInfer

     * Call simplifyInfer to simplify the constraints and decide what to
       quantify over. We pass in the level used for the RHS constraints,
       here called rhs_tclvl.

    This ensures that the implication constraint we generate, if any,
    has a strictly-increased level compared to the ambient level outside
    the let binding.

    Note [Inferring principal types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We don't always infer principal types. For instance, the inferred type for

    > f x = show [x]

    is

    > f :: Show a => a -> String

    This is not the most general type if we allow flexible contexts.
    Indeed, if we try to write the following

    > g :: Show [a] => a -> String
    > g x = f x

    we get the error:

      * Could not deduce (Show a) arising from a use of `f'
        from the context: Show [a]

    Though replacing f x in the right-hand side of g with the definition
    of f x works, the call to f x does not. This is the hallmark of
    unprincip{led,al} types.

    Another example:

    > class C a
    > class D a where
    >   d :: a
    > instance C a => D a where
    >   d = undefined
    > h _ = d   -- argument is to avoid the monomorphism restriction

    The inferred type for h is

    > h :: C a => t -> a

    even though

    > h :: D a => t -> a

    is more general.

    The fix is easy: don't simplify constraints before inferring a type.
    That is, have the inferred type quantify over all constraints that arise
    in a definition's right-hand side, even if they are simplifiable.
    Unfortunately, this would yield all manner of unwieldy types,
    and so we won't do so.
    -}

    -- | How should we choose which constraints to quantify over?
    data InferMode = ApplyMR          -- ^ Apply the monomorphism restriction,
                                      -- never quantifying over any constraints
                   | EagerDefaulting  -- ^ See Note [TcRnExprMode] in "GHC.Tc.Module",
                                      -- the :type +d case; this mode refuses
                                      -- to quantify over any defaultable constraint
                   | NoRestrictions   -- ^ Quantify over any constraint that
                                      -- satisfies pickQuantifiablePreds

    instance Outputable InferMode where
      ppr :: InferMode -> SDoc
    ppr InferMode
    ApplyMR         = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ApplyMR"
      ppr InferMode
    EagerDefaulting = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "EagerDefaulting"
      ppr InferMode
    NoRestrictions  = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "NoRestrictions"

    simplifyInfer :: TcLevel               -- Used when generating the constraints
                  -> InferMode
                  -> [TcIdSigInst]         -- Any signatures (possibly partial)
                  -> [(Name, TcTauType)]   -- Variables to be generalised,
                                           -- and their tau-types
                  -> WantedConstraints
                  -> TcM ([TcTyVar],    -- Quantify over these type variables
                          [EvVar],      -- ... and these constraints (fully zonked)
                          TcEvBinds,    -- ... binding these evidence variables
                          Bool)         -- True <=> the residual constraints are insoluble

    simplifyInfer :: TcLevel
    -> InferMode
    -> [TcIdSigInst]
    -> [(Name, Type)]
    -> WantedConstraints
    -> TcM ([TyVar], [TyVar], TcEvBinds, Bool)
    simplifyInfer TcLevel
    rhs_tclvl InferMode
    infer_mode [TcIdSigInst]
    sigs [(Name, Type)]
    name_taus WantedConstraints
    wanteds
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanteds
       = do { -- When quantifying, we want to preserve any order of variables as they
              -- appear in partial signatures. cf. decideQuantifiedTyVars
              let psig_tv_tys :: [Type]
    psig_tv_tys = [ TyVar -> Type
    mkTyVarTy TyVar
    tv | TcIdSigInst
    sig <- [TcIdSigInst]
    partial_sigs
                                               , (Name
    _,Bndr TyVar
    tv Specificity
    _) <- TcIdSigInst -> [(Name, InvisTVBinder)]
    sig_inst_skols TcIdSigInst
    sig ]
                  psig_theta :: [Type]
    psig_theta  = [ Type
    pred | TcIdSigInst
    sig <- [TcIdSigInst]
    partial_sigs
                                       , Type
    pred <- TcIdSigInst -> [Type]
    sig_inst_theta TcIdSigInst
    sig ]

           ; dep_vars <- [Type] -> TcM CandidatesQTvs
    candidateQTyVarsOfTypes ([Type]
    psig_tv_tys [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    psig_theta [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ ((Name, Type) -> Type) -> [(Name, Type)] -> [Type]
    forall a b. (a -> b) -> [a] -> [b]
    map (Name, Type) -> Type
    forall a b. (a, b) -> b
    snd [(Name, Type)]
    name_taus)

           ; skol_info <- mkSkolemInfo (InferSkol name_taus)
           ; qtkvs <- quantifyTyVars skol_info DefaultNonStandardTyVars dep_vars
           ; traceTc "simplifyInfer: empty WC" (ppr name_taus $$ ppr qtkvs)
           ; return (qtkvs, [], emptyTcEvBinds, False) }

      | Bool
    otherwise
      = do { String -> SDoc -> TcM ()
    traceTc String
    "simplifyInfer {"  (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
    "sigs =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [TcIdSigInst] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [TcIdSigInst]
    sigs
                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "binds =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [(Name, Type)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(Name, Type)]
    name_taus
                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "rhs_tclvl =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TcLevel -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TcLevel
    rhs_tclvl
                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "infer_mode =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> InferMode -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr InferMode
    infer_mode
                 , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "(unzonked) wanted =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> WantedConstraints -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wanteds
                 ]

           ; let psig_theta :: [Type]
    psig_theta = (TcIdSigInst -> [Type]) -> [TcIdSigInst] -> [Type]
    forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
    concatMap TcIdSigInst -> [Type]
    sig_inst_theta [TcIdSigInst]
    partial_sigs

           -- First do full-blown solving
           -- NB: we must gather up all the bindings from doing
           -- this solving; hence (runTcSWithEvBinds ev_binds_var).
           -- And note that since there are nested implications,
           -- calling solveWanteds will side-effect their evidence
           -- bindings, so we can't just revert to the input
           -- constraint.

           ; ev_binds_var <- TcM EvBindsVar
    TcM.newTcEvBinds
           ; psig_evs     <- newWanteds AnnOrigin psig_theta
           ; wanted_transformed
                <- setTcLevel rhs_tclvl $
                   runTcSWithEvBinds ev_binds_var $
                   solveWanteds (mkSimpleWC psig_evs `andWC` wanteds)
                   -- psig_evs : see Note [Add signature contexts as wanteds]
                   -- See Note [Inferring principal types]

           -- Find quant_pred_candidates, the predicates that
           -- we'll consider quantifying over
           -- NB1: wanted_transformed does not include anything provable from
           --      the psig_theta; it's just the extra bit
           -- NB2: We do not do any defaulting when inferring a type, this can lead
           --      to less polymorphic types, see Note [Default while Inferring]
           ; wanted_transformed <- TcM.liftZonkM $ TcM.zonkWC wanted_transformed
           ; let definite_error = WantedConstraints -> Bool
    insolubleWC WantedConstraints
    wanted_transformed
                                  -- See Note [Quantification with errors]
                 quant_pred_candidates
                   | Bool
    definite_error = []
                   | Bool
    otherwise      = Cts -> [Type]
    ctsPreds (Bool -> WantedConstraints -> Cts
    approximateWC Bool
    False WantedConstraints
    wanted_transformed)

           -- Decide what type variables and constraints to quantify
           -- NB: quant_pred_candidates is already fully zonked
           -- NB: bound_theta are constraints we want to quantify over,
           --     including the psig_theta, which we always quantify over
           -- NB: bound_theta are fully zonked
           -- rec {..}: see Note [Keeping SkolemInfo inside a SkolemTv]
           --           in GHC.Tc.Utils.TcType
           ; rec { (qtvs, bound_theta, co_vars) <- decideQuantification skol_info infer_mode rhs_tclvl
                                                         name_taus partial_sigs
                                                         quant_pred_candidates
                 ; bound_theta_vars <- mapM TcM.newEvVar bound_theta

                 ; let full_theta = (TyVar -> Type) -> [TyVar] -> [Type]
    forall a b. (a -> b) -> [a] -> [b]
    map TyVar -> Type
    idType [TyVar]
    bound_theta_vars
                 ; skol_info <- mkSkolemInfo (InferSkol [ (name, mkPhiTy full_theta ty)
                                                        | (name, ty) <- name_taus ])
           }

           -- Now emit the residual constraint
           ; emitResidualConstraints rhs_tclvl ev_binds_var
                                     name_taus co_vars qtvs bound_theta_vars
                                     wanted_transformed

             -- All done!
           ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ text "quant_pred_candidates =" <+> ppr quant_pred_candidates
                  , text "psig_theta ="     <+> ppr psig_theta
                  , text "bound_theta ="    <+> pprCoreBinders bound_theta_vars
                  , text "qtvs ="           <+> ppr qtvs
                  , text "definite_error =" <+> ppr definite_error ]

           ; return ( qtvs, bound_theta_vars, TcEvBinds ev_binds_var, definite_error ) }
             -- NB: bound_theta_vars must be fully zonked
      where
        partial_sigs :: [TcIdSigInst]
    partial_sigs = (TcIdSigInst -> Bool) -> [TcIdSigInst] -> [TcIdSigInst]
    forall a. (a -> Bool) -> [a] -> [a]
    filter TcIdSigInst -> Bool
    isPartialSig [TcIdSigInst]
    sigs

    --------------------
    emitResidualConstraints :: TcLevel -> EvBindsVar
                            -> [(Name, TcTauType)]
                            -> CoVarSet -> [TcTyVar] -> [EvVar]
                            -> WantedConstraints -> TcM ()
    -- Emit the remaining constraints from the RHS.
    emitResidualConstraints :: TcLevel
    -> EvBindsVar
    -> [(Name, Type)]
    -> VarSet
    -> [TyVar]
    -> [TyVar]
    -> WantedConstraints
    -> TcM ()
    emitResidualConstraints TcLevel
    rhs_tclvl EvBindsVar
    ev_binds_var
                            [(Name, Type)]
    name_taus VarSet
    co_vars [TyVar]
    qtvs [TyVar]
    full_theta_vars WantedConstraints
    wanteds
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanteds
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

      | Bool
    otherwise
      = do { wanted_simple <- ZonkM Cts -> TcM Cts
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM (ZonkM Cts -> TcM Cts) -> ZonkM Cts -> TcM Cts
    forall a b. (a -> b) -> a -> b
    $ Cts -> ZonkM Cts
    TcM.zonkSimples (WantedConstraints -> Cts
    wc_simple WantedConstraints
    wanteds)
           ; let (outer_simple, inner_simple) = partitionBag is_mono wanted_simple
                 is_mono Ct
    ct
                   | Just TyVar
    ct_ev_id <- Ct -> Maybe TyVar
    wantedEvId_maybe Ct
    ct
                   = TyVar
    ct_ev_id TyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    co_vars
                   | Bool
    otherwise
                   = Bool
    False
                 -- Reason for the partition:
                 -- see Note [Emitting the residual implication in simplifyInfer]

    -- Already done by defaultTyVarsAndSimplify
    --      ; _ <- TcM.promoteTyVarSet (tyCoVarsOfCts outer_simple)

            ; let inner_wanted = WantedConstraints
    wanteds { wc_simple = inner_simple }
            ; implics <- if isEmptyWC inner_wanted
                         then return emptyBag
                         else do implic1 <- newImplication
                                 return $ unitBag $
                                          implic1  { ic_tclvl     = rhs_tclvl
                                                   , ic_skols     = qtvs
                                                   , ic_given     = full_theta_vars
                                                   , ic_wanted    = inner_wanted
                                                   , ic_binds     = ev_binds_var
                                                   , ic_given_eqs = MaybeGivenEqs
                                                   , ic_info      = skol_info }

            ; emitConstraints (emptyWC { wc_simple = outer_simple
                                       , wc_impl   = implics }) }
      where
        full_theta :: [Type]
    full_theta = (TyVar -> Type) -> [TyVar] -> [Type]
    forall a b. (a -> b) -> [a] -> [b]
    map TyVar -> Type
    idType [TyVar]
    full_theta_vars
        skol_info :: SkolemInfoAnon
    skol_info = [(Name, Type)] -> SkolemInfoAnon
    InferSkol [ (Name
    name, [Type] -> Type -> Type
    HasDebugCallStack => [Type] -> Type -> Type
    mkPhiTy [Type]
    full_theta Type
    ty)
                              | (Name
    name, Type
    ty) <- [(Name, Type)]
    name_taus ]
        -- We don't add the quantified variables here, because they are
        -- also bound in ic_skols and we want them to be tidied
        -- uniformly.

    --------------------
    findInferredDiff :: TcThetaType -> TcThetaType -> TcM TcThetaType
    -- Given a partial type signature f :: (C a, D a, _) => blah
    -- and the inferred constraints (X a, D a, Y a, C a)
    -- compute the difference, which is what will fill in the "_" underscore,
    -- In this case the diff is (X a, Y a).
    findInferredDiff :: [Type] -> [Type] -> TcM [Type]
    findInferredDiff [Type]
    annotated_theta [Type]
    inferred_theta
      | [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    annotated_theta   -- Short cut the common case when the user didn't
      = [Type] -> TcM [Type]
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [Type]
    inferred_theta  -- write any constraints in the partial signature
      | Bool
    otherwise
      = TcM [Type] -> TcM [Type]
    forall r. TcM r -> TcM r
    pushTcLevelM_ (TcM [Type] -> TcM [Type]) -> TcM [Type] -> TcM [Type]
    forall a b. (a -> b) -> a -> b
    $
        do { lcl_env   <- TcRnIf TcGblEnv TcLclEnv TcLclEnv
    forall gbl lcl. TcRnIf gbl lcl lcl
    TcM.getLclEnv
           ; given_ids <- mapM TcM.newEvVar annotated_theta
           ; wanteds   <- newWanteds AnnOrigin inferred_theta
           ; let given_loc = TcLevel -> SkolemInfoAnon -> CtLocEnv -> CtLoc
    mkGivenLoc TcLevel
    topTcLevel (SkolemInfo -> SkolemInfoAnon
    getSkolemInfo SkolemInfo
    HasCallStack => SkolemInfo
    unkSkol) (TcLclEnv -> CtLocEnv
    mkCtLocEnv TcLclEnv
    lcl_env)
                 given_cts = CtLoc -> [TyVar] -> [Ct]
    mkGivens CtLoc
    given_loc [TyVar]
    given_ids

           ; (residual, _) <- runTcS $
                              do { _ <- solveSimpleGivens given_cts
                                 ; solveSimpleWanteds (listToBag (map mkNonCanonical wanteds)) }
             -- NB: There are no meta tyvars fromn this level annotated_theta
             -- because we have either promoted them or unified them
             -- See `Note [Quantification and partial signatures]` Wrinkle 2

           ; return (map (box_pred . ctPred) $
                     bagToList               $
                     wc_simple residual) }
      where
         box_pred :: PredType -> PredType
         box_pred :: Type -> Type
    box_pred Type
    pred = case Type -> Pred
    classifyPredType Type
    pred of
                            EqPred EqRel
    rel Type
    ty1 Type
    ty2
                              | Just (Class
    cls,[Type]
    tys) <- EqRel -> Type -> Type -> Maybe (Class, [Type])
    boxEqPred EqRel
    rel Type
    ty1 Type
    ty2
                              -> Class -> [Type] -> Type
    mkClassPred Class
    cls [Type]
    tys
                              | Bool
    otherwise
                              -> String -> SDoc -> Type
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "findInferredDiff" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    pred)
                            Pred
    _other -> Type
    pred

    {- Note [Emitting the residual implication in simplifyInfer]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       f = e
    where f's type is inferred to be something like (a, Proxy k (Int |> co))
    and we have an as-yet-unsolved, or perhaps insoluble, constraint
       [W] co :: Type ~ k
    We can't form types like (forall co. blah), so we can't generalise over
    the coercion variable, and hence we can't generalise over things free in
    its kind, in the case 'k'.  But we can still generalise over 'a'.  So
    we'll generalise to
       f :: forall a. (a, Proxy k (Int |> co))
    Now we do NOT want to form the residual implication constraint
       forall a. [W] co :: Type ~ k
    because then co's eventual binding (which will be a value binding if we
    use -fdefer-type-errors) won't scope over the entire binding for 'f' (whose
    type mentions 'co').  Instead, just as we don't generalise over 'co', we
    should not bury its constraint inside the implication.  Instead, we must
    put it outside.

    That is the reason for the partitionBag in emitResidualConstraints,
    which takes the CoVars free in the inferred type, and pulls their
    constraints out.  (NB: this set of CoVars should be closed-over-kinds.)

    All rather subtle; see #14584.

    Note [Add signature contexts as wanteds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this (#11016):
      f2 :: (?x :: Int) => _
      f2 = ?x

    or this
      class C a b | a -> b
      g :: C p q => p -> q
      f3 :: C Int b => _
      f3 = g (3::Int)

    We'll use plan InferGen because there are holes in the type.  But:
     * For f2 we want to have the (?x :: Int) constraint floating around
       so that the functional dependencies kick in.  Otherwise the
       occurrence of ?x on the RHS produces constraint (?x :: alpha), and
       we won't unify alpha:=Int.

     * For f3 want the (C Int b) constraint from the partial signature
       to meet the (C Int beta) constraint we get from the call to g; again,
       fundeps

    Solution: in simplifyInfer, we add the constraints from the signature
    as extra Wanteds.

    Why Wanteds?  Wouldn't it be neater to treat them as Givens?  Alas
    that would mess up (GivenInv) in Note [TcLevel invariants].  Consider
        f :: (Eq a, _) => blah1
        f = ....g...
        g :: (Eq b, _) => blah2
        g = ...f...

    Then we have two psig_theta constraints (Eq a[tv], Eq b[tv]), both with
    TyVarTvs inside.  Ultimately a[tv] := b[tv], but only when we've solved
    all those constraints.  And both have level 1, so we can't put them as
    Givens when solving at level 1.

    Best to treat them as Wanteds.

    But see also #20076, which would be solved if they were Givens.

    ************************************************************************
    *                                                                      *
                    Quantification
    *                                                                      *
    ************************************************************************

    Note [Deciding quantification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If the monomorphism restriction does not apply, then we quantify as follows:

    * Step 1: decidePromotedTyVars.
      Take the global tyvars, and "grow" them using functional dependencies
         E.g.  if x:alpha is in the environment, and alpha ~ [beta] (which can
              happen because alpha is untouchable here) then do not quantify over
              beta, because alpha fixes beta, and beta is effectively free in
              the environment too; this logic extends to general fundeps, not
              just equalities

      We also account for the monomorphism restriction; if it applies,
      add the free vars of all the constraints.

      Result is mono_tvs; we will promote all of these to the outer levek,
      and certainly not quantify over them.

    * Step 2: defaultTyVarsAndSimplify.
      Default any non-promoted tyvars (i.e ones that are definitely
      not going to become further constrained), and re-simplify the
      candidate constraints.

      Motivation for re-simplification (#7857): imagine we have a
      constraint (C (a->b)), where 'a :: TYPE l1' and 'b :: TYPE l2' are
      not free in the envt, and instance forall (a::*) (b::*). (C a) => C
      (a -> b) The instance doesn't match while l1,l2 are polymorphic, but
      it will match when we default them to LiftedRep.

      This is all very tiresome.

      This step also promotes the mono_tvs from Step 1. See
      Note [Promote monomorphic tyvars]. In fact, the *only*
      use of the mono_tvs from Step 1 is to promote them here.
      This promotion effectively stops us from quantifying over them
      later, in Step 3. Because the actual variables to quantify
      over are determined in Step 3 (not in Step 1), it is OK for
      the mono_tvs to be missing some variables free in the
      environment. This is why removing the psig_qtvs is OK in
      decidePromotedTyVars. Test case for this scenario: T14479.

    * Step 3: decideQuantifiedTyVars.
      Decide which variables to quantify over, as follows:

      - Take the free vars of the partial-type-signature types and constraints,
        and the tau-type (zonked_tau_tvs), and then "grow"
        them using all the constraints.  These are grown_tcvs.
        See Note [growThetaTyVars vs closeWrtFunDeps].

      - Use quantifyTyVars to quantify over the free variables of all the types
        involved, but only those in the grown_tcvs.

      Result is qtvs.

    * Step 4: Filter the constraints using pickQuantifiablePreds and the
      qtvs. We have to zonk the constraints first, so they "see" the
      freshly created skolems.

    Note [Unconditionally resimplify constraints when quantifying]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    During quantification (in defaultTyVarsAndSimplify, specifically), we re-invoke
    the solver to simplify the constraints before quantifying them. We do this for
    two reasons, enumerated below. We could, in theory, detect when either of these
    cases apply and simplify only then, but collecting this information is bothersome,
    and simplifying redundantly causes no real harm. Note that this code path
    happens only for definitions
      * without a type signature
      * when -XMonoLocalBinds does not apply
      * with unsolved constraints
    and so the performance cost will be small.

    1. Defaulting

    Defaulting the variables handled by defaultTyVar may unlock instance simplifications.
    Example (typecheck/should_compile/T20584b):

      with (t :: Double) (u :: String) = printf "..." t u

    We know the types of t and u, but we do not know the return type of `with`. So, we
    assume `with :: alpha`, where `alpha :: TYPE rho`. The type of printf is
      printf :: PrintfType r => String -> r
    The occurrence of printf is instantiated with a fresh var beta. We then get
      beta := Double -> String -> alpha
    and
      [W] PrintfType (Double -> String -> alpha)

    Module Text.Printf exports
      instance (PrintfArg a, PrintfType r) => PrintfType (a -> r)
    and it looks like that instance should apply.

    But I have elided some key details: (->) is polymorphic over multiplicity and
    runtime representation. Here it is in full glory:
      [W] PrintfType ((Double :: Type) %m1 -> (String :: Type) %m2 -> (alpha :: TYPE rho))
      instance (PrintfArg a, PrintfType r) => PrintfType ((a :: Type) %Many -> (r :: Type))

    Because we do not know that m1 is Many, we cannot use the instance. (Perhaps a better instance
    would have an explicit equality constraint to the left of =>, but that's not what we have.)
    Then, in defaultTyVarsAndSimplify, we get m1 := Many, m2 := Many, and rho := LiftedRep.
    Yet it's too late to simplify the quantified constraint, and thus GHC infers
      wait :: PrintfType (Double -> String -> t) => Double -> String -> t
    which is silly. Simplifying again after defaulting solves this problem.

    2. Interacting functional dependencies

    Suppose we have

      class C a b | a -> b

    and we are running simplifyInfer over

      forall[2] x. () => [W] C a beta1[1]
      forall[2] y. () => [W] C a beta2[1]

    These are two implication constraints, both of which contain a
    wanted for the class C. Neither constraint mentions the bound
    skolem. We might imagine that these constraints could thus float
    out of their implications and then interact, causing beta1 to unify
    with beta2, but constraints do not currently float out of implications.

    Unifying the beta1 and beta2 is important. Without doing so, then we might
    infer a type like (C a b1, C a b2) => a -> a, which will fail to pass the
    ambiguity check, which will say (rightly) that it cannot unify b1 with b2, as
    required by the fundep interactions. This happens in the parsec library, and
    in test case typecheck/should_compile/FloatFDs.

    If we re-simplify, however, the two fundep constraints will interact, causing
    a unification between beta1 and beta2, and all will be well. The key step
    is that this simplification happens *after* the call to approximateWC in
    simplifyInfer.

    -}

    decideQuantification
      :: SkolemInfo
      -> InferMode
      -> TcLevel
      -> [(Name, TcTauType)]   -- Variables to be generalised
      -> [TcIdSigInst]         -- Partial type signatures (if any)
      -> [PredType]            -- Candidate theta; already zonked
      -> TcM ( [TcTyVar]       -- Quantify over these (skolems)
             , [PredType]      -- and this context (fully zonked)
             , CoVarSet)
    -- See Note [Deciding quantification]
    decideQuantification :: SkolemInfo
    -> InferMode
    -> TcLevel
    -> [(Name, Type)]
    -> [TcIdSigInst]
    -> [Type]
    -> TcM ([TyVar], [Type], VarSet)
    decideQuantification SkolemInfo
    skol_info InferMode
    infer_mode TcLevel
    rhs_tclvl [(Name, Type)]
    name_taus [TcIdSigInst]
    psigs [Type]
    candidates
      = do { -- Step 1: find the mono_tvs
           ; (candidates, co_vars, mono_tvs0)
                 <- InferMode
    -> [(Name, Type)]
    -> [TcIdSigInst]
    -> [Type]
    -> TcM ([Type], VarSet, VarSet)
    decidePromotedTyVars InferMode
    infer_mode [(Name, Type)]
    name_taus [TcIdSigInst]
    psigs [Type]
    candidates

           -- Step 2: default any non-mono tyvars, and re-simplify
           -- This step may do some unification, but result candidates is zonked
           ; candidates <- defaultTyVarsAndSimplify rhs_tclvl candidates

           -- Step 3: decide which kind/type variables to quantify over
           ; qtvs <- decideQuantifiedTyVars skol_info name_taus psigs candidates

           -- Step 4: choose which of the remaining candidate
           --         predicates to actually quantify over
           -- NB: decideQuantifiedTyVars turned some meta tyvars
           -- into quantified skolems, so we have to zonk again
           ; (candidates, psig_theta) <- TcM.liftZonkM $
              do { candidates <- TcM.zonkTcTypes candidates
                 ; psig_theta <- TcM.zonkTcTypes (concatMap sig_inst_theta psigs)
                 ; return (candidates, psig_theta) }
           ; min_theta  <- pickQuantifiablePreds (mkVarSet qtvs) mono_tvs0 candidates

           -- Take account of partial type signatures
           -- See Note [Constraints in partial type signatures]
           ; let min_psig_theta = (Type -> Type) -> [Type] -> [Type]
    forall a. (a -> Type) -> [a] -> [a]
    mkMinimalBySCs Type -> Type
    forall a. a -> a
    id [Type]
    psig_theta
           ; theta <- if
               | null psigs -> return min_theta                 -- Case (P3)
               | not (all has_extra_constraints_wildcard psigs) -- Case (P2)
                 -> return min_psig_theta
               | otherwise                                      -- Case (P1)
                 -> do { diff <- findInferredDiff min_psig_theta min_theta
                       ; return (min_psig_theta ++ diff) }

           ; traceTc "decideQuantification"
               (vcat [ text "infer_mode:" <+> ppr infer_mode
                     , text "candidates:" <+> ppr candidates
                     , text "psig_theta:" <+> ppr psig_theta
                     , text "co_vars:"    <+> ppr co_vars
                     , text "qtvs:"       <+> ppr qtvs
                     , text "theta:"      <+> ppr theta ])
           ; return (qtvs, theta, co_vars) }

      where
        has_extra_constraints_wildcard :: TcIdSigInst -> Bool
    has_extra_constraints_wildcard (TISI { sig_inst_wcx :: TcIdSigInst -> Maybe Type
    sig_inst_wcx = Just {} }) = Bool
    True
        has_extra_constraints_wildcard TcIdSigInst
    _                                 = Bool
    False

    {- Note [Constraints in partial type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have decided to quantify over min_theta, say (Eq a, C a, Ix a).
    Then we distinguish three cases:

    (P1) No partial type signatures: just quantify over min_theta

    (P2) Partial type signatures with no extra_constraints wildcard:
          e.g.   f :: (Eq a, C a) => a -> _
         Quantify over psig_theta: the user has explicitly specified the
         entire context.

         That may mean we have an unsolved residual constraint (Ix a) arising
         from the RHS of the function. But so be it: the user said (Eq a, C a).

    (P3) Partial type signature with an extra_constraints wildcard.
          e.g.   f :: (Eq a, C a, _) => a -> a
        Quantify over (psig_theta ++ diff)
          where diff = min_theta - psig_theta, using findInferredDiff.
        In our example, diff = Ix a

    Some rationale and observations

    * See Note [When the MR applies] in GHC.Tc.Gen.Bind.

    * We always want to quantify over psig_theta (if present).  The user specified
      it!  And pickQuantifiableCandidates might have dropped some
      e.g. CallStack constraints.  c.f #14658
           equalities (a ~ Bool)

    * In case (P3) we ask that /all/ the signatures have an extra-constraints
      wildcard.  It's a bit arbitrary; not clear what the "right" thing is.

    * In (P2) we encounter #20076:
         f :: Eq [a] => a -> _
         f x = [x] == [x]
      From the RHS we get [W] Eq [a].  We simplify those Wanteds in simplifyInfer,
      to get (Eq a).  But then we quantify over the user-specified (Eq [a]), leaving
      a residual implication constraint (forall a. Eq [a] => [W] Eq a), which is
      insoluble.  Idea: in simplifyInfer we could put the /un-simplified/ constraints
      in the residual -- at least in the case like #20076 where the partial signature
      fully specifies the final constraint. Maybe: a battle for another day.

    * It's helpful to use the same "find difference" algorithm, `findInferredDiff`,
      here as we use in GHC.Tc.Gen.Bind.chooseInferredQuantifiers (#20921)

      At least for single functions we would like to quantify f over precisely the
      same theta as <quant-theta>, so that we get to take the short-cut path in
      `GHC.Tc.Gen.Bind.mkExport`, and avoid calling `tcSubTypeSigma` for impedance
      matching. Why avoid?  Because it falls over for ambiguous types (#20921).

      We can get precisely the same theta by using the same algorithm,
      `findInferredDiff`.

    * All of this goes wrong if we have (a) mutual recursion, (b) multiple
      partial type signatures, (c) with different constraints, and (d)
      ambiguous types.  Something like
        f :: forall a. Eq a => F a -> _
        f x = (undefined :: a) == g x undefined
        g :: forall b. Show b => F b -> _ -> b
        g x y = let _ = (f y, show x) in x
      But that's a battle for another day.
    -}

    decidePromotedTyVars :: InferMode
                         -> [(Name,TcType)]
                         -> [TcIdSigInst]
                         -> [PredType]
                         -> TcM ([PredType], CoVarSet, TcTyVarSet)
    -- We are about to generalise over type variables at level N
    -- Each must be either
    --    (P) promoted
    --    (D) defaulted
    --    (Q) quantified
    -- This function finds (P), the type variables that we are going to promote:
    --   (a) Mentioned in a constraint we can't generalise (the MR)
    --   (b) Mentioned in the kind of a CoVar; we can't quantify over a CoVar,
    --       so we must not quantify over a type variable free in its kind
    --   (c) Connected by an equality or fundep to
    --          * a type variable at level < N, or
    --          * A tyvar subject to (a), (b) or (c)
    -- Having found all such level-N tyvars that we can't generalise,
    -- promote them, to eliminate them from further consideration.
    --
    -- Also return CoVars that appear free in the final quantified types
    --   we can't quantify over these, and we must make sure they are in scope
    decidePromotedTyVars :: InferMode
    -> [(Name, Type)]
    -> [TcIdSigInst]
    -> [Type]
    -> TcM ([Type], VarSet, VarSet)
    decidePromotedTyVars InferMode
    infer_mode [(Name, Type)]
    name_taus [TcIdSigInst]
    psigs [Type]
    candidates
      = do { tc_lvl <- TcM TcLevel
    TcM.getTcLevel
           ; (no_quant, maybe_quant) <- pick infer_mode candidates

           -- If possible, we quantify over partial-sig qtvs, so they are
           -- not mono. Need to zonk them because they are meta-tyvar TyVarTvs
           ; (psig_qtvs, psig_theta, taus) <- TcM.liftZonkM $
              do { psig_qtvs <- zonkTcTyVarsToTcTyVars $ binderVars $
                                concatMap (map snd . sig_inst_skols) psigs
                 ; psig_theta <- mapM TcM.zonkTcType $
                                 concatMap sig_inst_theta psigs
                 ; taus <- mapM (TcM.zonkTcType . snd) name_taus
                 ; return (psig_qtvs, psig_theta, taus) }

           ; let psig_tys = [TyVar] -> [Type]
    mkTyVarTys [TyVar]
    psig_qtvs [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    psig_theta

                 -- (b) The co_var_tvs are tvs mentioned in the types of covars or
                 -- coercion holes. We can't quantify over these covars, so we
                 -- must include the variable in their types in the mono_tvs.
                 -- E.g.  If we can't quantify over co :: k~Type, then we can't
                 --       quantify over k either!  Hence closeOverKinds
                 -- Recall that coVarsOfTypes also returns coercion holes
                 co_vars = [Type] -> VarSet
    coVarsOfTypes ([Type]
    psig_tys [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    taus [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    candidates)
                 co_var_tvs = VarSet -> VarSet
    closeOverKinds VarSet
    co_vars

                 mono_tvs0 = (TyVar -> Bool) -> VarSet -> VarSet
    filterVarSet (Bool -> Bool
    not (Bool -> Bool) -> (TyVar -> Bool) -> TyVar -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TcLevel -> TyVar -> Bool
    isQuantifiableTv TcLevel
    tc_lvl) (VarSet -> VarSet) -> VarSet -> VarSet
    forall a b. (a -> b) -> a -> b
    $
                             [Type] -> VarSet
    tyCoVarsOfTypes [Type]
    candidates
                   -- We need to grab all the non-quantifiable tyvars in the
                   -- types so that we can grow this set to find other
                   -- non-quantifiable tyvars. This can happen with something like
                   --    f x y = ...
                   --      where z = x 3
                   -- The body of z tries to unify the type of x (call it alpha[1])
                   -- with (beta[2] -> gamma[2]). This unification fails because
                   -- alpha is untouchable, leaving [W] alpha[1] ~ (beta[2] -> gamma[2]).
                   -- We need to know not to quantify over beta or gamma, because they
                   -- are in the equality constraint with alpha. Actual test case:
                   -- typecheck/should_compile/tc213

                 mono_tvs1 = VarSet
    mono_tvs0 VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    co_var_tvs

                   -- mono_tvs1 is now the set of variables from an outer scope
                   -- (that's mono_tvs0) and the set of covars, closed over kinds.
                   -- Given this set of variables we know we will not quantify,
                   -- we want to find any other variables that are determined by this
                   -- set, by functional dependencies or equalities. We thus use
                   -- closeWrtFunDeps to find all further variables determined by this root
                   -- set. See Note [growThetaTyVars vs closeWrtFunDeps]

                 non_ip_candidates = (Type -> Bool) -> [Type] -> [Type]
    forall a. (a -> Bool) -> [a] -> [a]
    filterOut Type -> Bool
    isIPLikePred [Type]
    candidates
                   -- implicit params don't really determine a type variable
                   -- (that is, we might have IP "c" Bool and IP "c" Int in different
                   -- places within the same program), and
                   -- skipping this causes implicit params to monomorphise too many
                   -- variables; see Note [Inheriting implicit parameters] in GHC.Tc.Solver.
                   -- Skipping causes typecheck/should_compile/tc219 to fail.

                 mono_tvs2 = [Type] -> VarSet -> VarSet
    closeWrtFunDeps [Type]
    non_ip_candidates VarSet
    mono_tvs1
                   -- mono_tvs2 now contains any variable determined by the "root
                   -- set" of monomorphic tyvars in mono_tvs1.

                 constrained_tvs = (TyVar -> Bool) -> VarSet -> VarSet
    filterVarSet (TcLevel -> TyVar -> Bool
    isQuantifiableTv TcLevel
    tc_lvl) (VarSet -> VarSet) -> VarSet -> VarSet
    forall a b. (a -> b) -> a -> b
    $
                                   [Type] -> VarSet -> VarSet
    closeWrtFunDeps [Type]
    non_ip_candidates ([Type] -> VarSet
    tyCoVarsOfTypes [Type]
    no_quant)
                                    VarSet -> VarSet -> VarSet
    `minusVarSet` VarSet
    mono_tvs2
                 -- constrained_tvs: the tyvars that we are not going to
                 -- quantify /solely/ because of the monomorphism restriction
                 --
                 -- (`minusVarSet` mono_tvs2): a type variable is only
                 --   "constrained" (so that the MR bites) if it is not
                 --   free in the environment (#13785) or is determined
                 --   by some variable that is free in the env't

                 mono_tvs = (VarSet
    mono_tvs2 VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    constrained_tvs)
                            VarSet -> [TyVar] -> VarSet
    `delVarSetList` [TyVar]
    psig_qtvs
                 -- (`delVarSetList` psig_qtvs): if the user has explicitly
                 --   asked for quantification, then that request "wins"
                 --   over the MR.
                 --
                 -- What if a psig variable is also free in the environment
                 -- (i.e. says "no" to isQuantifiableTv)? That's OK: explanation
                 -- in Step 2 of Note [Deciding quantification].

               -- Warn about the monomorphism restriction
           ; when (case infer_mode of { InferMode
    ApplyMR -> Bool
    True; InferMode
    _ -> Bool
    False}) $ do
               let dia = [Name] -> TcRnMessage
    TcRnMonomorphicBindings (((Name, Type) -> Name) -> [(Name, Type)] -> [Name]
    forall a b. (a -> b) -> [a] -> [b]
    map (Name, Type) -> Name
    forall a b. (a, b) -> a
    fst [(Name, Type)]
    name_taus)
               diagnosticTc (constrained_tvs `intersectsVarSet` tyCoVarsOfTypes taus) dia

           -- Promote the mono_tvs: see Note [Promote monomorphic tyvars]
           ; _ <- promoteTyVarSet mono_tvs

           ; traceTc "decidePromotedTyVars" $ vcat
               [ text "infer_mode =" <+> ppr infer_mode
               , text "psigs =" <+> ppr psigs
               , text "psig_qtvs =" <+> ppr psig_qtvs
               , text "mono_tvs0 =" <+> ppr mono_tvs0
               , text "no_quant =" <+> ppr no_quant
               , text "maybe_quant =" <+> ppr maybe_quant
               , text "mono_tvs =" <+> ppr mono_tvs
               , text "co_vars =" <+> ppr co_vars ]

           ; return (maybe_quant, co_vars, mono_tvs0) }
      where
        pick :: InferMode -> [PredType] -> TcM ([PredType], [PredType])
        -- Split the candidates into ones we definitely
        -- won't quantify, and ones that we might
        pick :: InferMode -> [Type] -> TcM ([Type], [Type])
    pick InferMode
    ApplyMR         [Type]
    cand = ([Type], [Type]) -> TcM ([Type], [Type])
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ([Type]
    cand, [])
        pick InferMode
    NoRestrictions  [Type]
    cand = ([Type], [Type]) -> TcM ([Type], [Type])
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ([], [Type]
    cand)
        pick InferMode
    EagerDefaulting [Type]
    cand = do { os <- Extension -> TcM Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.OverloadedStrings
                                       ; return (partition (is_int_ct os) cand) }

        -- is_int_ct returns True for a constraint we should /not/ quantify
        -- For EagerDefaulting, do not quantify over
        -- over any interactive class constraint
        is_int_ct :: Bool -> Type -> Bool
    is_int_ct Bool
    ovl_strings Type
    pred
          = case Type -> Pred
    classifyPredType Type
    pred of
               ClassPred Class
    cls [Type]
    _ -> Bool -> Class -> Bool
    isInteractiveClass Bool
    ovl_strings Class
    cls
               Pred
    _               -> Bool
    False

    -------------------
    defaultTyVarsAndSimplify :: TcLevel
                             -> [PredType]          -- Assumed zonked
                             -> TcM [PredType]      -- Guaranteed zonked
    -- Default any tyvar free in the constraints;
    -- and re-simplify in case the defaulting allows further simplification
    defaultTyVarsAndSimplify :: TcLevel -> [Type] -> TcM [Type]
    defaultTyVarsAndSimplify TcLevel
    rhs_tclvl [Type]
    candidates
      = do {  -- Default any kind/levity vars
           ; DV {dv_kvs = cand_kvs, dv_tvs = cand_tvs}
                    <- [Type] -> TcM CandidatesQTvs
    candidateQTyVarsOfTypes [Type]
    candidates
             -- NB1: decidePromotedTyVars has promoted any type variable fixed by the
             --      type envt, so they won't be chosen by candidateQTyVarsOfTypes
             -- NB2: Defaulting for variables free in tau_tys is done later, by quantifyTyVars
             --      Hence looking only at 'candidates'
             -- NB3: Any covars should already be handled by
             --      the logic in decidePromotedTyVars, which looks at
             --      the constraints generated

           ; poly_kinds  <- xoptM LangExt.PolyKinds
           ; let default_kv | Bool
    poly_kinds = TyVar -> TcM Bool
    default_tv
                            | Bool
    otherwise  = DefaultingStrategy -> TyVar -> TcM Bool
    defaultTyVar DefaultingStrategy
    DefaultKindVars
                 default_tv = DefaultingStrategy -> TyVar -> TcM Bool
    defaultTyVar (NonStandardDefaultingStrategy -> DefaultingStrategy
    NonStandardDefaulting NonStandardDefaultingStrategy
    DefaultNonStandardTyVars)
           ; mapM_ default_kv (dVarSetElems cand_kvs)
           ; mapM_ default_tv (dVarSetElems (cand_tvs `minusDVarSet` cand_kvs))

           ; simplify_cand candidates
           }
      where
        -- See Note [Unconditionally resimplify constraints when quantifying]
        simplify_cand :: [Type] -> TcM [Type]
    simplify_cand [] = [Type] -> TcM [Type]
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return []  -- Fast path
        simplify_cand [Type]
    candidates
          = do { clone_wanteds <- CtOrigin -> [Type] -> TcM [CtEvidence]
    newWanteds CtOrigin
    DefaultOrigin [Type]
    candidates
               ; WC { wc_simple = simples } <- setTcLevel rhs_tclvl $
                                               simplifyWantedsTcM clone_wanteds
                  -- Discard evidence; simples is fully zonked

               ; let new_candidates = Cts -> [Type]
    ctsPreds Cts
    simples
               ; traceTc "Simplified after defaulting" $
                          vcat [ text "Before:" <+> ppr candidates
                               , text "After:"  <+> ppr new_candidates ]
               ; return new_candidates }

    ------------------
    decideQuantifiedTyVars
       :: SkolemInfo
       -> [(Name,TcType)]   -- Annotated theta and (name,tau) pairs
       -> [TcIdSigInst]     -- Partial signatures
       -> [PredType]        -- Candidates, zonked
       -> TcM [TyVar]
    -- Fix what tyvars we are going to quantify over, and quantify them
    decideQuantifiedTyVars :: SkolemInfo
    -> [(Name, Type)] -> [TcIdSigInst] -> [Type] -> TcM [TyVar]
    decideQuantifiedTyVars SkolemInfo
    skol_info [(Name, Type)]
    name_taus [TcIdSigInst]
    psigs [Type]
    candidates
      = do {     -- Why psig_tys? We try to quantify over everything free in here
                 -- See Note [Quantification and partial signatures]
                 --     Wrinkles 2 and 3
           ; (psig_tv_tys, psig_theta, tau_tys) <- ZonkM ([Type], [Type], [Type]) -> TcM ([Type], [Type], [Type])
    forall a. ZonkM a -> TcM a
    TcM.liftZonkM (ZonkM ([Type], [Type], [Type]) -> TcM ([Type], [Type], [Type]))
    -> ZonkM ([Type], [Type], [Type]) -> TcM ([Type], [Type], [Type])
    forall a b. (a -> b) -> a -> b
    $
             do { psig_tv_tys <- (TyVar -> ZonkM Type) -> [TyVar] -> ZonkM [Type]
    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 TyVar -> ZonkM Type
    TcM.zonkTcTyVar [ TyVar
    tv | TcIdSigInst
    sig <- [TcIdSigInst]
    psigs
                                                           , (Name
    _,Bndr TyVar
    tv Specificity
    _) <- TcIdSigInst -> [(Name, InvisTVBinder)]
    sig_inst_skols TcIdSigInst
    sig ]
                ; psig_theta  <- mapM TcM.zonkTcType [ pred | sig <- psigs
                                                            , pred <- sig_inst_theta sig ]
                ; tau_tys     <- mapM (TcM.zonkTcType . snd) name_taus
                ; return (psig_tv_tys, psig_theta, tau_tys) }

           ; let -- Try to quantify over variables free in these types
                 psig_tys = [Type]
    psig_tv_tys [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    psig_theta
                 seed_tys = [Type]
    psig_tys [Type] -> [Type] -> [Type]
    forall a. [a] -> [a] -> [a]
    ++ [Type]
    tau_tys

                 -- Now "grow" those seeds to find ones reachable via 'candidates'
                 -- See Note [growThetaTyVars vs closeWrtFunDeps]
                 grown_tcvs = [Type] -> VarSet -> VarSet
    growThetaTyVars [Type]
    candidates ([Type] -> VarSet
    tyCoVarsOfTypes [Type]
    seed_tys)

           -- Now we have to classify them into kind variables and type variables
           -- (sigh) just for the benefit of -XNoPolyKinds; see quantifyTyVars
           --
           -- Keep the psig_tys first, so that candidateQTyVarsOfTypes produces
           -- them in that order, so that the final qtvs quantifies in the same
           -- order as the partial signatures do (#13524)
           ; dv@DV {dv_kvs = cand_kvs, dv_tvs = cand_tvs} <- candidateQTyVarsOfTypes $
                                                             psig_tys ++ candidates ++ tau_tys
           ; let pick     = (DTyVarSet -> VarSet -> DTyVarSet
    `dVarSetIntersectVarSet` VarSet
    grown_tcvs)
                 dvs_plus = CandidatesQTvs
    dv { dv_kvs = pick cand_kvs, dv_tvs = pick cand_tvs }

           ; traceTc "decideQuantifiedTyVars" (vcat
               [ text "tau_tys =" <+> ppr tau_tys
               , text "candidates =" <+> ppr candidates
               , text "cand_kvs =" <+> ppr cand_kvs
               , text "cand_tvs =" <+> ppr cand_tvs
               , text "tau_tys =" <+> ppr tau_tys
               , text "seed_tys =" <+> ppr seed_tys
               , text "seed_tcvs =" <+> ppr (tyCoVarsOfTypes seed_tys)
               , text "grown_tcvs =" <+> ppr grown_tcvs
               , text "dvs =" <+> ppr dvs_plus])

           ; quantifyTyVars skol_info DefaultNonStandardTyVars dvs_plus }

    ------------------
    -- | When inferring types, should we quantify over a given predicate?
    -- See Note [pickQuantifiablePreds]
    pickQuantifiablePreds
      :: TyVarSet           -- Quantifying over these
      -> TcTyVarSet         -- mono_tvs0: variables mentioned a candidate
                            --   constraint that come from some outer level
      -> TcThetaType        -- Proposed constraints to quantify
      -> TcM TcThetaType    -- A subset that we can actually quantify
    -- This function decides whether a particular constraint should be
    -- quantified over, given the type variables that are being quantified
    pickQuantifiablePreds :: VarSet -> VarSet -> [Type] -> TcM [Type]
    pickQuantifiablePreds VarSet
    qtvs VarSet
    mono_tvs0 [Type]
    theta
      = do { tc_lvl <- TcM TcLevel
    TcM.getTcLevel
           ; let is_nested = Bool -> Bool
    not (TcLevel -> Bool
    isTopTcLevel TcLevel
    tc_lvl)
           ; return (mkMinimalBySCs id $  -- See Note [Minimize by Superclasses]
                     mapMaybe (pick_me is_nested) theta) }
      where
        pick_me :: Bool -> Type -> Maybe Type
    pick_me Bool
    is_nested Type
    pred
          = let pred_tvs :: VarSet
    pred_tvs = Type -> VarSet
    tyCoVarsOfType Type
    pred
                mentions_qtvs :: Bool
    mentions_qtvs = VarSet
    pred_tvs VarSet -> VarSet -> Bool
    `intersectsVarSet` VarSet
    qtvs
            in case Type -> Pred
    classifyPredType Type
    pred of

              ClassPred Class
    cls [Type]
    tys
                | Just {} <- Class -> [Type] -> Maybe FastString
    isCallStackPred Class
    cls [Type]
    tys
                  -- NEVER infer a CallStack constraint.  Otherwise we let
                  -- the constraints bubble up to be solved from the outer
                  -- context, or be defaulted when we reach the top-level.
                  -- See Note [Overview of implicit CallStacks] in GHC.Tc.Types.Evidence
                -> Maybe Type
    forall a. Maybe a
    Nothing

                | Class -> Bool
    isIPClass Class
    cls
                -> Type -> Maybe Type
    forall a. a -> Maybe a
    Just Type
    pred -- See Note [Inheriting implicit parameters]

                | Bool -> Bool
    not Bool
    mentions_qtvs
                -> Maybe Type
    forall a. Maybe a
    Nothing   -- Don't quantify over predicates that don't
                             -- mention any of the quantified type variables

                | Bool
    is_nested
                -> Type -> Maybe Type
    forall a. a -> Maybe a
    Just Type
    pred

                -- From here on, we are thinking about top-level defns only

                | VarSet
    pred_tvs VarSet -> VarSet -> Bool
    `subVarSet` (VarSet
    qtvs VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    mono_tvs0)
                  -- See Note [Do not quantify over constraints that determine a variable]
                -> Type -> Maybe Type
    forall a. a -> Maybe a
    Just Type
    pred

                | Bool
    otherwise
                -> Maybe Type
    forall a. Maybe a
    Nothing

              EqPred EqRel
    eq_rel Type
    ty1 Type
    ty2
                | Bool
    mentions_qtvs
                , EqRel -> Type -> Type -> Bool
    quantify_equality EqRel
    eq_rel Type
    ty1 Type
    ty2
                , Just (Class
    cls, [Type]
    tys) <- EqRel -> Type -> Type -> Maybe (Class, [Type])
    boxEqPred EqRel
    eq_rel Type
    ty1 Type
    ty2
                  -- boxEqPred: See Note [Lift equality constraints when quantifying]
                -> Type -> Maybe Type
    forall a. a -> Maybe a
    Just (Class -> [Type] -> Type
    mkClassPred Class
    cls [Type]
    tys)
                | Bool
    otherwise
                -> Maybe Type
    forall a. Maybe a
    Nothing

              IrredPred {} | Bool
    mentions_qtvs -> Type -> Maybe Type
    forall a. a -> Maybe a
    Just Type
    pred
                           | Bool
    otherwise     -> Maybe Type
    forall a. Maybe a
    Nothing

              ForAllPred {} -> Maybe Type
    forall a. Maybe a
    Nothing

        -- See Note [Quantifying over equality constraints]
        quantify_equality :: EqRel -> Type -> Type -> Bool
    quantify_equality EqRel
    NomEq  Type
    ty1 Type
    ty2 = Type -> Bool
    quant_fun Type
    ty1 Bool -> Bool -> Bool
    || Type -> Bool
    quant_fun Type
    ty2
        quantify_equality EqRel
    ReprEq Type
    _   Type
    _   = Bool
    True

        quant_fun :: Type -> Bool
    quant_fun Type
    ty
          = case HasCallStack => Type -> Maybe (TyCon, [Type])
    Type -> Maybe (TyCon, [Type])
    tcSplitTyConApp_maybe Type
    ty of
              Just (TyCon
    tc, [Type]
    tys) | TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc
                             -> [Type] -> VarSet
    tyCoVarsOfTypes [Type]
    tys VarSet -> VarSet -> Bool
    `intersectsVarSet` VarSet
    qtvs
              Maybe (TyCon, [Type])
    _ -> Bool
    False

    ------------------
    growThetaTyVars :: ThetaType -> TyCoVarSet -> TyCoVarSet
    -- See Note [growThetaTyVars vs closeWrtFunDeps]
    growThetaTyVars :: [Type] -> VarSet -> VarSet
    growThetaTyVars [Type]
    theta VarSet
    tcvs
      | [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    theta = VarSet
    tcvs
      | Bool
    otherwise  = (VarSet -> VarSet) -> VarSet -> VarSet
    transCloVarSet VarSet -> VarSet
    mk_next VarSet
    seed_tcvs
      where
        seed_tcvs :: VarSet
    seed_tcvs = VarSet
    tcvs VarSet -> VarSet -> VarSet
    `unionVarSet` [Type] -> VarSet
    tyCoVarsOfTypes [Type]
    ips
        ([Type]
    ips, [Type]
    non_ips) = (Type -> Bool) -> [Type] -> ([Type], [Type])
    forall a. (a -> Bool) -> [a] -> ([a], [a])
    partition Type -> Bool
    isIPLikePred [Type]
    theta
                             -- See Note [Inheriting implicit parameters]

        mk_next :: VarSet -> VarSet -- Maps current set to newly-grown ones
        mk_next :: VarSet -> VarSet
    mk_next VarSet
    so_far = (Type -> VarSet -> VarSet) -> VarSet -> [Type] -> VarSet
    forall a b. (a -> b -> b) -> b -> [a] -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr (VarSet -> Type -> VarSet -> VarSet
    grow_one VarSet
    so_far) VarSet
    emptyVarSet [Type]
    non_ips
        grow_one :: VarSet -> Type -> VarSet -> VarSet
    grow_one VarSet
    so_far Type
    pred VarSet
    tcvs
           | VarSet
    pred_tcvs VarSet -> VarSet -> Bool
    `intersectsVarSet` VarSet
    so_far = VarSet
    tcvs VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    pred_tcvs
           | Bool
    otherwise                           = VarSet
    tcvs
           where
             pred_tcvs :: VarSet
    pred_tcvs = Type -> VarSet
    tyCoVarsOfType Type
    pred

    {- Note [Promote monomorphic tyvars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Promote any type variables that are free in the environment.  Eg
       f :: forall qtvs. bound_theta => zonked_tau
    The free vars of f's type become free in the envt, and hence will show
    up whenever 'f' is called.  They may currently at rhs_tclvl, but they
    had better be unifiable at the outer_tclvl!  Example: envt mentions
    alpha[1]
               tau_ty = beta[2] -> beta[2]
               constraints = alpha ~ [beta]
    we don't quantify over beta (since it is fixed by envt)
    so we must promote it!  The inferred type is just
      f :: beta -> beta

    NB: promoteTyVarSet ignores coercion variables

    Note [pickQuantifiablePreds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When pickQuantifiablePreds is called we have decided what type
    variables to quantify over, `qtvs`. The only quesion is: which of the
    unsolved candidate predicates should we quantify over?  Call them
    `picked_theta`.

    Note that will leave behind a residual implication
         forall qtvs. picked_theta => unsolved_constraints
    For the members of unsolved_constraints that we select for picked_theta
    it is easy to solve, by identity.  For the others we just hope that
    we can solve them.

    So which of the candidates should we pick to quantify over?  In some
    situations we distinguish top-level from nested bindings.  The point
    about nested binding is that
     (a) the types may mention type variables free in the environment
     (b) all of the call sites are statically visible, reducing the
         worries about "spooky action at a distance".

    First, never pick a constraint that doesn't mention any of the quantified
    variables `qtvs`.  Picking such a constraint essentially moves the solving of
    the constraint from this function definition to call sites.  But because the
    constraint mentions no quantified variables, call sites have no advantage
    over the definition site. Well, not quite: there could be new constraints
    brought into scope by a pattern-match against a constrained (e.g. GADT)
    constructor.  Example

          data T a where { T1 :: T1 Bool; ... }

          f :: forall a. a -> T a -> blah
          f x t = let g y = x&&y    -- This needs a~Bool
                in case t of
                      T1 -> g True
                      ....

    At g's call site we have `a~Bool`, so we /could/ infer
         g :: forall . (a~Bool) => Bool -> Bool  -- qtvs = {}

    This is all very contrived, and probably just postponse type errors to
    the call site.  If that's what you want, write a type signature.

    Actually, implicit parameters is an exception to the "no quantified vars"
    rule (see Note [Inheriting implicit parameters]) so we can't actually
    simply test this case first.

    Now we consider the different sorts of constraints:

    * For ClassPred constraints:

      * Never pick a CallStack constraint.
        See Note [Overview of implicit CallStacks]

      * Always pick an implicit-parameter constraint.
        Note [Inheriting implicit parameters]

      * For /top-level/ class constraints see
        Note [Do not quantify over constraints that determine a variable]

    * For EqPred constraints see Note [Quantifying over equality constraints]

    * For IrredPred constraints, we allow anything that mentions the quantified
      type variables.

    * A ForAllPred should not appear: the candidates come from approximateWC.

    Notice that we do /not/ consult -XFlexibleContexts here.  For example,
    we allow `pickQuantifiablePreds` to quantify over a constraint like
    `Num [a]`; then if we don't have `-XFlexibleContexts` we'll get an
    error from `checkValidType` but (critically) it includes the helpful
    suggestion of adding `-XFlexibleContexts`.  See #10608, #10351.

    Note [Lift equality constraints when quantifying]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We can't quantify over a constraint (t1 ~# t2) because that isn't a
    predicate type; see Note [Types for coercions, predicates, and evidence]
    in GHC.Core.TyCo.Rep.

    So we have to 'lift' it to (t1 ~ t2).  Similarly (~R#) must be lifted
    to Coercible.

    This tiresome lifting is the reason that pick_me (in
    pickQuantifiablePreds) returns a Maybe rather than a Bool.

    Note [Inheriting implicit parameters]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:

            f x = (x::Int) + ?y

    where f is *not* a top-level binding.
    From the RHS of f we'll get the constraint (?y::Int).
    There are two types we might infer for f:

            f :: Int -> Int

    (so we get ?y from the context of f's definition), or

            f :: (?y::Int) => Int -> Int

    At first you might think the first was better, because then
    ?y behaves like a free variable of the definition, rather than
    having to be passed at each call site.  But of course, the WHOLE
    IDEA is that ?y should be passed at each call site (that's what
    dynamic binding means) so we'd better infer the second.

    BOTTOM LINE: when *inferring types* you must quantify over implicit
    parameters, *even if* they don't mention the bound type variables.
    Reason: because implicit parameters, uniquely, have local instance
    declarations. See pickQuantifiablePreds.

    Note [Quantifying over equality constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Should we quantify over an equality constraint (s ~ t)
    in pickQuantifiablePreds?

    * It is always /sound/ to quantify over a constraint -- those
      quantified constraints will need to be proved at each call site.

    * We definitely don't want to quantify over (Maybe a ~ Bool), to get
         f :: forall a. (Maybe a ~ Bool) => blah
      That simply postpones a type error from the function definition site to
      its call site.  Fortunately we have already filtered out insoluble
      constraints: see `definite_error` in `simplifyInfer`.

    * What about (a ~ T alpha b), where we are about to quantify alpha, `a` and
      `b` are in-scope skolems, and `T` is a data type.  It's pretty unlikely
      that this will be soluble at a call site, so we don't quantify over it.

    * What about `(F beta ~ Int)` where we are going to quantify `beta`?
      Should we quantify over the (F beta ~ Int), to get
         f :: forall b. (F b ~ Int) => blah
      Aha!  Perhaps yes, because at the call site we will instantiate `b`, and
      perhaps we have `instance F Bool = Int`. So we *do* quantify over a
      type-family equality where the arguments mention the quantified variables.

    This is all a bit ad-hoc.

    Note [Do not quantify over constraints that determine a variable]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (typecheck/should_compile/tc231), where we're trying to infer
    the type of a top-level declaration. We have
      class Zork s a b | a -> b
    and the candidate constraint at the end of simplifyInfer is
      [W] Zork alpha[1] (Z [Char]) beta[1]
    We definitely want to quantify over `alpha` (which is mentioned in the
    tau-type).

    But we do *not* want to quantify over `beta`: it is determined by the
    functional dependency on Zork: note that the second argument to Zork
    in the Wanted is a variable-free `Z [Char]`.  Quantifying over it
    would be "Henry Ford polymorphism".  (Presumably we don't have an
    instance in scope that tells us what `beta` actually is.)  Instead
    we promote `beta[1]` to `beta[0]`, in `decidePromotedTyVars`.

    The question here: do we want to quantify over the constraint, to
    give the type
       forall a. Zork a (Z [Char]) beta[0] => blah
    Definitely not.  Since we're not quantifying over beta, it has been
    promoted; and then will be zapped to Any in the final zonk.  So we end
    up with a (perhaps exported) type involving
      forall a. Zork a (Z [Char]) Any => blah
    No no no:

      Key principle: we never want to show the programmer
                     a type with `Any` in it.

    What we really want (to catch the Zork example) is this:

       Quantify over the constraint only if all its free variables are
       (a) quantified, or
       (b) appears in the type of something in the environment (mono_tvs0).

    To understand (b) consider

      class C a b where { op :: a -> b -> () }

      mr = 3                      -- mr :: alpha
      f1 x = op x mr              -- f1 :: forall b. b -> (), plus [W] C b alpha
      intify = mr + (4 :: Int)

    In `f1` should we quantify over that `(C b alpha)`?  Answer: since `alpha`
    is free in the type envt, yes we should.  After all, if we'd typechecked
    `intify` first, we'd have set `alpha := Int`, and /then/ we'd certainly
    quantify.  The delicate Zork situation applies when beta is completely
    unconstrained (not free in the environment) -- except by the fundep.

    Another way to put it: let's say `alpha` is in `mono_tvs0`. It must be that
    some variable `x` has `alpha` free in its type. If we are at top-level (and we
    are, because nested decls don't go through this path all), then `x` must also
    be at top-level. And, by induction, `x` will not have Any in its type when all
    is said and done. The induction is well-founded because, if `x` is mutually
    recursive with the definition at hand, then their constraints get processed
    together (or `x` has a type signature, in which case the type doesn't have
    `Any`). So the key thing is that we must not introduce a new top-level
    unconstrained variable here.

    However this regrettably-subtle reasoning is needed only for /top-level/
    declarations.  For /nested/ decls we can see all the calls, so we'll
    instantiate that quantifed `Zork a (Z [Char]) beta` constraint at call sites,
    and either solve it or not (probably not).  We won't be left with a
    still-callable function with Any in its type.  So for nested definitions we
    don't make this tricky test.

    Historical note: we had a different, and more complicated test
    before, but it was utterly wrong: #23199.

    Note [Quantification and partial signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When choosing type variables to quantify, the basic plan is to
    quantify over all type variables that are
     * free in the tau_tvs, and
     * not forced to be monomorphic (mono_tvs),
       for example by being free in the environment.

    However, in the case of a partial type signature, we are doing inference
    *in the presence of a type signature*. For example:
       f :: _ -> a
       f x = ...
    or
       g :: (Eq _a) => _b -> _b
    In both cases we use plan InferGen, and hence call simplifyInfer.  But
    those 'a' variables are skolems (actually TyVarTvs), and we should be
    sure to quantify over them.  This leads to several wrinkles:

    * Wrinkle 1.  In the case of a type error
         f :: _ -> Maybe a
         f x = True && x
      The inferred type of 'f' is f :: Bool -> Bool, but there's a
      left-over error of form (Maybe a ~ Bool).  The error-reporting
      machine expects to find a binding site for the skolem 'a', so we
      add it to the quantified tyvars.

    * Wrinkle 2.  Consider the partial type signature
         f :: (Eq _) => Int -> Int
         f x = x
      In normal cases that makes sense; e.g.
         g :: Eq _a => _a -> _a
         g x = x
      where the signature makes the type less general than it could
      be. But for 'f' we must therefore quantify over the user-annotated
      constraints, to get
         f :: forall a. Eq a => Int -> Int
      (thereby correctly triggering an ambiguity error later).  If we don't
      we'll end up with a strange open type
         f :: Eq alpha => Int -> Int
      which isn't ambiguous but is still very wrong.

      Bottom line: Try to quantify over any variable free in psig_theta,
      just like the tau-part of the type.

    * Wrinkle 3 (#13482). Also consider
        f :: forall a. _ => Int -> Int
        f x = if (undefined :: a) == undefined then x else 0
      Here we get an (Eq a) constraint, but it's not mentioned in the
      psig_theta nor the type of 'f'.  But we still want to quantify
      over 'a' even if the monomorphism restriction is on.

    * Wrinkle 4 (#14479)
        foo :: Num a => a -> a
        foo xxx = g xxx
          where
            g :: forall b. Num b => _ -> b
            g y = xxx + y

      In the signature for 'g', we cannot quantify over 'b' because it turns out to
      get unified with 'a', which is free in g's environment.  So we carefully
      refrain from bogusly quantifying, in GHC.Tc.Solver.decidePromotedTyVars.  We
      report the error later, in GHC.Tc.Gen.Bind.chooseInferredQuantifiers.

    Note [growThetaTyVars vs closeWrtFunDeps]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    GHC has two functions, growThetaTyVars and closeWrtFunDeps, both with
    the same type and similar behavior. This Note outlines the differences
    and why we use one or the other.

    Both functions take a list of constraints. We will call these the
    *candidates*.

    closeWrtFunDeps takes a set of "determined" type variables and finds the
    closure of that set with respect to the functional dependencies
    within the class constraints in the set of candidates. So, if we
    have

      class C a b | a -> b
      class D a b   -- no fundep
      candidates = {C (Maybe a) (Either b c), D (Maybe a) (Either d e)}

    then closeWrtFunDeps {a} will return the set {a,b,c}.
    This is because, if `a` is determined, then `b` and `c` are, too,
    by functional dependency. closeWrtFunDeps called with any seed set not including
    `a` will just return its argument, as only `a` determines any other
    type variable (in this example).

    growThetaTyVars operates similarly, but it behaves as if every
    constraint has a functional dependency among all its arguments.
    So, continuing our example, growThetaTyVars {a} will return
    {a,b,c,d,e}. Put another way, growThetaTyVars grows the set of
    variables to include all variables that are mentioned in the same
    constraint (transitively).

    We use closeWrtFunDeps in places where we need to know which variables are
    *always* determined by some seed set. This includes
      * when determining the mono-tyvars in decidePromotedTyVars. If `a`
        is going to be monomorphic, we need b and c to be also: they
        are determined by the choice for `a`.
      * when checking instance coverage, in
        GHC.Tc.Instance.FunDeps.checkInstCoverage

    On the other hand, we use growThetaTyVars where we need to know
    which variables *might* be determined by some seed set. This includes
      * deciding quantification (GHC.Tc.Gen.Bind.chooseInferredQuantifiers
        and decideQuantifiedTyVars
    How can `a` determine (say) `d` in the example above without a fundep?
    Suppose we have
      instance (b ~ a, c ~ a) => D (Maybe [a]) (Either b c)
    Now, if `a` turns out to be a list, it really does determine b and c.
    The danger in overdoing quantification is the creation of an ambiguous
    type signature, but this is conveniently caught in the validity checker.

    Note [Quantification with errors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we find that the RHS of the definition has some absolutely-insoluble
    constraints (including especially "variable not in scope"), we

    * Abandon all attempts to find a context to quantify over,
      and instead make the function fully-polymorphic in whatever
      type we have found

    * Return a flag from simplifyInfer, indicating that we found an
      insoluble constraint.  This flag is used to suppress the ambiguity
      check for the inferred type, which may well be bogus, and which
      tends to obscure the real error.  This fix feels a bit clunky,
      but I failed to come up with anything better.

    Reasons:
        - Avoid downstream errors
        - Do not perform an ambiguity test on a bogus type, which might well
          fail spuriously, thereby obfuscating the original insoluble error.
          #14000 is an example

    I tried an alternative approach: simply failM, after emitting the
    residual implication constraint; the exception will be caught in
    GHC.Tc.Gen.Bind.tcPolyBinds, which gives all the binders in the group the type
    (forall a. a).  But that didn't work with -fdefer-type-errors, because
    the recovery from failM emits no code at all, so there is no function
    to run!   But -fdefer-type-errors aspires to produce a runnable program.

    Note [Default while Inferring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Our current plan is that defaulting only happens at simplifyTop and
    not simplifyInfer.  This may lead to some insoluble deferred constraints.
    Example:

    instance D g => C g Int b

    constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha
    type inferred       = gamma -> gamma

    Now, if we try to default (alpha := Int) we will be able to refine the implication to
      (forall b. 0 => C gamma Int b)
    which can then be simplified further to
      (forall b. 0 => D gamma)
    Finally, we /can/ approximate this implication with (D gamma) and infer the quantified
    type:  forall g. D g => g -> g

    Instead what will currently happen is that we will get a quantified type
    (forall g. g -> g) and an implication:
           forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha

    Which, even if the simplifyTop defaults (alpha := Int) we will still be left with an
    unsolvable implication:
           forall g. 0 => (forall b. 0 => D g)

    The concrete example would be:
           h :: C g a s => g -> a -> ST s a
           f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1)

    But it is quite tedious to do defaulting and resolve the implication constraints, and
    we have not observed code breaking because of the lack of defaulting in inference, so
    we don't do it for now.

    Note [Minimize by Superclasses]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we quantify over a constraint, in simplifyInfer we need to
    quantify over a constraint that is minimal in some sense: For
    instance, if the final wanted constraint is (Eq alpha, Ord alpha),
    we'd like to quantify over Ord alpha, because we can just get Eq alpha
    from superclass selection from Ord alpha. This minimization is what
    mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
    to check the original wanted.

    Note [Avoid unnecessary constraint simplification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        -------- NB NB NB (Jun 12) -------------
        This note not longer applies; see the notes with #4361.
        But I'm leaving it in here so we remember the issue.)
        ----------------------------------------
    When inferring the type of a let-binding, with simplifyInfer,
    try to avoid unnecessarily simplifying class constraints.
    Doing so aids sharing, but it also helps with delicate
    situations like

       instance C t => C [t] where ..

       f :: C [t] => ....
       f x = let g y = ...(constraint C [t])...
             in ...
    When inferring a type for 'g', we don't want to apply the
    instance decl, because then we can't satisfy (C t).  So we
    just notice that g isn't quantified over 't' and partition
    the constraints before simplifying.

    This only half-works, but then let-generalisation only half-works.

    *********************************************************************************
    *                                                                                 *
    *                                 Main Simplifier                                 *
    *                                                                                 *
    ***********************************************************************************

    -}

    simplifyWantedsTcM :: [CtEvidence] -> TcM WantedConstraints
    -- Solve the specified Wanted constraints
    -- Discard the evidence binds
    -- Postcondition: fully zonked
    simplifyWantedsTcM :: [CtEvidence] -> IOEnv (Env TcGblEnv TcLclEnv) WantedConstraints
    simplifyWantedsTcM [CtEvidence]
    wanted
      = do { String -> SDoc -> TcM ()
    traceTc String
    "simplifyWantedsTcM {" ([CtEvidence] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [CtEvidence]
    wanted)
           ; (result, _) <- TcS WantedConstraints -> TcM (WantedConstraints, EvBindMap)
    forall a. TcS a -> TcM (a, EvBindMap)
    runTcS (WantedConstraints -> TcS WantedConstraints
    solveWanteds ([CtEvidence] -> WantedConstraints
    mkSimpleWC [CtEvidence]
    wanted))
           ; result <- TcM.liftZonkM $ TcM.zonkWC result
           ; traceTc "simplifyWantedsTcM }" (ppr result)
           ; return result }

    solveWanteds :: WantedConstraints -> TcS WantedConstraints
    solveWanteds :: WantedConstraints -> TcS WantedConstraints
    solveWanteds wc :: WantedConstraints
    wc@(WC { wc_errors :: WantedConstraints -> Bag DelayedError
    wc_errors = Bag DelayedError
    errs })
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wc  -- Fast path
      = WantedConstraints -> TcS WantedConstraints
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return WantedConstraints
    wc
      | Bool
    otherwise
      = do { cur_lvl <- TcS TcLevel
    TcS.getTcLevel
           ; traceTcS "solveWanteds {" $
             vcat [ text "Level =" <+> ppr cur_lvl
                  , ppr wc ]

           ; dflags <- getDynFlags
           ; solved_wc <- simplify_loop 0 (solverIterations dflags) True wc

           ; errs' <- simplifyDelayedErrors errs
           ; let final_wc = WantedConstraints
    solved_wc { wc_errors = errs' }

           ; ev_binds_var <- getTcEvBindsVar
           ; bb <- TcS.getTcEvBindsMap ev_binds_var
           ; traceTcS "solveWanteds }" $
                     vcat [ text "final wc =" <+> ppr final_wc
                          , text "current evbinds  =" <+> ppr (evBindMapBinds bb) ]

           ; return final_wc }

    simplify_loop :: Int -> IntWithInf -> Bool
                  -> WantedConstraints -> TcS WantedConstraints
    -- Do a round of solving, and call maybe_simplify_again to iterate
    -- The 'definitely_redo_implications' flags is False if the only reason we
    -- are iterating is that we have added some new Wanted superclasses
    -- hoping for fundeps to help us; see Note [Superclass iteration]
    --
    -- Does not affect wc_holes at all; reason: wc_holes never affects anything
    -- else, so we do them once, at the end in solveWanteds
    simplify_loop :: Int
    -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints
    simplify_loop Int
    n IntWithInf
    limit Bool
    definitely_redo_implications
                  wc :: WantedConstraints
    wc@(WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics })
      = do { SDoc -> TcS ()
    csTraceTcS (SDoc -> TcS ()) -> SDoc -> TcS ()
    forall a b. (a -> b) -> a -> b
    $
             String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "simplify_loop iteration=" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> Int -> SDoc
    forall doc. IsLine doc => Int -> doc
    int Int
    n
             SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> (SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc
    parens (SDoc -> SDoc) -> SDoc -> SDoc
    forall a b. (a -> b) -> a -> b
    $ [SDoc] -> SDoc
    forall doc. IsLine doc => [doc] -> doc
    hsep [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "definitely_redo =" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Bool -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Bool
    definitely_redo_implications SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <> SDoc
    forall doc. IsLine doc => doc
    comma
                                , Int -> SDoc
    forall doc. IsLine doc => Int -> doc
    int (Cts -> Int
    forall a. Bag a -> Int
    lengthBag Cts
    simples) SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "simples to solve" ])
           ; String -> SDoc -> TcS ()
    traceTcS String
    "simplify_loop: wc =" (WantedConstraints -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr WantedConstraints
    wc)

           ; (unifs1, wc1) <- TcS WantedConstraints -> TcS (Int, WantedConstraints)
    forall a. TcS a -> TcS (Int, a)
    reportUnifications (TcS WantedConstraints -> TcS (Int, WantedConstraints))
    -> TcS WantedConstraints -> TcS (Int, WantedConstraints)
    forall a b. (a -> b) -> a -> b
    $  -- See Note [Superclass iteration]
                              Cts -> TcS WantedConstraints
    solveSimpleWanteds Cts
    simples
                    -- Any insoluble constraints are in 'simples' and so get rewritten
                    -- See Note [Rewrite insolubles] in GHC.Tc.Solver.InertSet

           ; wc2 <- if not definitely_redo_implications  -- See Note [Superclass iteration]
                       && unifs1 == 0                    -- for this conditional
                       && isEmptyBag (wc_impl wc1)
                    then return (wc { wc_simple = wc_simple wc1 })  -- Short cut
                    else do { implics2 <- solveNestedImplications $
                                          implics `unionBags` (wc_impl wc1)
                            ; return (wc { wc_simple = wc_simple wc1
                                         , wc_impl = implics2 }) }

           ; unif_happened <- resetUnificationFlag
           ; csTraceTcS $ text "unif_happened" <+> ppr unif_happened
             -- Note [The Unification Level Flag] in GHC.Tc.Solver.Monad
           ; maybe_simplify_again (n+1) limit unif_happened wc2 }

    maybe_simplify_again :: Int -> IntWithInf -> Bool
                         -> WantedConstraints -> TcS WantedConstraints
    maybe_simplify_again :: Int
    -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints
    maybe_simplify_again Int
    n IntWithInf
    limit Bool
    unif_happened wc :: WantedConstraints
    wc@(WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples })
      | Int
    n Int -> IntWithInf -> Bool
    `intGtLimit` IntWithInf
    limit
      = do { -- Add an error (not a warning) if we blow the limit,
             -- Typically if we blow the limit we are going to report some other error
             -- (an unsolved constraint), and we don't want that error to suppress
             -- the iteration limit warning!
             TcRnMessage -> TcS ()
    addErrTcS (TcRnMessage -> TcS ()) -> TcRnMessage -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ Cts -> IntWithInf -> WantedConstraints -> TcRnMessage
    TcRnSimplifierTooManyIterations Cts
    simples IntWithInf
    limit WantedConstraints
    wc
           ; WantedConstraints -> TcS WantedConstraints
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return WantedConstraints
    wc }

      | Bool
    unif_happened
      = Int
    -> IntWithInf -> Bool -> WantedConstraints -> TcS WantedConstraints
    simplify_loop Int
    n IntWithInf
    limit Bool
    True WantedConstraints
    wc

      | WantedConstraints -> Bool
    superClassesMightHelp WantedConstraints
    wc
      = -- We still have unsolved goals, and apparently no way to solve them,
        -- so try expanding superclasses at this level, both Given and Wanted
        do { pending_given <- TcS [Ct]
    getPendingGivenScs
           ; let (pending_wanted, simples1) = getPendingWantedScs simples
           ; if null pending_given && null pending_wanted
               then return wc  -- After all, superclasses did not help
               else
        do { new_given  <- makeSuperClasses pending_given
           ; new_wanted <- makeSuperClasses pending_wanted
           ; solveSimpleGivens new_given -- Add the new Givens to the inert set
           ; traceTcS "maybe_simplify_again" (vcat [ text "pending_given" <+> ppr pending_given
                                                   , text "new_given" <+> ppr new_given
                                                   , text "pending_wanted" <+> ppr pending_wanted
                                                   , text "new_wanted" <+> ppr new_wanted ])
           ; simplify_loop n limit (not (null pending_given)) $
             wc { wc_simple = simples1 `unionBags` listToBag new_wanted } } }
             -- (not (null pending_given)): see Note [Superclass iteration]

      | Bool
    otherwise
      = WantedConstraints -> TcS WantedConstraints
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return WantedConstraints
    wc

    {- Note [Superclass iteration]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this implication constraint
        forall a.
           [W] d: C Int beta
           forall b. blah
    where
      class D a b | a -> b
      class D a b => C a b
    We will expand d's superclasses, giving [W] D Int beta, in the hope of geting
    fundeps to unify beta.  Doing so is usually fruitless (no useful fundeps),
    and if so it seems a pity to waste time iterating the implications (forall b. blah)
    (If we add new Given superclasses it's a different matter: it's really worth looking
    at the implications.)

    Hence the definitely_redo_implications flag to simplify_loop.  It's usually
    True, but False in the case where the only reason to iterate is new Wanted
    superclasses.  In that case we check whether the new Wanteds actually led to
    any new unifications, and iterate the implications only if so.
    -}

    {- Note [Expanding Recursive Superclasses and ExpansionFuel]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the class declaration (T21909)

        class C [a] => C a where
           foo :: a -> Int

    and suppose during type inference we obtain an implication constraint:

        forall a. C a => C [[a]]

    To solve this implication constraint, we first expand one layer of the superclass
    of Given constraints, but not for Wanted constraints.
    (See Note [Eagerly expand given superclasses] and Note [Why adding superclasses can help]
    in GHC.Tc.Solver.Dict.) We thus get:

        [G] g1 :: C a
        [G] g2 :: C [a]    -- new superclass layer from g1
        [W] w1 :: C [[a]]

    Now, we cannot solve `w1` directly from `g1` or `g2` as we may not have
    any instances for C. So we expand a layer of superclasses of each Wanteds and Givens
    that we haven't expanded yet.
    This is done in `maybe_simplify_again`. And we get:

        [G] g1 :: C a
        [G] g2 :: C [a]
        [G] g3 :: C [[a]]    -- new superclass layer from g2, can solve w1
        [W] w1 :: C [[a]]
        [W] w2 :: C [[[a]]]  -- new superclass layer from w1, not solvable

    Now, although we can solve `w1` using `g3` (obtained from expanding `g2`),
    we have a new wanted constraint `w2` (obtained from expanding `w1`) that cannot be solved.
    We thus make another go at solving in `maybe_simplify_again` by expanding more
    layers of superclasses. This looping is futile as Givens will never be able to catch up with Wanteds.

    Side Note: In principle we don't actually need to /solve/ `w2`, as it is a superclass of `w1`
    but we only expand it to expose any functional dependencies (see Note [The superclass story])
    But `w2` is a wanted constraint, so we will try to solve it like any other,
    even though ultimately we will discard its evidence.

    Solution: Simply bound the maximum number of layers of expansion for
    Givens and Wanteds, with ExpansionFuel.  Give the Givens more fuel
    (say 3 layers) than the Wanteds (say 1 layer). Now the Givens will
    win.  The Wanteds don't need much fuel: we are only expanding at all
    to expose functional dependencies, and wantedFuel=1 means we will
    expand a full recursive layer.  If the superclass hierarchy is
    non-recursive (the normal case) one layer is therefore full expansion.

    The default value for wantedFuel = Constants.max_WANTEDS_FUEL = 1.
    The default value for givenFuel  = Constants.max_GIVENS_FUEL = 3.
    Both are configurable via the `-fgivens-fuel` and `-fwanteds-fuel`
    compiler flags.

    There are two preconditions for the default fuel values:
       (1) default givenFuel >= default wantedsFuel
       (2) default givenFuel < solverIterations

    Precondition (1) ensures that we expand givens at least as many times as we expand wanted constraints
    preferably givenFuel > wantedsFuel to avoid issues like T21909 while
    the precondition (2) ensures that we do not reach the solver iteration limit and fail with a
    more meaningful error message (see T19627)

    This also applies for quantified constraints; see `-fqcs-fuel` compiler flag and `QCI.qci_pend_sc` field.
    -}

    solveNestedImplications :: Bag Implication
                            -> TcS (Bag Implication)
    -- Precondition: the TcS inerts may contain unsolved simples which have
    -- to be converted to givens before we go inside a nested implication.
    solveNestedImplications :: Bag Implication -> TcS (Bag Implication)
    solveNestedImplications Bag Implication
    implics
      | Bag Implication -> Bool
    forall a. Bag a -> Bool
    isEmptyBag Bag Implication
    implics
      = Bag Implication -> TcS (Bag Implication)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Bag Implication
    forall a. Bag a
    emptyBag)
      | Bool
    otherwise
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "solveNestedImplications starting {" SDoc
    forall doc. IsOutput doc => doc
    empty
           ; unsolved_implics <- (Implication -> TcS (Maybe Implication))
    -> Bag Implication -> TcS (Bag (Maybe Implication))
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> Bag a -> m (Bag b)
    mapBagM Implication -> TcS (Maybe Implication)
    solveImplication Bag Implication
    implics

           -- ... and we are back in the original TcS inerts
           -- Notice that the original includes the _insoluble_simples so it was safe to ignore
           -- them in the beginning of this function.
           ; traceTcS "solveNestedImplications end }" $
                      vcat [ text "unsolved_implics =" <+> ppr unsolved_implics ]

           ; return (catBagMaybes unsolved_implics) }

    solveImplication :: Implication    -- Wanted
                     -> TcS (Maybe Implication) -- Simplified implication (empty or singleton)
    -- Precondition: The TcS monad contains an empty worklist and given-only inerts
    -- which after trying to solve this implication we must restore to their original value
    solveImplication :: Implication -> TcS (Maybe Implication)
    solveImplication imp :: Implication
    imp@(Implic { ic_tclvl :: Implication -> TcLevel
    ic_tclvl  = TcLevel
    tclvl
                                 , ic_binds :: Implication -> EvBindsVar
    ic_binds  = EvBindsVar
    ev_binds_var
                                 , ic_given :: Implication -> [TyVar]
    ic_given  = [TyVar]
    given_ids
                                 , ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WantedConstraints
    wanteds
                                 , ic_info :: Implication -> SkolemInfoAnon
    ic_info   = SkolemInfoAnon
    info
                                 , ic_status :: Implication -> ImplicStatus
    ic_status = ImplicStatus
    status })
      | ImplicStatus -> Bool
    isSolvedStatus ImplicStatus
    status
      = Maybe Implication -> TcS (Maybe Implication)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (Implication -> Maybe Implication
    forall a. a -> Maybe a
    Just Implication
    imp)  -- Do nothing

      | Bool
    otherwise  -- Even for IC_Insoluble it is worth doing more work
                   -- The insoluble stuff might be in one sub-implication
                   -- and other unsolved goals in another; and we want to
                   -- solve the latter as much as possible
      = do { inerts <- TcS InertSet
    getInertSet
           ; traceTcS "solveImplication {" (ppr imp $$ text "Inerts" <+> ppr inerts)

           -- commented out; see `where` clause below
           -- ; when debugIsOn check_tc_level

             -- Solve the nested constraints
           ; (has_given_eqs, given_insols, residual_wanted)
                <- nestImplicTcS ev_binds_var tclvl $
                   do { let loc    = TcLevel -> SkolemInfoAnon -> CtLocEnv -> CtLoc
    mkGivenLoc TcLevel
    tclvl SkolemInfoAnon
    info (Implication -> CtLocEnv
    ic_env Implication
    imp)
                            givens = CtLoc -> [TyVar] -> [Ct]
    mkGivens CtLoc
    loc [TyVar]
    given_ids
                      ; solveSimpleGivens givens

                      ; residual_wanted <- solveWanteds wanteds

                      ; (has_eqs, given_insols) <- getHasGivenEqs tclvl
                            -- Call getHasGivenEqs /after/ solveWanteds, because
                            -- solveWanteds can augment the givens, via expandSuperClasses,
                            -- to reveal given superclass equalities

                      ; return (has_eqs, given_insols, residual_wanted) }

           ; traceTcS "solveImplication 2"
               (ppr given_insols $$ ppr residual_wanted)
           ; let final_wanted = WantedConstraints
    residual_wanted WantedConstraints -> InertIrreds -> WantedConstraints
    `addInsols` InertIrreds
    given_insols
                 -- Don't lose track of the insoluble givens,
                 -- which signal unreachable code; put them in ic_wanted

           ; res_implic <- setImplicationStatus (imp { ic_given_eqs = has_given_eqs
                                                     , ic_wanted = final_wanted })

           ; evbinds <- TcS.getTcEvBindsMap ev_binds_var
           ; tcvs    <- TcS.getTcEvTyCoVars ev_binds_var
           ; traceTcS "solveImplication end }" $ vcat
                 [ text "has_given_eqs =" <+> ppr has_given_eqs
                 , text "res_implic =" <+> ppr res_implic
                 , text "implication evbinds =" <+> ppr (evBindMapBinds evbinds)
                 , text "implication tvcs =" <+> ppr tcvs ]

           ; return res_implic }

        -- TcLevels must be strictly increasing (see (ImplicInv) in
        -- Note [TcLevel invariants] in GHC.Tc.Utils.TcType),
        -- and in fact I think they should always increase one level at a time.

        -- Though sensible, this check causes lots of testsuite failures. It is
        -- remaining commented out for now.
        {-
        check_tc_level = do { cur_lvl <- TcS.getTcLevel
                            ; massertPpr (tclvl == pushTcLevel cur_lvl)
                                         (text "Cur lvl =" <+> ppr cur_lvl $$ text "Imp lvl =" <+> ppr tclvl) }
        -}

    ----------------------
    setImplicationStatus :: Implication -> TcS (Maybe Implication)
    -- Finalise the implication returned from solveImplication,
    -- setting the ic_status field
    -- Precondition: the ic_status field is not already IC_Solved
    -- Return Nothing if we can discard the implication altogether
    setImplicationStatus :: Implication -> TcS (Maybe Implication)
    setImplicationStatus implic :: Implication
    implic@(Implic { ic_status :: Implication -> ImplicStatus
    ic_status     = ImplicStatus
    status
                                        , ic_info :: Implication -> SkolemInfoAnon
    ic_info       = SkolemInfoAnon
    info
                                        , ic_wanted :: Implication -> WantedConstraints
    ic_wanted     = WantedConstraints
    wc
                                        , ic_given :: Implication -> [TyVar]
    ic_given      = [TyVar]
    givens })
     | Bool -> SDoc -> Bool -> Bool
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (Bool -> Bool
    not (ImplicStatus -> Bool
    isSolvedStatus ImplicStatus
    status)) (SkolemInfoAnon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr SkolemInfoAnon
    info) (Bool -> Bool) -> Bool -> Bool
    forall a b. (a -> b) -> a -> b
    $
       -- Precondition: we only set the status if it is not already solved
       Bool -> Bool
    not (WantedConstraints -> Bool
    isSolvedWC WantedConstraints
    pruned_wc)
     = do { String -> SDoc -> TcS ()
    traceTcS String
    "setImplicationStatus(not-all-solved) {" (Implication -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Implication
    implic)

          ; implic <- Implication -> TcS Implication
    neededEvVars Implication
    implic

          ; let new_status | WantedConstraints -> Bool
    insolubleWC WantedConstraints
    pruned_wc = ImplicStatus
    IC_Insoluble
                           | Bool
    otherwise             = ImplicStatus
    IC_Unsolved
                new_implic = Implication
    implic { ic_status = new_status
                                    , ic_wanted = pruned_wc }

          ; traceTcS "setImplicationStatus(not-all-solved) }" (ppr new_implic)

          ; return $ Just new_implic }

     | Bool
    otherwise  -- Everything is solved
                  -- Set status to IC_Solved,
                  -- and compute the dead givens and outer needs
                  -- See Note [Tracking redundant constraints]
     = do { String -> SDoc -> TcS ()
    traceTcS String
    "setImplicationStatus(all-solved) {" (Implication -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Implication
    implic)

          ; implic@(Implic { ic_need_inner = need_inner
                           , ic_need_outer = need_outer }) <- Implication -> TcS Implication
    neededEvVars Implication
    implic

          ; bad_telescope <- checkBadTelescope implic

          ; let warn_givens = SkolemInfoAnon -> VarSet -> [TyVar] -> [TyVar]
    findUnnecessaryGivens SkolemInfoAnon
    info VarSet
    need_inner [TyVar]
    givens

                discard_entire_implication  -- Can we discard the entire implication?
                  =  [TyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TyVar]
    warn_givens           -- No warning from this implication
                  Bool -> Bool -> Bool
    && Bool -> Bool
    not Bool
    bad_telescope
                  Bool -> Bool -> Bool
    && WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    pruned_wc        -- No live children
                  Bool -> Bool -> Bool
    && VarSet -> Bool
    isEmptyVarSet VarSet
    need_outer   -- No needed vars to pass up to parent

                final_status
                  | Bool
    bad_telescope = ImplicStatus
    IC_BadTelescope
                  | Bool
    otherwise     = IC_Solved { ics_dead :: [TyVar]
    ics_dead = [TyVar]
    warn_givens }
                final_implic = Implication
    implic { ic_status = final_status
                                      , ic_wanted = pruned_wc }

          ; traceTcS "setImplicationStatus(all-solved) }" $
            vcat [ text "discard:" <+> ppr discard_entire_implication
                 , text "new_implic:" <+> ppr final_implic ]

          ; return $ if discard_entire_implication
                     then Nothing
                     else Just final_implic }
     where
       WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics, wc_errors :: WantedConstraints -> Bag DelayedError
    wc_errors = Bag DelayedError
    errs } = WantedConstraints
    wc

       pruned_implics :: Bag Implication
    pruned_implics = (Implication -> Bool) -> Bag Implication -> Bag Implication
    forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag Implication -> Bool
    keep_me Bag Implication
    implics
       pruned_wc :: WantedConstraints
    pruned_wc = WC { wc_simple :: Cts
    wc_simple = Cts
    simples
                      , wc_impl :: Bag Implication
    wc_impl   = Bag Implication
    pruned_implics
                      , wc_errors :: Bag DelayedError
    wc_errors = Bag DelayedError
    errs }   -- do not prune holes; these should be reported

       keep_me :: Implication -> Bool
       keep_me :: Implication -> Bool
    keep_me Implication
    ic
         | IC_Solved { ics_dead :: ImplicStatus -> [TyVar]
    ics_dead = [TyVar]
    dead_givens } <- Implication -> ImplicStatus
    ic_status Implication
    ic
                              -- Fully solved
         , [TyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TyVar]
    dead_givens   -- No redundant givens to report
         , Bag Implication -> Bool
    forall a. Bag a -> Bool
    isEmptyBag (WantedConstraints -> Bag Implication
    wc_impl (Implication -> WantedConstraints
    ic_wanted Implication
    ic))
               -- And no children that might have things to report
         = Bool
    False       -- Tnen we don't need to keep it
         | Bool
    otherwise
         = Bool
    True        -- Otherwise, keep it

    findUnnecessaryGivens :: SkolemInfoAnon -> VarSet -> [EvVar] -> [EvVar]
    findUnnecessaryGivens :: SkolemInfoAnon -> VarSet -> [TyVar] -> [TyVar]
    findUnnecessaryGivens SkolemInfoAnon
    info VarSet
    need_inner [TyVar]
    givens
      | Bool -> Bool
    not (SkolemInfoAnon -> Bool
    warnRedundantGivens SkolemInfoAnon
    info)   -- Don't report redundant constraints at all
      = []

      | Bool -> Bool
    not ([TyVar] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TyVar]
    unused_givens)         -- Some givens are literally unused
      = [TyVar]
    unused_givens

      | Bool
    otherwise                       -- All givens are used, but some might
      = [TyVar]
    redundant_givens                -- still be redundant e.g. (Eq a, Ord a)

      where
        in_instance_decl :: Bool
    in_instance_decl = case SkolemInfoAnon
    info of { InstSkol {} -> Bool
    True; SkolemInfoAnon
    _ -> Bool
    False }
                           -- See Note [Redundant constraints in instance decls]

        unused_givens :: [TyVar]
    unused_givens = (TyVar -> Bool) -> [TyVar] -> [TyVar]
    forall a. (a -> Bool) -> [a] -> [a]
    filterOut TyVar -> Bool
    is_used [TyVar]
    givens

        is_used :: TyVar -> Bool
    is_used TyVar
    given =  TyVar -> Bool
    is_type_error TyVar
    given
                      Bool -> Bool -> Bool
    || TyVar
    given TyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    need_inner
                      Bool -> Bool -> Bool
    || (Bool
    in_instance_decl Bool -> Bool -> Bool
    && Type -> Bool
    is_improving (TyVar -> Type
    idType TyVar
    given))

        minimal_givens :: [TyVar]
    minimal_givens = (TyVar -> Type) -> [TyVar] -> [TyVar]
    forall a. (a -> Type) -> [a] -> [a]
    mkMinimalBySCs TyVar -> Type
    evVarPred [TyVar]
    givens
        is_minimal :: TyVar -> Bool
    is_minimal = (TyVar -> VarSet -> Bool
    `elemVarSet` [TyVar] -> VarSet
    mkVarSet [TyVar]
    minimal_givens)
        redundant_givens :: [TyVar]
    redundant_givens
          | Bool
    in_instance_decl = []
          | Bool
    otherwise        = (TyVar -> Bool) -> [TyVar] -> [TyVar]
    forall a. (a -> Bool) -> [a] -> [a]
    filterOut TyVar -> Bool
    is_minimal [TyVar]
    givens

        -- See #15232
        is_type_error :: TyVar -> Bool
    is_type_error TyVar
    id = Type -> Bool
    isTopLevelUserTypeError (TyVar -> Type
    idType TyVar
    id)

        is_improving :: Type -> Bool
    is_improving Type
    pred -- (transSuperClasses p) does not include p
          = (Type -> Bool) -> [Type] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    any Type -> Bool
    isImprovementPred (Type
    pred Type -> [Type] -> [Type]
    forall a. a -> [a] -> [a]
    : Type -> [Type]
    transSuperClasses Type
    pred)

    {- Note [Redundant constraints in instance decls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Instance declarations are special in two ways:

    * We don't report unused givens if they can give rise to improvement.
      Example (#10100):
        class Add a b ab | a b -> ab, a ab -> b
        instance Add Zero b b
        instance Add a b ab => Add (Succ a) b (Succ ab)
      The context (Add a b ab) for the instance is clearly unused in terms
      of evidence, since the dictionary has no fields.  But it is still
      needed!  With the context, a wanted constraint
         Add (Succ Zero) beta (Succ Zero)
      we will reduce to (Add Zero beta Zero), and thence we get beta := Zero.
      But without the context we won't find beta := Zero.

      This only matters in instance declarations.

    * We don't report givens that are a superclass of another given. E.g.
           class Ord r => UserOfRegs r a where ...
           instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
      The (Ord r) is not redundant, even though it is a superclass of
      (UserOfRegs r CmmReg).  See Note [Recursive superclasses] in GHC.Tc.TyCl.Instance.

      Again this is specific to instance declarations.
    -}

    checkBadTelescope :: Implication -> TcS Bool
    -- True <=> the skolems form a bad telescope
    -- See Note [Checking telescopes] in GHC.Tc.Types.Constraint
    checkBadTelescope :: Implication -> TcS Bool
    checkBadTelescope (Implic { ic_info :: Implication -> SkolemInfoAnon
    ic_info  = SkolemInfoAnon
    info
                              , ic_skols :: Implication -> [TyVar]
    ic_skols = [TyVar]
    skols })
      | SkolemInfoAnon -> Bool
    checkTelescopeSkol SkolemInfoAnon
    info
      = do{ skols <- (TyVar -> TcS TyVar) -> [TyVar] -> TcS [TyVar]
    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 TyVar -> TcS TyVar
    TcS.zonkTyCoVarKind [TyVar]
    skols
          ; return (go emptyVarSet (reverse skols))}

      | Bool
    otherwise
      = Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False

      where
        go :: TyVarSet   -- skolems that appear *later* than the current ones
           -> [TcTyVar]  -- ordered skolems, in reverse order
           -> Bool       -- True <=> there is an out-of-order skolem
        go :: VarSet -> [TyVar] -> Bool
    go VarSet
    _ [] = Bool
    False
        go VarSet
    later_skols (TyVar
    one_skol : [TyVar]
    earlier_skols)
          | Type -> VarSet
    tyCoVarsOfType (TyVar -> Type
    tyVarKind TyVar
    one_skol) VarSet -> VarSet -> Bool
    `intersectsVarSet` VarSet
    later_skols
          = Bool
    True
          | Bool
    otherwise
          = VarSet -> [TyVar] -> Bool
    go (VarSet
    later_skols VarSet -> TyVar -> VarSet
    `extendVarSet` TyVar
    one_skol) [TyVar]
    earlier_skols

    warnRedundantGivens :: SkolemInfoAnon -> Bool
    warnRedundantGivens :: SkolemInfoAnon -> Bool
    warnRedundantGivens (SigSkol UserTypeCtxt
    ctxt Type
    _ [(Name, TyVar)]
    _)
      = case UserTypeCtxt
    ctxt of
           FunSigCtxt Name
    _ ReportRedundantConstraints
    rrc -> ReportRedundantConstraints -> Bool
    reportRedundantConstraints ReportRedundantConstraints
    rrc
           ExprSigCtxt ReportRedundantConstraints
    rrc  -> ReportRedundantConstraints -> Bool
    reportRedundantConstraints ReportRedundantConstraints
    rrc
           UserTypeCtxt
    _                -> Bool
    False

      -- To think about: do we want to report redundant givens for
      -- pattern synonyms, PatSynSigSkol? c.f #9953, comment:21.
    warnRedundantGivens (InstSkol {}) = Bool
    True
    warnRedundantGivens SkolemInfoAnon
    _             = Bool
    False

    neededEvVars :: Implication -> TcS Implication
    -- Find all the evidence variables that are "needed",
    -- and delete dead evidence bindings
    --   See Note [Tracking redundant constraints]
    --   See Note [Delete dead Given evidence bindings]
    --
    --   - Start from initial_seeds (from nested implications)
    --
    --   - Add free vars of RHS of all Wanted evidence bindings
    --     and coercion variables accumulated in tcvs (all Wanted)
    --
    --   - Generate 'needed', the needed set of EvVars, by doing transitive
    --     closure through Given bindings
    --     e.g.   Needed {a,b}
    --            Given  a = sc_sel a2
    --            Then a2 is needed too
    --
    --   - Prune out all Given bindings that are not needed
    --
    --   - From the 'needed' set, delete ev_bndrs, the binders of the
    --     evidence bindings, to give the final needed variables
    --
    neededEvVars :: Implication -> TcS Implication
    neededEvVars implic :: Implication
    implic@(Implic { ic_given :: Implication -> [TyVar]
    ic_given = [TyVar]
    givens
                                , ic_binds :: Implication -> EvBindsVar
    ic_binds = EvBindsVar
    ev_binds_var
                                , ic_wanted :: Implication -> WantedConstraints
    ic_wanted = WC { wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics }
                                , ic_need_inner :: Implication -> VarSet
    ic_need_inner = VarSet
    old_needs })
     = do { ev_binds <- EvBindsVar -> TcS EvBindMap
    TcS.getTcEvBindsMap EvBindsVar
    ev_binds_var
          ; tcvs     <- TcS.getTcEvTyCoVars ev_binds_var

          ; let seeds1        = (Implication -> VarSet -> VarSet)
    -> VarSet -> Bag Implication -> VarSet
    forall a b. (a -> b -> b) -> b -> Bag a -> b
    forall (t :: * -> *) a b.
    Foldable t =>
    (a -> b -> b) -> b -> t a -> b
    foldr Implication -> VarSet -> VarSet
    add_implic_seeds VarSet
    old_needs Bag Implication
    implics
                seeds2        = (EvBind -> VarSet -> VarSet) -> VarSet -> EvBindMap -> VarSet
    forall a. (EvBind -> a -> a) -> a -> EvBindMap -> a
    nonDetStrictFoldEvBindMap EvBind -> VarSet -> VarSet
    add_wanted VarSet
    seeds1 EvBindMap
    ev_binds
                                -- It's OK to use a non-deterministic fold here
                                -- because add_wanted is commutative
                seeds3        = VarSet
    seeds2 VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    tcvs
                need_inner    = EvBindMap -> VarSet -> VarSet
    findNeededEvVars EvBindMap
    ev_binds VarSet
    seeds3
                live_ev_binds = (EvBind -> Bool) -> EvBindMap -> EvBindMap
    filterEvBindMap (VarSet -> EvBind -> Bool
    needed_ev_bind VarSet
    need_inner) EvBindMap
    ev_binds
                need_outer    = VarSet -> EvBindMap -> VarSet
    varSetMinusEvBindMap VarSet
    need_inner EvBindMap
    live_ev_binds
                                VarSet -> [TyVar] -> VarSet
    `delVarSetList` [TyVar]
    givens

          ; TcS.setTcEvBindsMap ev_binds_var live_ev_binds
               -- See Note [Delete dead Given evidence bindings]

          ; traceTcS "neededEvVars" $
            vcat [ text "old_needs:" <+> ppr old_needs
                 , text "seeds3:" <+> ppr seeds3
                 , text "tcvs:" <+> ppr tcvs
                 , text "ev_binds:" <+> ppr ev_binds
                 , text "live_ev_binds:" <+> ppr live_ev_binds ]

          ; return (implic { ic_need_inner = need_inner
                           , ic_need_outer = need_outer }) }
     where
       add_implic_seeds :: Implication -> VarSet -> VarSet
    add_implic_seeds (Implic { ic_need_outer :: Implication -> VarSet
    ic_need_outer = VarSet
    needs }) VarSet
    acc
          = VarSet
    needs VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    acc

       needed_ev_bind :: VarSet -> EvBind -> Bool
    needed_ev_bind VarSet
    needed (EvBind { eb_lhs :: EvBind -> TyVar
    eb_lhs = TyVar
    ev_var
                                     , eb_info :: EvBind -> EvBindInfo
    eb_info = EvBindInfo
    info })
         | EvBindGiven{} <- EvBindInfo
    info = TyVar
    ev_var TyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    needed
         | Bool
    otherwise = Bool
    True   -- Keep all wanted bindings

       add_wanted :: EvBind -> VarSet -> VarSet
       add_wanted :: EvBind -> VarSet -> VarSet
    add_wanted (EvBind { eb_info :: EvBind -> EvBindInfo
    eb_info = EvBindInfo
    info, eb_rhs :: EvBind -> EvTerm
    eb_rhs = EvTerm
    rhs }) VarSet
    needs
         | EvBindGiven{} <- EvBindInfo
    info = VarSet
    needs  -- Add the rhs vars of the Wanted bindings only
         | Bool
    otherwise = EvTerm -> VarSet
    evVarsOfTerm EvTerm
    rhs VarSet -> VarSet -> VarSet
    `unionVarSet` VarSet
    needs

    -------------------------------------------------
    simplifyDelayedErrors :: Bag DelayedError -> TcS (Bag DelayedError)
    -- Simplify any delayed errors: e.g. type and term holes
    -- NB: At this point we have finished with all the simple
    --     constraints; they are in wc_simple, not in the inert set.
    --     So those Wanteds will not rewrite these delayed errors.
    --     That's probably no bad thing.
    --
    --     However if we have [W] alpha ~ Maybe a, [W] alpha ~ Int
    --     and _ : alpha, then we'll /unify/ alpha with the first of
    --     the Wanteds we get, and thereby report (_ : Maybe a) or
    --     (_ : Int) unpredictably, depending on which we happen to see
    --     first.  Doesn't matter much; there is a type error anyhow.
    --     T17139 is a case in point.
    simplifyDelayedErrors :: Bag DelayedError -> TcS (Bag DelayedError)
    simplifyDelayedErrors = (DelayedError -> TcS DelayedError)
    -> Bag DelayedError -> TcS (Bag DelayedError)
    forall (m :: * -> *) a b.
    Monad m =>
    (a -> m b) -> Bag a -> m (Bag b)
    mapBagM DelayedError -> TcS DelayedError
    simpl_err
      where
        simpl_err :: DelayedError -> TcS DelayedError
        simpl_err :: DelayedError -> TcS DelayedError
    simpl_err (DE_Hole Hole
    hole) = Hole -> DelayedError
    DE_Hole (Hole -> DelayedError) -> TcS Hole -> TcS DelayedError
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    <$> Hole -> TcS Hole
    simpl_hole Hole
    hole
        simpl_err err :: DelayedError
    err@(DE_NotConcrete {}) = DelayedError -> TcS DelayedError
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return DelayedError
    err

        simpl_hole :: Hole -> TcS Hole

         -- See Note [Do not simplify ConstraintHoles]
        simpl_hole :: Hole -> TcS Hole
    simpl_hole h :: Hole
    h@(Hole { hole_sort :: Hole -> HoleSort
    hole_sort = HoleSort
    ConstraintHole }) = Hole -> TcS Hole
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Hole
    h

         -- other wildcards should be simplified for printing
         -- we must do so here, and not in the error-message generation
         -- code, because we have all the givens already set up
        simpl_hole h :: Hole
    h@(Hole { hole_ty :: Hole -> Type
    hole_ty = Type
    ty, hole_loc :: Hole -> CtLoc
    hole_loc = CtLoc
    loc })
          = do { ty' <- CtLoc -> Type -> TcS Type
    rewriteType CtLoc
    loc Type
    ty
               ; traceTcS "simpl_hole" (ppr ty $$ ppr ty')
               ; return (h { hole_ty = ty' }) }

    {- Note [Delete dead Given evidence bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    As a result of superclass expansion, we speculatively
    generate evidence bindings for Givens. E.g.
       f :: (a ~ b) => a -> b -> Bool
       f x y = ...
    We'll have
       [G] d1 :: (a~b)
    and we'll speculatively generate the evidence binding
       [G] d2 :: (a ~# b) = sc_sel d

    Now d2 is available for solving.  But it may not be needed!  Usually
    such dead superclass selections will eventually be dropped as dead
    code, but:

     * It won't always be dropped (#13032).  In the case of an
       unlifted-equality superclass like d2 above, we generate
           case heq_sc d1 of d2 -> ...
       and we can't (in general) drop that case expression in case
       d1 is bottom.  So it's technically unsound to have added it
       in the first place.

     * Simply generating all those extra superclasses can generate lots of
       code that has to be zonked, only to be discarded later.  Better not
       to generate it in the first place.

       Moreover, if we simplify this implication more than once
       (e.g. because we can't solve it completely on the first iteration
       of simpl_loop), we'll generate all the same bindings AGAIN!

    Easy solution: take advantage of the work we are doing to track dead
    (unused) Givens, and use it to prune the Given bindings too.  This is
    all done by neededEvVars.

    This led to a remarkable 25% overall compiler allocation decrease in
    test T12227.

    But we don't get to discard all redundant equality superclasses, alas;
    see #15205.

    Note [Do not simplify ConstraintHoles]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Before printing the inferred value for a type hole (a _ wildcard in
    a partial type signature), we simplify it w.r.t. any Givens. This
    makes for an easier-to-understand diagnostic for the user.

    However, we do not wish to do this for extra-constraint holes. Here is
    the example for why (partial-sigs/should_compile/T12844):

      bar :: _ => FooData rngs
      bar = foo

      data FooData rngs

      class Foo xs where foo :: (Head xs ~ '(r,r')) => FooData xs

      type family Head (xs :: [k]) where Head (x ': xs) = x

    GHC correctly infers that the extra-constraints wildcard on `bar`
    should be (Head rngs ~ '(r, r'), Foo rngs). It then adds this
    constraint as a Given on the implication constraint for `bar`. (This
    implication is emitted by emitResidualConstraints.) The Hole for the _
    is stored within the implication's WantedConstraints.  When
    simplifyHoles is called, that constraint is already assumed as a
    Given. Simplifying with respect to it turns it into ('(r, r') ~ '(r,
    r'), Foo rngs), which is disastrous.

    Furthermore, there is no need to simplify here: extra-constraints wildcards
    are filled in with the output of the solver, in chooseInferredQuantifiers
    (choose_psig_context), so they are already simplified. (Contrast to normal
    type holes, which are just bound to a meta-variable.) Avoiding the poor output
    is simple: just don't simplify extra-constraints wildcards.

    This is the only reason we need to track ConstraintHole separately
    from TypeHole in HoleSort.

    See also Note [Extra-constraint holes in partial type signatures]
    in GHC.Tc.Gen.HsType.

    Note [Tracking redundant constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    With Opt_WarnRedundantConstraints, GHC can report which
    constraints of a type signature (or instance declaration) are
    redundant, and can be omitted.  Here is an overview of how it
    works.

    This is all tested in typecheck/should_compile/T20602 (among
    others).

    ----- What is a redundant constraint?

    * The things that can be redundant are precisely the Given
      constraints of an implication.

    * A constraint can be redundant in two different ways:
      a) It is not needed by the Wanted constraints covered by the
         implication E.g.
           f :: Eq a => a -> Bool
           f x = True  -- Equality not used
      b) It is implied by other givens.  E.g.
           f :: (Eq a, Ord a)     => blah   -- Eq a unnecessary
           g :: (Eq a, a~b, Eq b) => blah   -- Either Eq a or Eq b unnecessary

    *  To find (a) we need to know which evidence bindings are 'wanted';
       hence the eb_is_given field on an EvBind.

    *  To find (b), we use mkMinimalBySCs on the Givens to see if any
       are unnecessary.

    ----- How tracking works

    (RC1) When two Givens are the same, we drop the evidence for the one
      that requires more superclass selectors. This is done
      according to 2(c) of Note [Replacement vs keeping] in GHC.Tc.Solver.InertSet.

    (RC2) The ic_need fields of an Implic records in-scope (given) evidence
      variables bound by the context, that were needed to solve this
      implication (so far).  See the declaration of Implication.

    (RC3) setImplicationStatus:
      When the constraint solver finishes solving all the wanteds in
      an implication, it sets its status to IC_Solved

      - The ics_dead field, of IC_Solved, records the subset of this
        implication's ic_given that are redundant (not needed).

      - We compute which evidence variables are needed by an implication
        in setImplicationStatus.  A variable is needed if
        a) it is free in the RHS of a Wanted EvBind,
        b) it is free in the RHS of an EvBind whose LHS is needed, or
        c) it is in the ics_need of a nested implication.

      - After computing which variables are needed, we then look at the
        remaining variables for internal redundancies. This is case (b)
        from above. This is also done in setImplicationStatus.
        Note that we only look for case (b) if case (a) shows up empty,
        as exemplified below.

      - We need to be careful not to discard an implication
        prematurely, even one that is fully solved, because we might
        thereby forget which variables it needs, and hence wrongly
        report a constraint as redundant.  But we can discard it once
        its free vars have been incorporated into its parent; or if it
        simply has no free vars. This careful discarding is also
        handled in setImplicationStatus.

    (RC4) We do not want to report redundant constraints for implications
      that come from quantified constraints.  Example #23323:
         data T a
         instance Show (T a) where ...  -- No context!
         foo :: forall f c. (forall a. c a => Show (f a)) => Proxy c -> f Int -> Int
         bar = foo @T @Eq

      The call to `foo` gives us
        [W] d : (forall a. Eq a => Show (T a))
      To solve this, GHC.Tc.Solver.Solve.solveForAll makes an implication constraint:
        forall a. Eq a =>  [W] ds : Show (T a)
      and because of the degnerate instance for `Show (T a)`, we don't need the `Eq a`
      constraint.  But we don't want to report it as redundant!

    * Examples:

        f, g, h :: (Eq a, Ord a) => a -> Bool
        f x = x == x
        g x = x > x
        h x = x == x && x > x

        All three will discover that they have two [G] Eq a constraints:
        one as given and one extracted from the Ord a constraint. They will
        both discard the latter, as noted above and in
        Note [Replacement vs keeping] in GHC.Tc.Solver.InertSet.

        The body of f uses the [G] Eq a, but not the [G] Ord a. It will
        report a redundant Ord a using the logic for case (a).

        The body of g uses the [G] Ord a, but not the [G] Eq a. It will
        report a redundant Eq a using the logic for case (a).

        The body of h uses both [G] Ord a and [G] Eq a. Case (a) will
        thus come up with nothing redundant. But then, the case (b)
        check will discover that Eq a is redundant and report this.

        If we did case (b) even when case (a) reports something, then
        we would report both constraints as redundant for f, which is
        terrible.

    ----- Reporting redundant constraints

    * GHC.Tc.Errors does the actual warning, in warnRedundantConstraints.

    * We don't report redundant givens for *every* implication; only
      for those which reply True to GHC.Tc.Solver.warnRedundantGivens:

       - For example, in a class declaration, the default method *can*
         use the class constraint, but it certainly doesn't *have* to,
         and we don't want to report an error there.  Ditto instance decls.

       - More subtly, in a function definition
           f :: (Ord a, Ord a, Ix a) => a -> a
           f x = rhs
         we do an ambiguity check on the type (which would find that one
         of the Ord a constraints was redundant), and then we check that
         the definition has that type (which might find that both are
         redundant).  We don't want to report the same error twice, so we
         disable it for the ambiguity check.  Hence using two different
         FunSigCtxts, one with the warn-redundant field set True, and the
         other set False in
            - GHC.Tc.Gen.Bind.tcSpecPrag
            - GHC.Tc.Gen.Bind.tcTySig

      This decision is taken in setImplicationStatus, rather than GHC.Tc.Errors
      so that we can discard implication constraints that we don't need.
      So ics_dead consists only of the *reportable* redundant givens.

    ----- Shortcomings

    Shortcoming 1.  Consider

      j :: (Eq a, a ~ b) => a -> Bool
      j x = x == x

      k :: (Eq a, b ~ a) => a -> Bool
      k x = x == x

    Currently (Nov 2021), j issues no warning, while k says that b ~ a
    is redundant. This is because j uses the a ~ b constraint to rewrite
    everything to be in terms of b, while k does none of that. This is
    ridiculous, but I (Richard E) don't see a good fix.

    Shortcoming 2.  Removing a redundant constraint can cause clients to fail to
    compile, by making the function more polymoprhic. Consider (#16154)

      f :: (a ~ Bool) => a -> Int
      f x = 3

      g :: String -> Int
      g s = f (read s)

    The constraint in f's signature is redundant; not used to typecheck
    `f`.  And yet if you remove it, `g` won't compile, because there'll
    be an ambiguous variable in `g`.
    -}

    -- | Like 'defaultTyVar', but in the TcS monad.
    defaultTyVarTcS :: TcTyVar -> TcS Bool
    defaultTyVarTcS :: TyVar -> TcS Bool
    defaultTyVarTcS TyVar
    the_tv
      | TyVar -> Bool
    isTyVarTyVar TyVar
    the_tv
        -- TyVarTvs should only be unified with a tyvar
        -- never with a type; c.f. GHC.Tc.Utils.TcMType.defaultTyVar
        -- and Note [Inferring kinds for type declarations] in GHC.Tc.TyCl
      = Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False
      | TyVar -> Bool
    isRuntimeRepVar TyVar
    the_tv
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "defaultTyVarTcS RuntimeRep" (TyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyVar
    the_tv)
           ; TyVar -> Type -> TcS ()
    unifyTyVar TyVar
    the_tv Type
    liftedRepTy
           ; Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True }
      | TyVar -> Bool
    isLevityVar TyVar
    the_tv
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "defaultTyVarTcS Levity" (TyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyVar
    the_tv)
           ; TyVar -> Type -> TcS ()
    unifyTyVar TyVar
    the_tv Type
    liftedDataConTy
           ; Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True }
      | TyVar -> Bool
    isMultiplicityVar TyVar
    the_tv
      = do { String -> SDoc -> TcS ()
    traceTcS String
    "defaultTyVarTcS Multiplicity" (TyVar -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyVar
    the_tv)
           ; TyVar -> Type -> TcS ()
    unifyTyVar TyVar
    the_tv Type
    ManyTy
           ; Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    True }
      | Bool
    otherwise
      = Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False  -- the common case

    approximateWC :: Bool   -- See Wrinkle (W3) in Note [ApproximateWC]
                  -> WantedConstraints
                  -> Cts
    -- Second return value is the depleted wc
    -- Postcondition: Wanted Cts
    -- See Note [ApproximateWC]
    -- See Note [floatKindEqualities vs approximateWC]
    approximateWC :: Bool -> WantedConstraints -> Cts
    approximateWC Bool
    float_past_equalities WantedConstraints
    wc
      = Bool -> VarSet -> WantedConstraints -> Cts
    float_wc Bool
    False VarSet
    emptyVarSet WantedConstraints
    wc
      where
        float_wc :: Bool           -- True <=> there are enclosing equalities
                 -> TcTyCoVarSet   -- Enclosing skolem binders
                 -> WantedConstraints -> Cts
        float_wc :: Bool -> VarSet -> WantedConstraints -> Cts
    float_wc Bool
    encl_eqs VarSet
    trapping_tvs (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    simples, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    implics })
          = (Ct -> Bool) -> Cts -> Cts
    forall a. (a -> Bool) -> Bag a -> Bag a
    filterBag (Bool -> VarSet -> Ct -> Bool
    is_floatable Bool
    encl_eqs VarSet
    trapping_tvs) Cts
    simples Cts -> Cts -> Cts
    forall a. Bag a -> Bag a -> Bag a
    `unionBags`
            (Implication -> Cts) -> Bag Implication -> Cts
    forall a b. (a -> Bag b) -> Bag a -> Bag b
    concatMapBag (Bool -> VarSet -> Implication -> Cts
    float_implic Bool
    encl_eqs VarSet
    trapping_tvs) Bag Implication
    implics

        float_implic :: Bool -> TcTyCoVarSet -> Implication -> Cts
        float_implic :: Bool -> VarSet -> Implication -> Cts
    float_implic Bool
    encl_eqs VarSet
    trapping_tvs Implication
    imp
          = Bool -> VarSet -> WantedConstraints -> Cts
    float_wc Bool
    new_encl_eqs VarSet
    new_trapping_tvs (Implication -> WantedConstraints
    ic_wanted Implication
    imp)
          where
            new_trapping_tvs :: VarSet
    new_trapping_tvs = VarSet
    trapping_tvs VarSet -> [TyVar] -> VarSet
    `extendVarSetList` Implication -> [TyVar]
    ic_skols Implication
    imp
            new_encl_eqs :: Bool
    new_encl_eqs = Bool
    encl_eqs Bool -> Bool -> Bool
    || Implication -> HasGivenEqs
    ic_given_eqs Implication
    imp HasGivenEqs -> HasGivenEqs -> Bool
    forall a. Eq a => a -> a -> Bool
    == HasGivenEqs
    MaybeGivenEqs

        is_floatable :: Bool -> VarSet -> Ct -> Bool
    is_floatable Bool
    encl_eqs VarSet
    skol_tvs Ct
    ct
           | Ct -> Bool
    isGivenCt Ct
    ct                                = Bool
    False
           | Ct -> Bool
    insolubleCt Ct
    ct                              = Bool
    False
           | Ct -> VarSet
    tyCoVarsOfCt Ct
    ct VarSet -> VarSet -> Bool
    `intersectsVarSet` VarSet
    skol_tvs = Bool
    False
           | Bool
    otherwise
           = case Type -> Pred
    classifyPredType (Ct -> Type
    ctPred Ct
    ct) of
               EqPred {}     -> Bool
    float_past_equalities Bool -> Bool -> Bool
    || Bool -> Bool
    not Bool
    encl_eqs
                                      -- See Wrinkle (W1)
               ClassPred {}  -> Bool
    True  -- See Wrinkle (W2)
               IrredPred {}  -> Bool
    True  -- ..both in Note [ApproximateWC]
               ForAllPred {} -> Bool
    False

    {- Note [ApproximateWC]
    ~~~~~~~~~~~~~~~~~~~~~~~
    approximateWC takes a constraint, typically arising from the RHS of a
    let-binding whose type we are *inferring*, and extracts from it some
    *simple* constraints that we might plausibly abstract over.  Of course
    the top-level simple constraints are plausible, but we also float constraints
    out from inside, if they are not captured by skolems.

    The same function is used when doing type-class defaulting (see the call
    to applyDefaultingRules) to extract constraints that might be defaulted.

    Wrinkle (W1)
      When inferring most-general types (in simplifyInfer), we
      do *not* float an equality constraint if the implication binds
      equality constraints, because that defeats the OutsideIn story.
      Consider data T a where TInt :: T Int MkT :: T a

             f TInt = 3::Int

      We get the implication (a ~ Int => res ~ Int), where so far we've decided
         f :: T a -> res
      We don't want to float (res~Int) out because then we'll infer
         f :: T a -> Int
      which is only on of the possible types. (GHC 7.6 accidentally *did*
      float out of such implications, which meant it would happily infer
      non-principal types.)

    Wrinkle (W2)
      We do allow /class/ constraints to float, even if
      the implication binds equalities.  This is a subtle point: see #23224.
      In principle, a class constraint might ultimately be satisfiable from
      a constraint bound by an implication (see #19106 for an example of this
      kind), but it's extremely obscure and I was unable to construct a
      concrete example.  In any case, in super-subtle cases where this might
      make a difference, you would be much better advised to simply write a
      type signature.

      I included IrredPred here too, for good measure.  In general,
      abstracting over more constraints does no harm.

    Wrinkle (W3)
      In findDefaultableGroups we are not worried about the
      most-general type; and we /do/ want to float out of equalities
      (#12797).  Hence the boolean flag to approximateWC.

    ------ Historical note -----------
    There used to be a second caveat, driven by #8155

       2. We do not float out an inner constraint that shares a type variable
          (transitively) with one that is trapped by a skolem.  Eg
              forall a.  F a ~ beta, Integral beta
          We don't want to float out (Integral beta).  Doing so would be bad
          when defaulting, because then we'll default beta:=Integer, and that
          makes the error message much worse; we'd get
              Can't solve  F a ~ Integer
          rather than
              Can't solve  Integral (F a)

          Moreover, floating out these "contaminated" constraints doesn't help
          when generalising either. If we generalise over (Integral b), we still
          can't solve the retained implication (forall a. F a ~ b).  Indeed,
          arguably that too would be a harder error to understand.

    But this transitive closure stuff gives rise to a complex rule for
    when defaulting actually happens, and one that was never documented.
    Moreover (#12923), the more complex rule is sometimes NOT what
    you want.  So I simply removed the extra code to implement the
    contamination stuff.  There was zero effect on the testsuite (not even #8155).
    ------ End of historical note -----------

    Note [DefaultTyVar]
    ~~~~~~~~~~~~~~~~~~~
    defaultTyVar is used on any un-instantiated meta type variables to
    default any RuntimeRep variables to LiftedRep.  This is important
    to ensure that instance declarations match.  For example consider

         instance Show (a->b)
         foo x = show (\_ -> True)

    Then we'll get a constraint (Show (p ->q)) where p has kind (TYPE r),
    and that won't match the typeKind (*) in the instance decl.  See tests
    tc217 and tc175.

    We look only at touchable type variables. No further constraints
    are going to affect these type variables, so it's time to do it by
    hand.  However we aren't ready to default them fully to () or
    whatever, because the type-class defaulting rules have yet to run.

    An alternate implementation would be to emit a Wanted constraint setting
    the RuntimeRep variable to LiftedRep, but this seems unnecessarily indirect.

    Note [Promote _and_ default when inferring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we are inferring a type, we simplify the constraint, and then use
    approximateWC to produce a list of candidate constraints.  Then we MUST

      a) Promote any meta-tyvars that have been floated out by
         approximateWC, to restore invariant (WantedInv) described in
         Note [TcLevel invariants] in GHC.Tc.Utils.TcType.

      b) Default the kind of any meta-tyvars that are not mentioned in
         in the environment.

    To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we
    have an instance (C ((x:*) -> Int)).  The instance doesn't match -- but it
    should!  If we don't solve the constraint, we'll stupidly quantify over
    (C (a->Int)) and, worse, in doing so skolemiseQuantifiedTyVar will quantify over
    (b:*) instead of (a:OpenKind), which can lead to disaster; see #7332.
    #7641 is a simpler example.

    Note [Promoting unification variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we float an equality out of an implication we must "promote" free
    unification variables of the equality, in order to maintain Invariant
    (WantedInv) from Note [TcLevel invariants] in GHC.Tc.Types.TcType.

    This is absolutely necessary. Consider the following example. We start
    with two implications and a class with a functional dependency.

        class C x y | x -> y
        instance C [a] [a]

        (I1)      [untch=beta]forall b. 0 => F Int ~ [beta]
        (I2)      [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c]

    We float (F Int ~ [beta]) out of I1, and we float (F Int ~ [[alpha]]) out of I2.
    They may react to yield that (beta := [alpha]) which can then be pushed inwards
    the leftover of I2 to get (C [alpha] [a]) which, using the FunDep, will mean that
    (alpha := a). In the end we will have the skolem 'b' escaping in the untouchable
    beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs:

        class C x y | x -> y where
         op :: x -> y -> ()

        instance C [a] [a]

        type family F a :: *

        h :: F Int -> ()
        h = undefined

        data TEx where
          TEx :: a -> TEx

        f (x::beta) =
            let g1 :: forall b. b -> ()
                g1 _ = h [x]
                g2 z = case z of TEx y -> (h [[undefined]], op x [y])
            in (g1 '3', g2 undefined)

    *********************************************************************************
    *                                                                               *
    *                          Defaulting and disambiguation                        *
    *                                                                               *
    *********************************************************************************

    Note [Defaulting plugins]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Defaulting plugins enable extending or overriding the defaulting
    behaviour. In `applyDefaulting`, before the built-in defaulting
    mechanism runs, the loaded defaulting plugins are passed the
    `WantedConstraints` and get a chance to propose defaulting assignments
    based on them.

    Proposals are represented as `[DefaultingProposal]` with each proposal
    consisting of a type variable to fill-in, the list of defaulting types to
    try in order, and a set of constraints to check at each try. This is
    the same representation (albeit in a nicely packaged-up data type) as
    the candidates generated by the built-in defaulting mechanism, so the
    actual trying of proposals is done by the same `disambigGroup` function.

    Wrinkle (DP1): The role of `WantedConstraints`

      Plugins are passed `WantedConstraints` that can perhaps be
      progressed on by defaulting. But a defaulting plugin is not a solver
      plugin, its job is to provide defaulting proposals, i.e. mappings of
      type variable to types. How do plugins know which type variables
      they are supposed to default?

      The `WantedConstraints` passed to the defaulting plugin are zonked
      beforehand to ensure all remaining metavariables are unfilled. Thus,
      the `WantedConstraints` serve a dual purpose: they are both the
      constraints of the given context that can act as hints to the
      defaulting, as well as the containers of the type variables under
      consideration for defaulting.

    Wrinkle (DP2): Interactions between defaulting mechanisms

      In the general case, we have multiple defaulting plugins loaded and
      there is also the built-in defaulting mechanism. In this case, we
      have to be careful to keep the `WantedConstraints` passed to the
      plugins up-to-date by zonking between successful defaulting
      rounds. Otherwise, two plugins might come up with a defaulting
      proposal for the same metavariable; if the first one is accepted by
      `disambigGroup` (thus the meta gets filled), the second proposal
      becomes invalid (see #23821 for an example).

    -}

    applyDefaultingRules :: WantedConstraints -> TcS Bool
    -- True <=> I did some defaulting, by unifying a meta-tyvar
    -- Input WantedConstraints are not necessarily zonked

    applyDefaultingRules :: WantedConstraints -> TcS Bool
    applyDefaultingRules WantedConstraints
    wanteds
      | WantedConstraints -> Bool
    isEmptyWC WantedConstraints
    wanteds
      = Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False
      | Bool
    otherwise
      = do { info@(default_tys, _) <- TcS ([Type], (Bool, Bool))
    getDefaultInfo
           ; wanteds               <- TcS.zonkWC wanteds

           ; tcg_env <- TcS.getGblEnv
           ; let plugins = TcGblEnv -> [FillDefaulting]
    tcg_defaulting_plugins TcGblEnv
    tcg_env

           -- Run any defaulting plugins
           -- See Note [Defaulting plugins] for an overview
           ; (wanteds, plugin_defaulted) <- if null plugins then return (wanteds, []) else
               do {
                 ; traceTcS "defaultingPlugins {" (ppr wanteds)
                 ; (wanteds, defaultedGroups) <- mapAccumLM run_defaulting_plugin wanteds plugins
                 ; traceTcS "defaultingPlugins }" (ppr defaultedGroups)
                 ; return (wanteds, defaultedGroups)
                 }

           ; let groups = ([Type], (Bool, Bool)) -> WantedConstraints -> [(TyVar, [Ct])]
    findDefaultableGroups ([Type], (Bool, Bool))
    info WantedConstraints
    wanteds

           ; traceTcS "applyDefaultingRules {" $
                      vcat [ text "wanteds =" <+> ppr wanteds
                           , text "groups  =" <+> ppr groups
                           , text "info    =" <+> ppr info ]

           ; something_happeneds <- mapM (disambigGroup wanteds default_tys) groups

           ; traceTcS "applyDefaultingRules }" (ppr something_happeneds)

           ; return $ or something_happeneds || or plugin_defaulted }
        where run_defaulting_plugin :: WantedConstraints
    -> FillDefaulting -> TcS (WantedConstraints, Bool)
    run_defaulting_plugin WantedConstraints
    wanteds FillDefaulting
    p =
                do { groups <- TcPluginM [DefaultingProposal] -> TcS [DefaultingProposal]
    forall a. TcPluginM a -> TcS a
    runTcPluginTcS (FillDefaulting
    p WantedConstraints
    wanteds)
                   ; defaultedGroups <-
                        filterM (\DefaultingProposal
    g -> WantedConstraints -> [Ct] -> [[(TyVar, Type)]] -> TcS Bool
    disambigMultiGroup
                                       WantedConstraints
    wanteds
                                       (DefaultingProposal -> [Ct]
    deProposalCts DefaultingProposal
    g)
                                       (DefaultingProposal -> [[(TyVar, Type)]]
    deProposals DefaultingProposal
    g))
                        groups
                   ; traceTcS "defaultingPlugin " $ ppr defaultedGroups
                   ; case defaultedGroups of
                     [] -> (WantedConstraints, Bool) -> TcS (WantedConstraints, Bool)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (WantedConstraints
    wanteds, Bool
    False)
                     [DefaultingProposal]
    _  -> do
                         -- If a defaulting plugin solves any tyvars, some of the wanteds
                         -- will have filled-in metavars by now (see wrinkle DP2 of
                         -- Note [Defaulting plugins]). So we re-zonk to make sure later
                         -- defaulting doesn't try to solve the same metavars.
                         wanteds' <- WantedConstraints -> TcS WantedConstraints
    TcS.zonkWC WantedConstraints
    wanteds
                         return (wanteds', True)
                   }

    findDefaultableGroups
        :: ( [Type]
           , (Bool,Bool) )     -- (Overloaded strings, extended default rules)
        -> WantedConstraints   -- Unsolved
        -> [(TyVar, [Ct])]
    findDefaultableGroups :: ([Type], (Bool, Bool)) -> WantedConstraints -> [(TyVar, [Ct])]
    findDefaultableGroups ([Type]
    default_tys, (Bool
    ovl_strings, Bool
    extended_defaults)) WantedConstraints
    wanteds
      | [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    default_tys
      = []
      | Bool
    otherwise
      = [ (TyVar
    tv, ((Ct, Class, TyVar) -> Ct) -> [(Ct, Class, TyVar)] -> [Ct]
    forall a b. (a -> b) -> [a] -> [b]
    map (Ct, Class, TyVar) -> Ct
    forall a b c. (a, b, c) -> a
    fstOf3 [(Ct, Class, TyVar)]
    group)
        | group' :: NonEmpty (Ct, Class, TyVar)
    group'@((Ct
    _,Class
    _,TyVar
    tv) :| [(Ct, Class, TyVar)]
    _) <- [NonEmpty (Ct, Class, TyVar)]
    unary_groups
        , let group :: [(Ct, Class, TyVar)]
    group = NonEmpty (Ct, Class, TyVar) -> [(Ct, Class, TyVar)]
    forall a. NonEmpty a -> [a]
    forall (t :: * -> *) a. Foldable t => t a -> [a]
    toList NonEmpty (Ct, Class, TyVar)
    group'
        , TyVar -> Bool
    defaultable_tyvar TyVar
    tv
        , [Class] -> Bool
    defaultable_classes (((Ct, Class, TyVar) -> Class) -> [(Ct, Class, TyVar)] -> [Class]
    forall a b. (a -> b) -> [a] -> [b]
    map (Ct, Class, TyVar) -> Class
    forall a b c. (a, b, c) -> b
    sndOf3 [(Ct, Class, TyVar)]
    group) ]
      where
        simples :: Cts
    simples                = Bool -> WantedConstraints -> Cts
    approximateWC Bool
    True WantedConstraints
    wanteds
        ([(Ct, Class, TyVar)]
    unaries, [Ct]
    non_unaries) = (Ct -> Either (Ct, Class, TyVar) Ct)
    -> [Ct] -> ([(Ct, Class, TyVar)], [Ct])
    forall a b c. (a -> Either b c) -> [a] -> ([b], [c])
    partitionWith Ct -> Either (Ct, Class, TyVar) Ct
    find_unary (Cts -> [Ct]
    forall a. Bag a -> [a]
    bagToList Cts
    simples)
        unary_groups :: [NonEmpty (Ct, Class, TyVar)]
    unary_groups           = ((Ct, Class, TyVar) -> (Ct, Class, TyVar) -> Ordering)
    -> [(Ct, Class, TyVar)] -> [NonEmpty (Ct, Class, TyVar)]
    forall a. (a -> a -> Ordering) -> [a] -> [NonEmpty a]
    equivClasses (Ct, Class, TyVar) -> (Ct, Class, TyVar) -> Ordering
    forall {a} {a} {b} {a} {b}.
    Ord a =>
    (a, b, a) -> (a, b, a) -> Ordering
    cmp_tv [(Ct, Class, TyVar)]
    unaries

        unary_groups :: [NonEmpty (Ct, Class, TcTyVar)] -- (C tv) constraints
        unaries      :: [(Ct, Class, TcTyVar)]          -- (C tv) constraints
        non_unaries  :: [Ct]                            -- and *other* constraints

            -- Finds unary type-class constraints
            -- But take account of polykinded classes like Typeable,
            -- which may look like (Typeable * (a:*))   (#8931)
        find_unary :: Ct -> Either (Ct, Class, TyVar) Ct
        find_unary :: Ct -> Either (Ct, Class, TyVar) Ct
    find_unary Ct
    cc
            | Just (Class
    cls,[Type]
    tys)   <- Type -> Maybe (Class, [Type])
    getClassPredTys_maybe (Ct -> Type
    ctPred Ct
    cc)
            , [Type
    ty] <- TyCon -> [Type] -> [Type]
    filterOutInvisibleTypes (Class -> TyCon
    classTyCon Class
    cls) [Type]
    tys
                  -- Ignore invisible arguments for this purpose
            , Just TyVar
    tv <- Type -> Maybe TyVar
    getTyVar_maybe Type
    ty
            , TyVar -> Bool
    isMetaTyVar TyVar
    tv  -- We might have runtime-skolems in GHCi, and
                              -- we definitely don't want to try to assign to those!
            = (Ct, Class, TyVar) -> Either (Ct, Class, TyVar) Ct
    forall a b. a -> Either a b
    Left (Ct
    cc, Class
    cls, TyVar
    tv)
        find_unary Ct
    cc = Ct -> Either (Ct, Class, TyVar) Ct
    forall a b. b -> Either a b
    Right Ct
    cc  -- Non unary or non dictionary

        bad_tvs :: TcTyCoVarSet  -- TyVars mentioned by non-unaries
        bad_tvs :: VarSet
    bad_tvs = (Ct -> VarSet) -> [Ct] -> VarSet
    forall a. (a -> VarSet) -> [a] -> VarSet
    mapUnionVarSet Ct -> VarSet
    tyCoVarsOfCt [Ct]
    non_unaries

        cmp_tv :: (a, b, a) -> (a, b, a) -> Ordering
    cmp_tv (a
    _,b
    _,a
    tv1) (a
    _,b
    _,a
    tv2) = a
    tv1 a -> a -> Ordering
    forall a. Ord a => a -> a -> Ordering
    `compare` a
    tv2

        defaultable_tyvar :: TcTyVar -> Bool
        defaultable_tyvar :: TyVar -> Bool
    defaultable_tyvar TyVar
    tv
            = let b1 :: Bool
    b1 = TyVar -> Bool
    isTyConableTyVar TyVar
    tv  -- Note [Avoiding spurious errors]
                  b2 :: Bool
    b2 = Bool -> Bool
    not (TyVar
    tv TyVar -> VarSet -> Bool
    `elemVarSet` VarSet
    bad_tvs)
              in Bool
    b1 Bool -> Bool -> Bool
    && (Bool
    b2 Bool -> Bool -> Bool
    || Bool
    extended_defaults) -- Note [Multi-parameter defaults]

        defaultable_classes :: [Class] -> Bool
        defaultable_classes :: [Class] -> Bool
    defaultable_classes [Class]
    clss
            | Bool
    extended_defaults = (Class -> Bool) -> [Class] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    any (Bool -> Class -> Bool
    isInteractiveClass Bool
    ovl_strings) [Class]
    clss
            | Bool
    otherwise         = (Class -> Bool) -> [Class] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all Class -> Bool
    is_std_class [Class]
    clss Bool -> Bool -> Bool
    && ((Class -> Bool) -> [Class] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    any (Bool -> Class -> Bool
    isNumClass Bool
    ovl_strings) [Class]
    clss)

        -- is_std_class adds IsString to the standard numeric classes,
        -- when -XOverloadedStrings is enabled
        is_std_class :: Class -> Bool
    is_std_class Class
    cls = Class -> Bool
    isStandardClass Class
    cls Bool -> Bool -> Bool
    ||
                           (Bool
    ovl_strings Bool -> Bool -> Bool
    && (Class
    cls Class -> Unique -> Bool
    forall a. Uniquable a => a -> Unique -> Bool
    `hasKey` Unique
    isStringClassKey))

    ------------------------------
    disambigGroup :: WantedConstraints -- ^ Original constraints, for diagnostic purposes
                  -> [Type]            -- ^ The default types
                  -> (TcTyVar, [Ct])   -- ^ All constraints sharing same type variable
                  -> TcS Bool   -- True <=> something happened, reflected in ty_binds

    disambigGroup :: WantedConstraints -> [Type] -> (TyVar, [Ct]) -> TcS Bool
    disambigGroup WantedConstraints
    orig_wanteds [Type]
    default_tys (TyVar
    the_tv, [Ct]
    wanteds)
      = WantedConstraints -> [Ct] -> [[(TyVar, Type)]] -> TcS Bool
    disambigMultiGroup WantedConstraints
    orig_wanteds [Ct]
    wanteds [[(TyVar
    the_tv, Type
    default_ty)] | Type
    default_ty <- [Type]
    default_tys]

    disambigMultiGroup :: WantedConstraints -- ^ Original constraints, for diagnostic purposes
                       -> [Ct]       -- ^ check these are solved by defaulting
                       -> [[(TcTyVar, Type)]]  -- ^ defaulting type assignments to try
                       -> TcS Bool   -- True <=> something happened, reflected in ty_binds
    disambigMultiGroup :: WantedConstraints -> [Ct] -> [[(TyVar, Type)]] -> TcS Bool
    disambigMultiGroup WantedConstraints
    orig_wanteds [Ct]
    wanteds = ([(TyVar, Type)] -> TcS Bool) -> [[(TyVar, Type)]] -> TcS Bool
    forall (m :: * -> *) (f :: * -> *) a.
    (Monad m, Foldable f) =>
    (a -> m Bool) -> f a -> m Bool
    anyM [(TyVar, Type)] -> TcS Bool
    propose
      where
        propose :: [(TyVar, Type)] -> TcS Bool
    propose [(TyVar, Type)]
    proposal
            = do { String -> SDoc -> TcS ()
    traceTcS String
    "disambigMultiGroup {" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ [(TyVar, Type)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(TyVar, Type)]
    proposal, [Ct] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Ct]
    wanteds ])
                 ; invalid_tvs <- (TyVar -> TcS Bool) -> [TyVar] -> TcS [TyVar]
    forall (m :: * -> *) a.
    Applicative m =>
    (a -> m Bool) -> [a] -> m [a]
    filterOutM TyVar -> TcS Bool
    TcS.isUnfilledMetaTyVar [TyVar]
    tvs
                 ; traverse_ (errInvalidDefaultedTyVar orig_wanteds proposal) (nonEmpty invalid_tvs)
                 ; fake_ev_binds_var <- TcS.newTcEvBinds
                 ; tclvl             <- TcS.getTcLevel
                 ; mb_subst <- nestImplicTcS fake_ev_binds_var (pushTcLevel tclvl) try_group

                 ; case mb_subst of
                       Just Subst
    subst -> -- Success: record the type variable bindings, and return
                           do { deep_tvs <- (TyVar -> TcS Bool) -> [TyVar] -> TcS [TyVar]
    forall (m :: * -> *) a.
    Applicative m =>
    (a -> m Bool) -> [a] -> m [a]
    filterM TyVar -> TcS Bool
    TcS.isUnfilledMetaTyVar ([TyVar] -> TcS [TyVar]) -> [TyVar] -> TcS [TyVar]
    forall a b. (a -> b) -> a -> b
    $ VarSet -> [TyVar]
    forall elt. UniqSet elt -> [elt]
    nonDetEltsUniqSet (VarSet -> [TyVar]) -> VarSet -> [TyVar]
    forall a b. (a -> b) -> a -> b
    $ VarSet -> VarSet
    closeOverKinds ([TyVar] -> VarSet
    mkVarSet [TyVar]
    tvs)
                              ; forM_ deep_tvs $ \ TyVar
    tv -> (Type -> TcS ()) -> Maybe Type -> TcS ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (TyVar -> Type -> TcS ()
    unifyTyVar TyVar
    tv) (VarEnv Type -> TyVar -> Maybe Type
    forall a. VarEnv a -> TyVar -> Maybe a
    lookupVarEnv (Subst -> VarEnv Type
    getTvSubstEnv Subst
    subst) TyVar
    tv)
                              ; wrapWarnTcS $ mapM_ (uncurry $ warnDefaulting wanteds) proposal
                              ; traceTcS "disambigMultiGroup succeeded }" (ppr proposal)
                              ; return True }
                       Maybe Subst
    Nothing -> -- Failure: try with the next defaulting group
                           do { String -> SDoc -> TcS ()
    traceTcS String
    "disambigMultiGroup failed, will try other default types }"
                                   ([(TyVar, Type)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(TyVar, Type)]
    proposal)
                              ; Bool -> TcS Bool
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Bool
    False } }
          where
            ([TyVar]
    tvs, [Type]
    default_tys) = [(TyVar, Type)] -> ([TyVar], [Type])
    forall a b. [(a, b)] -> ([a], [b])
    unzip [(TyVar, Type)]
    proposal

            try_group :: TcS (Maybe Subst)
    try_group
              | Just Subst
    subst <- Maybe Subst
    mb_subst
              = do { lcl_env <- TcS TcLclEnv
    TcS.getLclEnv
                   ; tc_lvl <- TcS.getTcLevel
                   ; let loc = TcLevel -> SkolemInfoAnon -> CtLocEnv -> CtLoc
    mkGivenLoc TcLevel
    tc_lvl (SkolemInfo -> SkolemInfoAnon
    getSkolemInfo SkolemInfo
    HasCallStack => SkolemInfo
    unkSkol) (TcLclEnv -> CtLocEnv
    mkCtLocEnv TcLclEnv
    lcl_env)
                   -- Equality constraints are possible due to type defaulting plugins
                   ; wanted_evs <- sequence [ newWantedNC loc rewriters pred'
                                            | wanted <- wanteds
                                            , CtWanted { ctev_pred = pred
                                                       , ctev_rewriters = rewriters }
                                                <- return (ctEvidence wanted)
                                            , let pred' = HasDebugCallStack => Subst -> Type -> Type
    Subst -> Type -> Type
    substTy Subst
    subst Type
    pred ]
                   ; residual_wc <- solveSimpleWanteds $ listToBag $ map mkNonCanonical wanted_evs
                   ; return $ if isEmptyWC residual_wc then Just subst else Nothing }

              | Bool
    otherwise
              = Maybe Subst -> TcS (Maybe Subst)
    forall a. a -> TcS a
    forall (m :: * -> *) a. Monad m => a -> m a
    return Maybe Subst
    forall a. Maybe a
    Nothing

            mb_subst :: Maybe Subst
    mb_subst = [Type] -> [Type] -> Maybe Subst
    tcMatchTyKis ([TyVar] -> [Type]
    mkTyVarTys [TyVar]
    tvs) [Type]
    default_tys
              -- Make sure the kinds match too; hence this call to tcMatchTyKi
              -- E.g. suppose the only constraint was (Typeable k (a::k))
              -- With the addition of polykinded defaulting we also want to reject
              -- ill-kinded defaulting attempts like (Eq []) or (Foldable Int) here.

    errInvalidDefaultedTyVar :: WantedConstraints -> [(TcTyVar, Type)] -> NonEmpty TcTyVar -> TcS ()
    errInvalidDefaultedTyVar :: WantedConstraints -> [(TyVar, Type)] -> NonEmpty TyVar -> TcS ()
    errInvalidDefaultedTyVar WantedConstraints
    wanteds [(TyVar, Type)]
    proposal NonEmpty TyVar
    problematic_tvs
      = TcRnMessage -> TcS ()
    forall a. TcRnMessage -> TcS a
    failTcS (TcRnMessage -> TcS ()) -> TcRnMessage -> TcS ()
    forall a b. (a -> b) -> a -> b
    $ [Ct] -> [(TyVar, Type)] -> NonEmpty TyVar -> TcRnMessage
    TcRnInvalidDefaultedTyVar [Ct]
    tidy_wanteds [(TyVar, Type)]
    tidy_proposal NonEmpty TyVar
    tidy_problems
      where
        proposal_tvs :: [TyVar]
    proposal_tvs = ((TyVar, Type) -> [TyVar]) -> [(TyVar, Type)] -> [TyVar]
    forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
    concatMap (\(TyVar
    tv, Type
    ty) -> TyVar
    tv TyVar -> [TyVar] -> [TyVar]
    forall a. a -> [a] -> [a]
    : Type -> [TyVar]
    tyCoVarsOfTypeList Type
    ty) [(TyVar, Type)]
    proposal
        tidy_env :: TidyEnv
    tidy_env = TidyEnv -> [TyVar] -> TidyEnv
    tidyFreeTyCoVars TidyEnv
    emptyTidyEnv ([TyVar] -> TidyEnv) -> [TyVar] -> TidyEnv
    forall a b. (a -> b) -> a -> b
    $ [TyVar]
    proposal_tvs [TyVar] -> [TyVar] -> [TyVar]
    forall a. [a] -> [a] -> [a]
    ++ NonEmpty TyVar -> [TyVar]
    forall a. NonEmpty a -> [a]
    NE.toList NonEmpty TyVar
    problematic_tvs
        tidy_wanteds :: [Ct]
    tidy_wanteds = (Ct -> Ct) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> [a] -> [b]
    map (TidyEnv -> Ct -> Ct
    tidyCt TidyEnv
    tidy_env) ([Ct] -> [Ct]) -> [Ct] -> [Ct]
    forall a b. (a -> b) -> a -> b
    $ WantedConstraints -> [Ct]
    flattenWC WantedConstraints
    wanteds
        tidy_proposal :: [(TyVar, Type)]
    tidy_proposal = [(TidyEnv -> TyVar -> TyVar
    tidyTyCoVarOcc TidyEnv
    tidy_env TyVar
    tv, TidyEnv -> Type -> Type
    tidyType TidyEnv
    tidy_env Type
    ty) | (TyVar
    tv, Type
    ty) <- [(TyVar, Type)]
    proposal]
        tidy_problems :: NonEmpty TyVar
    tidy_problems = (TyVar -> TyVar) -> NonEmpty TyVar -> NonEmpty TyVar
    forall a b. (a -> b) -> NonEmpty a -> NonEmpty b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap (TidyEnv -> TyVar -> TyVar
    tidyTyCoVarOcc TidyEnv
    tidy_env) NonEmpty TyVar
    problematic_tvs

        flattenWC :: WantedConstraints -> [Ct]
        flattenWC :: WantedConstraints -> [Ct]
    flattenWC (WC { wc_simple :: WantedConstraints -> Cts
    wc_simple = Cts
    cts, wc_impl :: WantedConstraints -> Bag Implication
    wc_impl = Bag Implication
    impls })
          = Cts -> [Ct]
    ctsElts Cts
    cts [Ct] -> [Ct] -> [Ct]
    forall a. [a] -> [a] -> [a]
    ++ (Implication -> [Ct]) -> Bag Implication -> [Ct]
    forall (t :: * -> *) a b. Foldable t => (a -> [b]) -> t a -> [b]
    concatMap (WantedConstraints -> [Ct]
    flattenWC (WantedConstraints -> [Ct])
    -> (Implication -> WantedConstraints) -> Implication -> [Ct]
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Implication -> WantedConstraints
    ic_wanted) Bag Implication
    impls

    -- In interactive mode, or with -XExtendedDefaultRules,
    -- we default Show a to Show () to avoid gratuitous errors on "show []"
    isInteractiveClass :: Bool   -- -XOverloadedStrings?
                       -> Class -> Bool
    isInteractiveClass :: Bool -> Class -> Bool
    isInteractiveClass Bool
    ovl_strings Class
    cls
        = Bool -> Class -> Bool
    isNumClass Bool
    ovl_strings Class
    cls Bool -> Bool -> Bool
    || (Class -> Unique
    classKey Class
    cls Unique -> [Unique] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Unique]
    interactiveClassKeys)

        -- isNumClass adds IsString to the standard numeric classes,
        -- when -XOverloadedStrings is enabled
    isNumClass :: Bool   -- -XOverloadedStrings?
               -> Class -> Bool
    isNumClass :: Bool -> Class -> Bool
    isNumClass Bool
    ovl_strings Class
    cls
      = Class -> Bool
    isNumericClass Class
    cls Bool -> Bool -> Bool
    || (Bool
    ovl_strings Bool -> Bool -> Bool
    && (Class
    cls Class -> Unique -> Bool
    forall a. Uniquable a => a -> Unique -> Bool
    `hasKey` Unique
    isStringClassKey))

    {-
    Note [Avoiding spurious errors]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When doing the unification for defaulting, we check for skolem
    type variables, and simply don't default them.  For example:
       f = (*)      -- Monomorphic
       g :: Num a => a -> a
       g x = f x x
    Here, we get a complaint when checking the type signature for g,
    that g isn't polymorphic enough; but then we get another one when
    dealing with the (Num a) context arising from f's definition;
    we try to unify a with Int (to default it), but find that it's
    already been unified with the rigid variable from g's type sig.

    Note [Multi-parameter defaults]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    With -XExtendedDefaultRules, we default only based on single-variable
    constraints, but do not exclude from defaulting any type variables which also
    appear in multi-variable constraints. This means that the following will
    default properly:

       default (Integer, Double)

       class A b (c :: Symbol) where
          a :: b -> Proxy c

       instance A Integer c where a _ = Proxy

       main = print (a 5 :: Proxy "5")

    Note that if we change the above instance ("instance A Integer") to
    "instance A Double", we get an error:

       No instance for (A Integer "5")

    This is because the first defaulted type (Integer) has successfully satisfied
    its single-parameter constraints (in this case Num).
    -}