    {-# LANGUAGE DerivingStrategies #-}
    {-# LANGUAGE LambdaCase #-}

    {-# OPTIONS_GHC -Wno-incomplete-record-updates #-}
    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns   #-}

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

    module GHC.Tc.Validity (
      Rank(..), UserTypeCtxt(..), checkValidType, checkValidMonoType,
      checkValidTheta,
      checkValidInstance, checkValidInstHead, validDerivPred,
      checkTySynRhs, checkEscapingKind,
      checkValidCoAxiom, checkValidCoAxBranch,
      checkFamPatBinders, checkTyFamEqnValidityInfo,
      checkValidTyFamEqn, checkValidAssocTyFamDeflt, checkConsistentFamInst,
      checkTyConTelescope,
      ) where

    import GHC.Prelude
    import GHC.Data.FastString

    import GHC.Data.Maybe

    -- friends:
    import GHC.Tc.Utils.Unify    ( tcSubTypeAmbiguity )
    import GHC.Tc.Solver         ( simplifyAmbiguityCheck )
    import GHC.Tc.Instance.Class ( matchGlobalInst, ClsInstResult(..), AssocInstInfo(..) )
    import GHC.Tc.Utils.TcType
    import GHC.Tc.Types.Origin
    import GHC.Tc.Types.Rank
    import GHC.Tc.Errors.Types
    import GHC.Tc.Utils.Monad
    import GHC.Tc.Instance.FunDeps
    import GHC.Tc.Instance.Family
    import GHC.Tc.Zonk.TcType

    import GHC.Builtin.Types
    import GHC.Builtin.Names
    import GHC.Builtin.Uniques  ( mkAlphaTyVarUnique )

    import GHC.Core.Type
    import GHC.Core.Unify ( typesAreApart, tcMatchTyX_BM, BindFlag(..) )
    import GHC.Core.Coercion
    import GHC.Core.Coercion.Axiom
    import GHC.Core.Class
    import GHC.Core.TyCon
    import GHC.Core.Predicate
    import GHC.Core.TyCo.FVs
    import GHC.Core.TyCo.Rep
    import GHC.Core.TyCo.Ppr
    import GHC.Core.FamInstEnv ( isDominatedBy, injectiveBranches
                               , InjectivityCheckResult(..) )

    import GHC.Hs
    import GHC.Driver.Session
    import qualified GHC.LanguageExtensions as LangExt

    import GHC.Types.Error
    import GHC.Types.Basic   ( TypeOrKind(..), UnboxedTupleOrSum(..)
                             , unboxedTupleOrSumExtension )
    import GHC.Types.Name
    import GHC.Types.Var.Env
    import GHC.Types.Var.Set
    import GHC.Types.Var     ( VarBndr(..), isInvisibleFunArg, mkTyVar, tyVarName )
    import GHC.Types.SourceFile
    import GHC.Types.SrcLoc
    import GHC.Types.TyThing ( TyThing(..) )
    import GHC.Types.Unique.Set( isEmptyUniqSet )

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

    import GHC.Data.List.SetOps

    import Language.Haskell.Syntax.Basic (FieldLabelString(..))

    import Control.Monad
    import Data.Foldable
    import Data.Function
    import Data.List        ( (\\) )
    import qualified Data.List.NonEmpty as NE
    import Data.List.NonEmpty ( NonEmpty(..) )

    {-
    ************************************************************************
    *                                                                      *
              Checking for ambiguity
    *                                                                      *
    ************************************************************************

    Note [The ambiguity check for type signatures]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    checkAmbiguity is a check on *user-supplied type signatures*.  It is
    *purely* there to report functions that cannot possibly be called.  So for
    example we want to reject:
       f :: C a => Int
    The idea is there can be no legal calls to 'f' because every call will
    give rise to an ambiguous constraint.  We could soundly omit the
    ambiguity check on type signatures entirely, at the expense of
    delaying ambiguity errors to call sites.  Indeed, the flag
    -XAllowAmbiguousTypes switches off the ambiguity check.

    What about things like this:
       class D a b | a -> b where ..
       h :: D Int b => Int
    The Int may well fix 'b' at the call site, so that signature should
    not be rejected.  Moreover, using *visible* fundeps is too
    conservative.  Consider
       class X a b where ...
       class D a b | a -> b where ...
       instance D a b => X [a] b where...
       h :: X a b => a -> a
    Here h's type looks ambiguous in 'b', but here's a legal call:
       ...(h [True])...
    That gives rise to a (X [Bool] beta) constraint, and using the
    instance means we need (D Bool beta) and that fixes 'beta' via D's
    fundep!

    Behind all these special cases there is a simple guiding principle.
    Consider

      f :: <type>
      f = ...blah...

      g :: <type>
      g = f

    You would think that the definition of g would surely typecheck!
    After all f has exactly the same type, and g=f. But in fact f's type
    is instantiated and the instantiated constraints are solved against
    the originals, so in the case of an ambiguous type it won't work.
    Consider our earlier example f :: C a => Int.  Then in g's definition,
    we'll instantiate to (C alpha) and try to deduce (C alpha) from (C a),
    and fail.

    So in fact we use this as our *definition* of ambiguity.  We use a
    very similar test for *inferred* types, to ensure that they are
    unambiguous. See Note [Impedance matching] in GHC.Tc.Gen.Bind.

    This test is very conveniently implemented by calling
        tcSubType <type> <type>
    This neatly takes account of the functional dependency stuff above,
    and implicit parameter (see Note [Implicit parameters and ambiguity]).
    And this is what checkAmbiguity does.

    Note [The squishiness of the ambiguity check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    What about this?
       g :: C [a] => Int
    Is every call to 'g' ambiguous?  After all, we might have
       instance C [a] where ...
    at the call site.  So maybe that type is ok!  Indeed even f's
    quintessentially ambiguous type might, just possibly be callable:
    with -XFlexibleInstances we could have
      instance C a where ...
    and now a call could be legal after all!  Well, we'll reject this
    unless the instance is available *here*.

    But even that's not quite right. Even a function with an utterly-ambiguous
    type like f :: Eq a => Int -> Int
    is still callable if you are prepared to use visible type application,
    thus (f @Bool x).

    In short, the ambiguity check is a good-faith attempt to say "you are likely
    to have trouble if your function has this type"; it is NOT the case that
    "you can't call this function without giving a type error".

    See also Note [Ambiguity check and deep subsumption] in GHC.Tc.Utils.Unify.

    Note [When to call checkAmbiguity]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We call checkAmbiguity
       (a) on user-specified type signatures
       (b) in checkValidType

    Conncerning (b), you might wonder about nested foralls.  What about
        f :: forall b. (forall a. Eq a => b) -> b
    The nested forall is ambiguous.  Originally we called checkAmbiguity
    in the forall case of check_type, but that had two bad consequences:
      * We got two error messages about (Eq b) in a nested forall like this:
           g :: forall a. Eq a => forall b. Eq b => a -> a
      * If we try to check for ambiguity of a nested forall like
        (forall a. Eq a => b), the implication constraint doesn't bind
        all the skolems, which results in "No skolem info" in error
        messages (see #10432).

    To avoid this, we call checkAmbiguity once, at the top, in checkValidType.
    (I'm still a bit worried about unbound skolems when the type mentions
    in-scope type variables.)

    In fact, because of the co/contra-variance implemented in tcSubType,
    this *does* catch function f above. too.

    Concerning (a) the ambiguity check is only used for *user* types, not
    for types coming from interface files.  The latter can legitimately
    have ambiguous types. Example

       class S a where s :: a -> (Int,Int)
       instance S Char where s _ = (1,1)
       f:: S a => [a] -> Int -> (Int,Int)
       f (_::[a]) x = (a*x,b)
            where (a,b) = s (undefined::a)

    Here the worker for f gets the type
            fw :: forall a. S a => Int -> (# Int, Int #)

    Note [Implicit parameters and ambiguity]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Only a *class* predicate can give rise to ambiguity
    An *implicit parameter* cannot.  For example:
            foo :: (?x :: [a]) => Int
            foo = length ?x
    is fine.  The call site will supply a particular 'x'

    Furthermore, the type variables fixed by an implicit parameter
    propagate to the others.  E.g.
            foo :: (Show a, ?x::[a]) => Int
            foo = show (?x++?x)
    The type of foo looks ambiguous.  But it isn't, because at a call site
    we might have
            let ?x = 5::Int in foo
    and all is well.  In effect, implicit parameters are, well, parameters,
    so we can take their type variables into account as part of the
    "tau-tvs" stuff.  This is done in the function 'GHC.Tc.Instance.FunDeps.grow'.
    -}

    checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
    checkAmbiguity :: UserTypeCtxt -> Type -> TcM ()
    checkAmbiguity UserTypeCtxt
    ctxt Type
    ty
      | UserTypeCtxt -> Bool
    wantAmbiguityCheck UserTypeCtxt
    ctxt
      = do { String -> SDoc -> TcM ()
    traceTc String
    "Ambiguity check for {" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)
             -- Solve the constraints eagerly because an ambiguous type
             -- can cause a cascade of further errors.  Since the free
             -- tyvars are skolemised, we can safely use tcSimplifyTop
           ; allow_ambiguous <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.AllowAmbiguousTypes
           ; unless allow_ambiguous $
             do { (_wrap, wanted) <- addErrCtxt (mk_msg allow_ambiguous) $
                                     captureConstraints $
                                     tcSubTypeAmbiguity ctxt ty ty
                                     -- See Note [Ambiguity check and deep subsumption]
                                     -- in GHC.Tc.Utils.Unify
                ; simplifyAmbiguityCheck ty wanted }

           ; traceTc "} Done ambiguity check for" (ppr ty) }

      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
     where
       mk_msg :: Bool -> SDoc
    mk_msg Bool
    allow_ambiguous
         = [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "In the ambiguity check for" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> SDoc
    what
                , Bool -> SDoc -> SDoc
    forall doc. IsOutput doc => Bool -> doc -> doc
    ppUnless Bool
    allow_ambiguous SDoc
    ambig_msg ]
       ambig_msg :: SDoc
    ambig_msg = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "To defer the ambiguity check to use sites, enable AllowAmbiguousTypes"
       what :: SDoc
    what | Just Name
    n <- UserTypeCtxt -> Maybe Name
    isSigMaybe UserTypeCtxt
    ctxt = SDoc -> SDoc
    quotes (Name -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Name
    n)
            | Bool
    otherwise                 = UserTypeCtxt -> SDoc
    pprUserTypeCtxt UserTypeCtxt
    ctxt

    wantAmbiguityCheck :: UserTypeCtxt -> Bool
    wantAmbiguityCheck :: UserTypeCtxt -> Bool
    wantAmbiguityCheck UserTypeCtxt
    ctxt
      = case UserTypeCtxt
    ctxt of  -- See Note [When we don't check for ambiguity]
          GhciCtxt {}  -> Bool
    False
          TySynCtxt {} -> Bool
    False
          UserTypeCtxt
    TypeAppCtxt  -> Bool
    False
          StandaloneKindSigCtxt{} -> Bool
    False
          UserTypeCtxt
    _            -> Bool
    True

    -- | Check whether the type signature contains custom type errors,
    -- and fail if so.
    --
    -- Note that some custom type errors are acceptable:
    --
    --   - in the RHS of a type synonym, e.g. to allow users to define
    --     type synonyms for custom type errors with large messages (#20181),
    --   - inside a type family application, as a custom type error
    --     might evaporate after performing type family reduction (#20241).
    checkUserTypeError :: UserTypeCtxt -> Type -> TcM ()
    -- Very unsatisfactorily (#11144) we need to tidy the type
    -- because it may have come from an /inferred/ signature, not a
    -- user-supplied one.  This is really only a half-baked fix;
    -- the other errors in checkValidType don't do tidying, and so
    -- may give bad error messages when given an inferred type.
    checkUserTypeError :: UserTypeCtxt -> Type -> TcM ()
    checkUserTypeError UserTypeCtxt
    ctxt Type
    ty
      | TySynCtxt {} <- UserTypeCtxt
    ctxt  -- Do not complain about TypeError on the
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()             -- RHS of type synonyms. See #20181

      | Just Type
    msg <- Type -> Maybe Type
    deepUserTypeError_maybe Type
    ty
      = do { env0 <- ZonkM TidyEnv -> TcM TidyEnv
    forall a. ZonkM a -> TcM a
    liftZonkM ZonkM TidyEnv
    tcInitTidyEnv
           ; let (env1, tidy_msg) = tidyOpenType env0 msg
           ; failWithTcM (env1, TcRnUserTypeError tidy_msg) }
      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    {- Note [When we don't check for ambiguity]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a few places we do not want to check a user-specified type for ambiguity

    * GhciCtxt: Allow ambiguous types in GHCi's :kind command
      E.g.   type family T a :: *  -- T :: forall k. k -> *
      Then :k T should work in GHCi, not complain that
      (T k) is ambiguous!

    * TySynCtxt: type T a b = C a b => blah
      It may be that when we /use/ T, we'll give an 'a' or 'b' that somehow
      cure the ambiguity.  So we defer the ambiguity check to the use site.

      There is also an implementation reason (#11608).  In the RHS of
      a type synonym we don't (currently) instantiate 'a' and 'b' with
      TcTyVars before calling checkValidType, so we get assertion failures
      from doing an ambiguity check on a type with TyVars in it.  Fixing this
      would not be hard, but let's wait till there's a reason.

    * TypeAppCtxt: visible type application
         f @ty
      No need to check ty for ambiguity

    * StandaloneKindSigCtxt: type T :: ksig
      Kinds need a different ambiguity check than types, and the currently
      implemented check is only good for types. See #14419, in particular
      https://gitlab.haskell.org/ghc/ghc/issues/14419#note_160844

    ************************************************************************
    *                                                                      *
              Checking validity of a user-defined type
    *                                                                      *
    ************************************************************************

    When dealing with a user-written type, we first translate it from an HsType
    to a Type, performing kind checking, and then check various things that should
    be true about it.  We don't want to perform these checks at the same time
    as the initial translation because (a) they are unnecessary for interface-file
    types and (b) when checking a mutually recursive group of type and class decls,
    we can't "look" at the tycons/classes yet.  Also, the checks are rather
    diverse, and used to really mess up the other code.

    One thing we check for is 'rank'.

            Rank 0:         monotypes (no foralls)
            Rank 1:         foralls at the front only, Rank 0 inside
            Rank 2:         foralls at the front, Rank 1 on left of fn arrow,

            basic ::= tyvar | T basic ... basic

            r2  ::= forall tvs. cxt => r2a
            r2a ::= r1 -> r2a | basic
            r1  ::= forall tvs. cxt => r0
            r0  ::= r0 -> r0 | basic

    Another thing is to check that type synonyms are saturated.
    This might not necessarily show up in kind checking.
            type A i = i
            data T k = MkT (k Int)
            f :: T A        -- BAD!
    -}

    checkValidType :: UserTypeCtxt -> Type -> TcM ()
    -- Checks that a user-written type is valid for the given context
    -- Assumes argument is fully zonked
    -- Assumes argument is well-kinded;
    --    that is, checkValidType doesn't need to do kind checking
    -- Not used for instance decls; checkValidInstance instead
    checkValidType :: UserTypeCtxt -> Type -> TcM ()
    checkValidType UserTypeCtxt
    ctxt Type
    ty
      = do { String -> SDoc -> TcM ()
    traceTc String
    "checkValidType" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "::" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (HasDebugCallStack => Type -> Type
    Type -> Type
    typeKind Type
    ty))
           ; rankn_flag  <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.RankNTypes
           ; impred_flag <- xoptM LangExt.ImpredicativeTypes
           ; let gen_rank :: Rank -> Rank
                 gen_rank Rank
    r | Bool
    rankn_flag = Rank
    ArbitraryRank
                            | Bool
    otherwise  = Rank
    r

                 rank1 = Rank -> Rank
    gen_rank Rank
    r1
                 rank0 = Rank -> Rank
    gen_rank Rank
    MonoTypeRankZero

                 r1 = Bool -> Rank -> Rank
    LimitedRank Bool
    True Rank
    MonoTypeRankZero

                 rank
                   = case UserTypeCtxt
    ctxt of
                     UserTypeCtxt
    DefaultDeclCtxt-> Rank
    MustBeMonoType
                     UserTypeCtxt
    PatSigCtxt     -> Rank
    rank0
                     RuleSigCtxt {} -> Rank
    rank1
                     TySynCtxt Name
    _    -> Rank
    rank0

                     ExprSigCtxt {} -> Rank
    rank1
                     UserTypeCtxt
    KindSigCtxt    -> Rank
    rank1
                     StandaloneKindSigCtxt{} -> Rank
    rank1
                     UserTypeCtxt
    TypeAppCtxt | Bool
    impred_flag -> Rank
    ArbitraryRank
                                 | Bool
    otherwise   -> Rank
    MonoTypeTyConArg
                        -- Normally, ImpredicativeTypes is handled in check_arg_type,
                        -- but visible type applications don't go through there.
                        -- So we do this check here.

                     FunSigCtxt {}  -> Rank
    rank1
                     InfSigCtxt {}  -> Rank
    rank1 -- Inferred types should obey the
                                             -- same rules as declared ones

                     ConArgCtxt Name
    _   -> Rank
    rank1 -- We are given the type of the entire
                                             -- constructor, hence rank 1
                     PatSynCtxt Name
    _   -> Rank
    rank1

                     ForSigCtxt Name
    _   -> Rank
    rank1
                     UserTypeCtxt
    SpecInstCtxt   -> Rank
    rank1
                     GhciCtxt {}    -> Rank
    ArbitraryRank

                     TyVarBndrKindCtxt Name
    _ -> Rank
    rank0
                     DataKindCtxt Name
    _      -> Rank
    rank1
                     TySynKindCtxt Name
    _     -> Rank
    rank1
                     TyFamResKindCtxt Name
    _  -> Rank
    rank1

                     UserTypeCtxt
    _              -> String -> Rank
    forall a. HasCallStack => String -> a
    panic String
    "checkValidType"
                                              -- Can't happen; not used for *user* sigs

           ; env <- liftZonkM $ tcInitOpenTidyEnv (tyCoVarsOfTypeList ty)
           ; expand <- initialExpandMode
           ; let ve = ValidityEnv{ ve_tidy_env :: TidyEnv
    ve_tidy_env = TidyEnv
    env, ve_ctxt :: UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt
                                 , ve_rank :: Rank
    ve_rank = Rank
    rank, ve_expand :: ExpandMode
    ve_expand = ExpandMode
    expand }

           -- Check the internal validity of the type itself
           -- Fail if bad things happen, else we misleading
           -- (and more complicated) errors in checkAmbiguity
           ; checkNoErrs $
             do { check_type ve ty
                ; checkUserTypeError ctxt ty
                ; traceTc "done ct" (ppr ty) }

           -- Check for ambiguous types.  See Note [When to call checkAmbiguity]
           -- NB: this will happen even for monotypes, but that should be cheap;
           --     and there may be nested foralls for the subtype test to examine
           ; checkAmbiguity ctxt ty

           ; traceTc "checkValidType done" (ppr ty <+> text "::" <+> ppr (typeKind ty)) }

    checkValidMonoType :: Type -> TcM ()
    -- Assumes argument is fully zonked
    checkValidMonoType :: Type -> TcM ()
    checkValidMonoType Type
    ty
      = do { env <- ZonkM TidyEnv -> TcM TidyEnv
    forall a. ZonkM a -> TcM a
    liftZonkM (ZonkM TidyEnv -> TcM TidyEnv) -> ZonkM TidyEnv -> TcM TidyEnv
    forall a b. (a -> b) -> a -> b
    $ [Var] -> ZonkM TidyEnv
    tcInitOpenTidyEnv (Type -> [Var]
    tyCoVarsOfTypeList Type
    ty)
           ; expand <- initialExpandMode
           ; let ve = ValidityEnv{ ve_tidy_env :: TidyEnv
    ve_tidy_env = TidyEnv
    env, ve_ctxt :: UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    SigmaCtxt
                                 , ve_rank :: Rank
    ve_rank = Rank
    MustBeMonoType, ve_expand :: ExpandMode
    ve_expand = ExpandMode
    expand }
           ; check_type ve ty }

    checkTySynRhs :: UserTypeCtxt -> TcType -> TcM ()
    checkTySynRhs :: UserTypeCtxt -> Type -> TcM ()
    checkTySynRhs UserTypeCtxt
    ctxt Type
    ty
      | Type -> Bool
    returnsConstraintKind Type
    actual_kind
      = do { ck <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.ConstraintKinds
           ; if ck
             then  when (isConstraintLikeKind actual_kind)
                        (do { dflags <- getDynFlags
                            ; expand <- initialExpandMode
                            ; check_pred_ty emptyTidyEnv dflags ctxt expand ty })
             else addErrTcM ( emptyTidyEnv
                            , TcRnIllegalConstraintSynonymOfKind (tidyType emptyTidyEnv actual_kind)
                            ) }

      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      where
        actual_kind :: Type
    actual_kind = HasDebugCallStack => Type -> Type
    Type -> Type
    typeKind Type
    ty

    {- Note [Check for escaping result kind]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider:
      type T :: TYPE (BoxedRep l)
      data T = MkT
    This is not OK: we get
      MkT :: forall l. T @l :: TYPE (BoxedRep l)
    which is ill-kinded.

    For ordinary /user-written/ type signatures f :: blah, we make this
    check as part of kind-checking the type signature in tcHsSigType; see
    Note [Escaping kind in type signatures] in GHC.Tc.Gen.HsType.

    But in two other places we need to check for an escaping result kind:

    * For data constructors we check the type piecemeal, and there is no
      very convenient place to do it.  For example, note that it only
      applies for /nullary/ constructors.  If we had
        data T = MkT Int
      then the type of MkT would be MkT :: forall l. Int -> T @l, which is fine.

      So we make the check in checkValidDataCon.

    * When inferring the type of a function, there is no user-written type
      that we are checking.  Forgetting this led to #22743.  Now we call
      checkEscapingKind in GHC.Tc.Gen.Bind.mkInferredPolyId

    Historical note: we used to do the escaping-kind check in
    checkValidType (#20929 discusses), but that is now redundant.
    -}

    checkEscapingKind :: Type -> TcM ()
    -- Give a sigma-type (forall a1 .. an. ty), where (ty :: ki),
    -- check that `ki` does not mention any of the binders a1..an.
    -- Otherwise the type is ill-kinded
    -- See Note [Check for escaping result kind]
    checkEscapingKind :: Type -> TcM ()
    checkEscapingKind Type
    poly_ty
      | ([Var]
    tvs, Type
    tau) <- Type -> ([Var], Type)
    splitForAllTyVars Type
    poly_ty
      , let tau_kind :: Type
    tau_kind = HasDebugCallStack => Type -> Type
    Type -> Type
    typeKind Type
    tau
      , Maybe Type
    Nothing <- [Var] -> Type -> Maybe Type
    occCheckExpand [Var]
    tvs Type
    tau_kind
        -- Ensure that none of the tvs occur in the kind of the forall
        -- /after/ expanding type synonyms.
        -- See Note [Phantom type variables in kinds] in GHC.Core.Type
      = TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ Type -> Type -> TcRnMessage
    TcRnForAllEscapeError Type
    poly_ty Type
    tau_kind
      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    funArgResRank :: Rank -> (Rank, Rank)             -- Function argument and result
    funArgResRank :: Rank -> (Rank, Rank)
    funArgResRank (LimitedRank Bool
    _ Rank
    arg_rank) = (Rank
    arg_rank, Bool -> Rank -> Rank
    LimitedRank (Rank -> Bool
    forAllAllowed Rank
    arg_rank) Rank
    arg_rank)
    funArgResRank Rank
    other_rank               = (Rank
    other_rank, Rank
    other_rank)

    forAllAllowed :: Rank -> Bool
    forAllAllowed :: Rank -> Bool
    forAllAllowed Rank
    ArbitraryRank             = Bool
    True
    forAllAllowed (LimitedRank Bool
    forall_ok Rank
    _) = Bool
    forall_ok
    forAllAllowed Rank
    _                         = Bool
    False

    -- | Indicates whether a 'UserTypeCtxt' represents type-level contexts,
    -- kind-level contexts, or both.
    data TypeOrKindCtxt
      = OnlyTypeCtxt
        -- ^ A 'UserTypeCtxt' that only represents type-level positions.
      | OnlyKindCtxt
        -- ^ A 'UserTypeCtxt' that only represents kind-level positions.
      | BothTypeAndKindCtxt
        -- ^ A 'UserTypeCtxt' that can represent both type- and kind-level positions.
      deriving TypeOrKindCtxt -> TypeOrKindCtxt -> Bool
    (TypeOrKindCtxt -> TypeOrKindCtxt -> Bool)
    -> (TypeOrKindCtxt -> TypeOrKindCtxt -> Bool) -> Eq TypeOrKindCtxt
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    $c== :: TypeOrKindCtxt -> TypeOrKindCtxt -> Bool
    == :: TypeOrKindCtxt -> TypeOrKindCtxt -> Bool
    $c/= :: TypeOrKindCtxt -> TypeOrKindCtxt -> Bool
    /= :: TypeOrKindCtxt -> TypeOrKindCtxt -> Bool
    Eq

    instance Outputable TypeOrKindCtxt where
      ppr :: TypeOrKindCtxt -> SDoc
    ppr TypeOrKindCtxt
    ctxt = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text (String -> SDoc) -> String -> SDoc
    forall a b. (a -> b) -> a -> b
    $ case TypeOrKindCtxt
    ctxt of
        TypeOrKindCtxt
    OnlyTypeCtxt        -> String
    "OnlyTypeCtxt"
        TypeOrKindCtxt
    OnlyKindCtxt        -> String
    "OnlyKindCtxt"
        TypeOrKindCtxt
    BothTypeAndKindCtxt -> String
    "BothTypeAndKindCtxt"

    -- | Determine whether a 'UserTypeCtxt' can represent type-level contexts,
    -- kind-level contexts, or both.
    typeOrKindCtxt :: UserTypeCtxt -> TypeOrKindCtxt
    typeOrKindCtxt :: UserTypeCtxt -> TypeOrKindCtxt
    typeOrKindCtxt (FunSigCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (InfSigCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (ExprSigCtxt {})     = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (TypeAppCtxt {})     = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (PatSynCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (PatSigCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (RuleSigCtxt {})     = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (ForSigCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (DefaultDeclCtxt {}) = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (InstDeclCtxt {})    = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (SpecInstCtxt {})    = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (GenSigCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (ClassSCCtxt {})     = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (SigmaCtxt {})       = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (DataTyCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (DerivClauseCtxt {}) = TypeOrKindCtxt
    OnlyTypeCtxt
    typeOrKindCtxt (ConArgCtxt {})      = TypeOrKindCtxt
    OnlyTypeCtxt
      -- Although data constructors can be promoted with DataKinds, we always
      -- validity-check them as though they are the types of terms. We may need
      -- to revisit this decision if we ever allow visible dependent quantification
      -- in the types of data constructors.

    typeOrKindCtxt (KindSigCtxt {})           = TypeOrKindCtxt
    OnlyKindCtxt
    typeOrKindCtxt (StandaloneKindSigCtxt {}) = TypeOrKindCtxt
    OnlyKindCtxt
    typeOrKindCtxt (TyVarBndrKindCtxt {})     = TypeOrKindCtxt
    OnlyKindCtxt
    typeOrKindCtxt (DataKindCtxt {})          = TypeOrKindCtxt
    OnlyKindCtxt
    typeOrKindCtxt (TySynKindCtxt {})         = TypeOrKindCtxt
    OnlyKindCtxt
    typeOrKindCtxt (TyFamResKindCtxt {})      = TypeOrKindCtxt
    OnlyKindCtxt

    -- These cases are also described in Note [No constraints in kinds], so any
    -- change here should be reflected in that note.
    typeOrKindCtxt (TySynCtxt {}) = TypeOrKindCtxt
    BothTypeAndKindCtxt
      -- Type synonyms can have types and kinds on their RHSs
    typeOrKindCtxt (GhciCtxt {})  = TypeOrKindCtxt
    BothTypeAndKindCtxt
      -- GHCi's :kind command accepts both types and kinds

    -- | Returns 'True' if the supplied 'UserTypeCtxt' is unambiguously not the
    -- context for a kind of a type.
    -- If the 'UserTypeCtxt' can refer to both types and kinds, this function
    -- conservatively returns 'True'.
    --
    -- An example of something that is unambiguously the kind of a type is the
    -- @Show a => a -> a@ in @type Foo :: Show a => a -> a@. On the other hand, the
    -- same type in @foo :: Show a => a -> a@ is unambiguously the type of a term,
    -- not the kind of a type, so it is permitted.
    typeLevelUserTypeCtxt :: UserTypeCtxt -> Bool
    typeLevelUserTypeCtxt :: UserTypeCtxt -> Bool
    typeLevelUserTypeCtxt UserTypeCtxt
    ctxt = case UserTypeCtxt -> TypeOrKindCtxt
    typeOrKindCtxt UserTypeCtxt
    ctxt of
      TypeOrKindCtxt
    OnlyTypeCtxt        -> Bool
    True
      TypeOrKindCtxt
    OnlyKindCtxt        -> Bool
    False
      TypeOrKindCtxt
    BothTypeAndKindCtxt -> Bool
    True

    -- | Returns 'True' if the supplied 'UserTypeCtxt' is unambiguously not the
    -- context for a kind of a type, where the arbitrary use of constraints is
    -- currently disallowed.
    allConstraintsAllowed :: UserTypeCtxt -> Bool
    allConstraintsAllowed :: UserTypeCtxt -> Bool
    allConstraintsAllowed = UserTypeCtxt -> Bool
    typeLevelUserTypeCtxt

    -- | Returns 'True' if the supplied 'UserTypeCtxt' is unambiguously not the
    -- context for a kind of a type, where all function arrows currently
    -- must be unrestricted.
    linearityAllowed :: UserTypeCtxt -> Bool
    linearityAllowed :: UserTypeCtxt -> Bool
    linearityAllowed = UserTypeCtxt -> Bool
    typeLevelUserTypeCtxt

    -- | Returns 'True' if the supplied 'UserTypeCtxt' is unambiguously not the
    -- context for the type of a term, where visible, dependent quantification is
    -- currently disallowed. If the 'UserTypeCtxt' can refer to both types and
    -- kinds, this function conservatively returns 'True'.
    --
    -- An example of something that is unambiguously the type of a term is the
    -- @forall a -> a -> a@ in @foo :: forall a -> a -> a@. On the other hand, the
    -- same type in @type family Foo :: forall a -> a -> a@ is unambiguously the
    -- kind of a type, not the type of a term, so it is permitted.
    --
    -- For more examples, see
    -- @testsuite/tests/dependent/should_compile/T16326_Compile*.hs@ (for places
    -- where VDQ is permitted) and
    -- @testsuite/tests/dependent/should_fail/T16326_Fail*.hs@ (for places where
    -- VDQ is disallowed).
    vdqAllowed :: UserTypeCtxt -> Bool
    vdqAllowed :: UserTypeCtxt -> Bool
    vdqAllowed UserTypeCtxt
    ctxt = case UserTypeCtxt -> TypeOrKindCtxt
    typeOrKindCtxt UserTypeCtxt
    ctxt of
      TypeOrKindCtxt
    OnlyTypeCtxt        -> Bool
    False
      TypeOrKindCtxt
    OnlyKindCtxt        -> Bool
    True
      TypeOrKindCtxt
    BothTypeAndKindCtxt -> Bool
    True

    {-
    Note [Correctness and performance of type synonym validity checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the type A arg1 arg2, where A is a type synonym. How should we check
    this type for validity? We have three distinct choices, corresponding to the
    three constructors of ExpandMode:

    1. Expand the application of A, and check the resulting type (`Expand`).
    2. Don't expand the application of A. Only check the arguments (`NoExpand`).
    3. Check the arguments *and* check the expanded type (`Both`).

    It's tempting to think that we could always just pick choice (3), but this
    results in serious performance issues when checking a type like in the
    signature for `f` below:

      type S = ...
      f :: S (S (S (S (S (S ....(S Int)...))))

    When checking the type of `f`, we'll check the outer `S` application with and
    without expansion, and in *each* of those checks, we'll check the next `S`
    application with and without expansion... the result is exponential blowup! So
    clearly we don't want to use `Both` 100% of the time.

    On the other hand, neither is it correct to use exclusively `Expand` or
    exclusively `NoExpand` 100% of the time:

    * If one always expands, then one can miss erroneous programs like the one in
      the `tcfail129` test case:

        type Foo a = String -> Maybe a
        type Bar m = m Int
        blah = undefined :: Bar Foo

      If we expand `Bar Foo` immediately, we'll miss the fact that the `Foo` type
      synonyms is unsaturated.
    * If one never expands and only checks the arguments, then one can miss
      erroneous programs like the one in #16059:

        type Foo b = Eq b => b
        f :: forall b (a :: Foo b). Int

      The kind of `a` contains a constraint, which is illegal, but this will only
      be caught if `Foo b` is expanded.

    Therefore, it's impossible to have these validity checks be simultaneously
    correct and performant if one sticks exclusively to a single `ExpandMode`. In
    that case, the solution is to vary the `ExpandMode`s! In more detail:

    1. When we start validity checking, we start with `Expand` if
       LiberalTypeSynonyms is enabled (see Note [Liberal type synonyms] for why we
       do this), and we start with `Both` otherwise. The `initialExpandMode`
       function is responsible for this.
    2. When expanding an application of a type synonym (in `check_syn_tc_app`), we
       determine which things to check based on the current `ExpandMode` argument.
       Importantly, if the current mode is `Both`, then we check the arguments in
       `NoExpand` mode and check the expanded type in `Both` mode.

       Switching to `NoExpand` when checking the arguments is vital to avoid
       exponential blowup. One consequence of this choice is that if you have
       the following type synonym in one module (with RankNTypes enabled):

         {-# LANGUAGE RankNTypes #-}
         module A where
         type A = forall a. a

       And you define the following in a separate module *without* RankNTypes
       enabled:

         module B where

         import A

         type Const a b = a
         f :: Const Int A -> Int

       Then `f` will be accepted, even though `A` (which is technically a rank-n
       type) appears in its type. We view this as an acceptable compromise, since
       `A` never appears in the type of `f` post-expansion. If `A` _did_ appear in
       a type post-expansion, such as in the following variant:

         g :: Const A A -> Int

       Then that would be rejected unless RankNTypes were enabled.
    -}

    -- | When validity-checking an application of a type synonym, should we
    -- check the arguments, check the expanded type, or both?
    -- See Note [Correctness and performance of type synonym validity checking]
    data ExpandMode
      = Expand   -- ^ Only check the expanded type.
      | NoExpand -- ^ Only check the arguments.
      | Both     -- ^ Check both the arguments and the expanded type.

    instance Outputable ExpandMode where
      ppr :: ExpandMode -> SDoc
    ppr ExpandMode
    e = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text (String -> SDoc) -> String -> SDoc
    forall a b. (a -> b) -> a -> b
    $ case ExpandMode
    e of
                       ExpandMode
    Expand   -> String
    "Expand"
                       ExpandMode
    NoExpand -> String
    "NoExpand"
                       ExpandMode
    Both     -> String
    "Both"

    -- | If @LiberalTypeSynonyms@ is enabled, we start in 'Expand' mode for the
    -- reasons explained in @Note [Liberal type synonyms]@. Otherwise, we start
    -- in 'Both' mode.
    initialExpandMode :: TcM ExpandMode
    initialExpandMode :: TcM ExpandMode
    initialExpandMode = do
      liberal_flag <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.LiberalTypeSynonyms
      pure $ if liberal_flag then Expand else Both

    -- | Information about a type being validity-checked.
    data ValidityEnv = ValidityEnv
      { ValidityEnv -> TidyEnv
    ve_tidy_env :: TidyEnv
      , ValidityEnv -> UserTypeCtxt
    ve_ctxt     :: UserTypeCtxt
      , ValidityEnv -> Rank
    ve_rank     :: Rank
      , ValidityEnv -> ExpandMode
    ve_expand   :: ExpandMode }

    instance Outputable ValidityEnv where
      ppr :: ValidityEnv -> SDoc
    ppr (ValidityEnv{ ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env, ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt
                      , ve_rank :: ValidityEnv -> Rank
    ve_rank = Rank
    rank, ve_expand :: ValidityEnv -> ExpandMode
    ve_expand = ExpandMode
    expand }) =
        SDoc -> Int -> SDoc -> SDoc
    hang (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ValidityEnv")
           Int
    2 ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ve_tidy_env" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> TidyEnv -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TidyEnv
    env
                   , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ve_ctxt"     SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> UserTypeCtxt -> SDoc
    pprUserTypeCtxt UserTypeCtxt
    ctxt
                   , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ve_rank"     SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Rank -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Rank
    rank
                   , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "ve_expand"   SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> ExpandMode -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ExpandMode
    expand ])

    ----------------------------------------
    check_type :: ValidityEnv -> Type -> TcM ()
    -- The args say what the *type context* requires, independent
    -- of *flag* settings.  You test the flag settings at usage sites.
    --
    -- Rank is allowed rank for function args
    -- Rank 0 means no for-alls anywhere

    check_type :: ValidityEnv -> Type -> TcM ()
    check_type ValidityEnv
    _ (TyVarTy Var
    _)
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    check_type ValidityEnv
    ve (AppTy Type
    ty1 Type
    ty2)
      = do  { ValidityEnv -> Type -> TcM ()
    check_type ValidityEnv
    ve Type
    ty1
            ; Bool -> ValidityEnv -> Type -> TcM ()
    check_arg_type Bool
    False ValidityEnv
    ve Type
    ty2 }

    check_type ValidityEnv
    ve ty :: Type
    ty@(TyConApp TyCon
    tc [Type]
    tys)
      | TyCon -> Bool
    isTypeSynonymTyCon TyCon
    tc Bool -> Bool -> Bool
    || TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc
      = ValidityEnv -> Type -> TyCon -> [Type] -> TcM ()
    check_syn_tc_app ValidityEnv
    ve Type
    ty TyCon
    tc [Type]
    tys

      -- Check for unboxed tuples and unboxed sums: these
      -- require the corresponding extension to be enabled.
      | TyCon -> Bool
    isUnboxedTupleTyCon TyCon
    tc
      = UnboxedTupleOrSum -> ValidityEnv -> Type -> [Type] -> TcM ()
    check_ubx_tuple_or_sum UnboxedTupleOrSum
    UnboxedTupleType ValidityEnv
    ve Type
    ty [Type]
    tys
      | TyCon -> Bool
    isUnboxedSumTyCon TyCon
    tc
      = UnboxedTupleOrSum -> ValidityEnv -> Type -> [Type] -> TcM ()
    check_ubx_tuple_or_sum UnboxedTupleOrSum
    UnboxedSumType   ValidityEnv
    ve Type
    ty [Type]
    tys

      | Bool
    otherwise
      = do { -- We require DataKinds to use a type constructor in a kind, unless it
             -- is exempted (e.g., Type, TYPE, etc., which is checked by
             -- isKindTyCon) or a `type data` type constructor.
             -- See Note [Checking for DataKinds].
             Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    unless (TyCon -> Bool
    isKindTyCon TyCon
    tc Bool -> Bool -> Bool
    || TyCon -> Bool
    isTypeDataTyCon TyCon
    tc) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             ValidityEnv -> Type -> TcM ()
    checkDataKinds ValidityEnv
    ve Type
    ty
           ; (Type -> TcM ()) -> [Type] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (Bool -> ValidityEnv -> Type -> TcM ()
    check_arg_type Bool
    False ValidityEnv
    ve) [Type]
    tys }

    check_type ValidityEnv
    ve ty :: Type
    ty@(LitTy {}) =
      -- Type-level literals are forbidden from appearing in kinds unless DataKinds
      -- is enabled. See Note [Checking for DataKinds].
      ValidityEnv -> Type -> TcM ()
    checkDataKinds ValidityEnv
    ve Type
    ty

    check_type ValidityEnv
    ve (CastTy Type
    ty KindCoercion
    _) = ValidityEnv -> Type -> TcM ()
    check_type ValidityEnv
    ve Type
    ty

    -- Check for rank-n types, such as (forall x. x -> x) or (Show x => x).
    --
    -- Critically, this case must come *after* the case for TyConApp.
    -- See Note [Liberal type synonyms].
    check_type ve :: ValidityEnv
    ve@(ValidityEnv{ ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env
                              , ve_rank :: ValidityEnv -> Rank
    ve_rank = Rank
    rank, ve_expand :: ValidityEnv -> ExpandMode
    ve_expand = ExpandMode
    expand }) Type
    ty
      | Bool -> Bool
    not ([TyVarBinder] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [TyVarBinder]
    tvbs Bool -> Bool -> Bool
    && [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    theta)
      = do  { String -> SDoc -> TcM ()
    traceTc String
    "check_type" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty SDoc -> SDoc -> SDoc
    forall doc. IsDoc doc => doc -> doc -> doc
    $$ Rank -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Rank
    rank)
            ; Bool -> (TidyEnv, TcRnMessage) -> TcM ()
    checkTcM (Rank -> Bool
    forAllAllowed Rank
    rank) (TidyEnv
    env, Rank -> Type -> TcRnMessage
    TcRnForAllRankErr Rank
    rank (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    ty))
                    -- Reject e.g. (Maybe (?x::Int => Int)),
                    -- with a decent error message

            ; ValidityEnv -> [Type] -> Type -> TcM ()
    checkConstraintsOK ValidityEnv
    ve [Type]
    theta Type
    ty
                    -- Reject forall (a :: Eq b => b). blah
                    -- In a kind signature we don't allow constraints

            ; ValidityEnv -> [TyVarBinder] -> Type -> TcM ()
    checkVdqOK ValidityEnv
    ve [TyVarBinder]
    tvbs Type
    ty
                    -- Reject visible, dependent quantification in the type of a
                    -- term (e.g., `f :: forall a -> a -> Maybe a`)

            ; TidyEnv -> UserTypeCtxt -> ExpandMode -> [Type] -> TcM ()
    check_valid_theta TidyEnv
    env' UserTypeCtxt
    SigmaCtxt ExpandMode
    expand [Type]
    theta
                    -- Allow     type T = ?x::Int => Int -> Int
                    -- but not   type T = ?x::Int

            ; ValidityEnv -> Type -> TcM ()
    check_type (ValidityEnv
    ve{ve_tidy_env = env'}) Type
    tau
                    -- Allow foralls to right of arrow

            -- Note: skolem-escape in types (e.g. forall r (a::r). a) is handled
            --       by tcHsSigType and the constraint solver, so no need to
            --       check it here; c.f. #20929
            }
      where
        ([TyVarBinder]
    tvbs, Type
    phi)   = Type -> ([TyVarBinder], Type)
    tcSplitForAllTyVarBinders Type
    ty
        ([Type]
    theta, Type
    tau)  = Type -> ([Type], Type)
    tcSplitPhiTy Type
    phi
        (TidyEnv
    env', [TyVarBinder]
    _)     = TidyEnv -> [TyVarBinder] -> (TidyEnv, [TyVarBinder])
    forall vis.
    TidyEnv -> [VarBndr Var vis] -> (TidyEnv, [VarBndr Var vis])
    tidyForAllTyBinders TidyEnv
    env [TyVarBinder]
    tvbs

    check_type (ve :: ValidityEnv
    ve@ValidityEnv{ ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env, ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt
                              , ve_rank :: ValidityEnv -> Rank
    ve_rank = Rank
    rank })
               ty :: Type
    ty@(FunTy FunTyFlag
    _ Type
    mult Type
    arg_ty Type
    res_ty)
      = do  { Bool -> (TidyEnv, TcRnMessage) -> TcM ()
    failIfTcM (Bool -> Bool
    not (UserTypeCtxt -> Bool
    linearityAllowed UserTypeCtxt
    ctxt) Bool -> Bool -> Bool
    && Bool -> Bool
    not (Type -> Bool
    isManyTy Type
    mult))
                         (TidyEnv
    env, Type -> TcRnMessage
    TcRnLinearFuncInKind (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    ty))
            ; ValidityEnv -> Type -> TcM ()
    check_type (ValidityEnv
    ve{ve_rank = arg_rank}) Type
    arg_ty
            ; ValidityEnv -> Type -> TcM ()
    check_type (ValidityEnv
    ve{ve_rank = res_rank}) Type
    res_ty }
      where
        (Rank
    arg_rank, Rank
    res_rank) = Rank -> (Rank, Rank)
    funArgResRank Rank
    rank

    check_type ValidityEnv
    _ ty :: Type
    ty@(ForAllTy {}) = String -> SDoc -> TcM ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "check_type" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)
    check_type ValidityEnv
    _ ty :: Type
    ty@(CoercionTy {}) = String -> SDoc -> TcM ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "check_type" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)

    ----------------------------------------
    check_syn_tc_app :: ValidityEnv
                     -> KindOrType -> TyCon -> [KindOrType] -> TcM ()
    -- Used for type synonyms and type synonym families,
    -- which must be saturated,
    -- but not data families, which need not be saturated
    check_syn_tc_app :: ValidityEnv -> Type -> TyCon -> [Type] -> TcM ()
    check_syn_tc_app (ve :: ValidityEnv
    ve@ValidityEnv{ ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt, ve_expand :: ValidityEnv -> ExpandMode
    ve_expand = ExpandMode
    expand })
                     Type
    ty TyCon
    tc [Type]
    tys
      | [Type]
    tys [Type] -> Int -> Bool
    forall a. [a] -> Int -> Bool
    `lengthAtLeast` Int
    tc_arity   -- Saturated
           -- Check that the synonym has enough args
           -- This applies equally to open and closed synonyms
           -- It's OK to have an *over-applied* type synonym
           --      data Tree a b = ...
           --      type Foo a = Tree [a]
           --      f :: Foo a b -> ...
      = case ExpandMode
    expand of
          ExpandMode
    _ |  TyCon -> Bool
    isTypeFamilyTyCon TyCon
    tc
            -> ExpandMode -> TcM ()
    check_args_only ExpandMode
    expand
          -- See Note [Correctness and performance of type synonym validity
          --           checking]
          ExpandMode
    Expand   -> ExpandMode -> TcM ()
    check_expansion_only ExpandMode
    expand
          ExpandMode
    NoExpand -> ExpandMode -> TcM ()
    check_args_only ExpandMode
    expand
          ExpandMode
    Both     -> ExpandMode -> TcM ()
    check_args_only ExpandMode
    NoExpand TcM () -> TcM () -> TcM ()
    forall a b.
    IOEnv (Env TcGblEnv TcLclEnv) a
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    forall (f :: * -> *) a b. Applicative f => f a -> f b -> f b
    *> ExpandMode -> TcM ()
    check_expansion_only ExpandMode
    Both

      | GhciCtxt Bool
    True <- UserTypeCtxt
    ctxt  -- Accept outermost under-saturated type synonym or
                               -- type family constructors in GHCi :kind commands.
                               -- See Note [Unsaturated type synonyms in GHCi]
      = ExpandMode -> TcM ()
    check_args_only ExpandMode
    expand

      | Bool
    otherwise
      = TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TyCon -> [Type] -> TcRnMessage
    tyConArityErr TyCon
    tc [Type]
    tys)
      where
        tc_arity :: Int
    tc_arity  = TyCon -> Int
    tyConArity TyCon
    tc

        check_arg :: ExpandMode -> KindOrType -> TcM ()
        check_arg :: ExpandMode -> Type -> TcM ()
    check_arg ExpandMode
    expand =
          Bool -> ValidityEnv -> Type -> TcM ()
    check_arg_type (TyCon -> Bool
    isTypeSynonymTyCon TyCon
    tc) (ValidityEnv
    ve{ve_expand = expand})

        check_args_only, check_expansion_only :: ExpandMode -> TcM ()
        check_args_only :: ExpandMode -> TcM ()
    check_args_only ExpandMode
    expand = (Type -> TcM ()) -> [Type] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (ExpandMode -> Type -> TcM ()
    check_arg ExpandMode
    expand) [Type]
    tys

        check_expansion_only :: ExpandMode -> TcM ()
    check_expansion_only ExpandMode
    expand
          = Bool -> SDoc -> TcM () -> TcM ()
    forall a. HasCallStack => Bool -> SDoc -> a -> a
    assertPpr (TyCon -> Bool
    isTypeSynonymTyCon TyCon
    tc) (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
            case Type -> Maybe Type
    coreView Type
    ty of
             Just Type
    ty' -> let err_ctxt :: SDoc
    err_ctxt = String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "In the expansion of type synonym"
                                        SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> SDoc -> SDoc
    quotes (TyCon -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr TyCon
    tc)
                         in SDoc -> TcM () -> TcM ()
    forall a. SDoc -> TcM a -> TcM a
    addErrCtxt SDoc
    err_ctxt (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
                            ValidityEnv -> Type -> TcM ()
    check_type (ValidityEnv
    ve{ve_expand = expand}) Type
    ty'
             Maybe Type
    Nothing  -> String -> SDoc -> TcM ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "check_syn_tc_app" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)

    {-
    Note [Unsaturated type synonyms in GHCi]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Generally speaking, GHC disallows unsaturated uses of type synonyms or type
    families. For instance, if one defines `type Const a b = a`, then GHC will not
    permit using `Const` unless it is applied to (at least) two arguments. There is
    an exception to this rule, however: GHCi's :kind command. For instance, it
    is quite common to look up the kind of a type constructor like so:

      λ> :kind Const
      Const :: j -> k -> j
      λ> :kind Const Int
      Const Int :: k -> Type

    Strictly speaking, the two uses of `Const` above are unsaturated, but this
    is an extremely benign (and useful) example of unsaturation, so we allow it
    here as a special case.

    That being said, we do not allow unsaturation carte blanche in GHCi. Otherwise,
    this GHCi interaction would be possible:

      λ> newtype Fix f = MkFix (f (Fix f))
      λ> type Id a = a
      λ> :kind Fix Id
      Fix Id :: Type

    This is rather dodgy, so we move to disallow this. We only permit unsaturated
    synonyms in GHCi if they are *top-level*—that is, if the synonym is the
    outermost type being applied. This allows `Const` and `Const Int` in the
    first example, but not `Fix Id` in the second example, as `Id` is not the
    outermost type being applied (`Fix` is).

    We track this outermost property in the GhciCtxt constructor of UserTypeCtxt.
    A field of True in GhciCtxt indicates that we're in an outermost position. Any
    time we invoke `check_arg` to check the validity of an argument, we switch the
    field to False.
    -}

    ----------------------------------------
    check_ubx_tuple_or_sum :: UnboxedTupleOrSum -> ValidityEnv -> KindOrType -> [KindOrType] -> TcM ()
    check_ubx_tuple_or_sum :: UnboxedTupleOrSum -> ValidityEnv -> Type -> [Type] -> TcM ()
    check_ubx_tuple_or_sum UnboxedTupleOrSum
    tup_or_sum (ve :: ValidityEnv
    ve@ValidityEnv{ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env}) Type
    ty [Type]
    tys
      = do  { ub_thing_allowed <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM (Extension -> TcRnIf TcGblEnv TcLclEnv Bool)
    -> Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall a b. (a -> b) -> a -> b
    $ UnboxedTupleOrSum -> Extension
    unboxedTupleOrSumExtension UnboxedTupleOrSum
    tup_or_sum
            ; checkTcM ub_thing_allowed
                (env, TcRnUnboxedTupleOrSumTypeFuncArg tup_or_sum (tidyType env ty))

              -- Unboxed tuples and sums are forbidden from appearing in kinds
              -- unless DataKinds is enabled. See Note [Checking for DataKinds].
            ; checkDataKinds ve ty

            ; impred <- xoptM LangExt.ImpredicativeTypes
            ; let rank' = if Bool
    impred then Rank
    ArbitraryRank else Rank
    MonoTypeTyConArg
                    -- c.f. check_arg_type
                    -- However, args are allowed to be unlifted, or
                    -- more unboxed tuples or sums, so can't use check_arg_ty
            ; mapM_ (check_type (ve{ve_rank = rank'})) tys }

    ----------------------------------------
    check_arg_type
      :: Bool -- ^ Is this the argument to a type synonym?
      -> ValidityEnv -> KindOrType -> TcM ()
    -- The sort of type that can instantiate a type variable,
    -- or be the argument of a type constructor.
    -- Not an unboxed tuple, but now *can* be a forall (since impredicativity)
    -- Other unboxed types are very occasionally allowed as type
    -- arguments depending on the kind of the type constructor
    --
    -- For example, we want to reject things like:
    --
    --      instance Ord a => Ord (forall s. T s a)
    -- and
    --      g :: T s (forall b.b)
    --
    -- NB: unboxed tuples can have polymorphic or unboxed args.
    --     This happens in the workers for functions returning
    --     product types with polymorphic components.
    --     But not in user code.
    -- Anyway, they are dealt with by a special case in check_tau_type

    check_arg_type :: Bool -> ValidityEnv -> Type -> TcM ()
    check_arg_type Bool
    _ ValidityEnv
    _ (CoercionTy {}) = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    check_arg_type Bool
    type_syn (ve :: ValidityEnv
    ve@ValidityEnv{ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt, ve_rank :: ValidityEnv -> Rank
    ve_rank = Rank
    rank}) Type
    ty
      = do  { impred <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.ImpredicativeTypes
            ; let rank' = case Rank
    rank of          -- Predictive => must be monotype
                            -- Rank-n arguments to type synonyms are OK, provided
                            -- that LiberalTypeSynonyms is enabled.
                            Rank
    _ | Bool
    type_syn       -> Rank
    MonoTypeSynArg
                            Rank
    MustBeMonoType     -> Rank
    MustBeMonoType  -- Monotype, regardless
                            Rank
    _other | Bool
    impred    -> Rank
    ArbitraryRank
                                   | Bool
    otherwise -> Rank
    MonoTypeTyConArg
                            -- Make sure that MustBeMonoType is propagated,
                            -- so that we don't suggest -XImpredicativeTypes in
                            --    (Ord (forall a.a)) => a -> a
                            -- and so that if it Must be a monotype, we check that it is!
                  ctxt' :: UserTypeCtxt
                  ctxt'
                    | GhciCtxt Bool
    _ <- UserTypeCtxt
    ctxt = Bool -> UserTypeCtxt
    GhciCtxt Bool
    False
                        -- When checking an argument, set the field of GhciCtxt to
                        -- False to indicate that we are no longer in an outermost
                        -- position (and thus unsaturated synonyms are no longer
                        -- allowed).
                        -- See Note [Unsaturated type synonyms in GHCi]
                    | Bool
    otherwise          = UserTypeCtxt
    ctxt

            ; check_type (ve{ve_ctxt = ctxt', ve_rank = rank'}) ty }

    ----------------------------------------
    checkConstraintsOK :: ValidityEnv -> ThetaType -> Type -> TcM ()
    checkConstraintsOK :: ValidityEnv -> [Type] -> Type -> TcM ()
    checkConstraintsOK ValidityEnv
    ve [Type]
    theta Type
    ty
      | [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    theta                         = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | UserTypeCtxt -> Bool
    allConstraintsAllowed (ValidityEnv -> UserTypeCtxt
    ve_ctxt ValidityEnv
    ve) = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Bool
    otherwise -- We are unambiguously in a kind; see
                  -- Note [No constraints in kinds]
      = (TidyEnv, TcRnMessage) -> TcM ()
    forall a. (TidyEnv, TcRnMessage) -> TcM a
    failWithTcM (TidyEnv
    env, Type -> TcRnMessage
    TcRnConstraintInKind (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    ty))
      where env :: TidyEnv
    env = ValidityEnv -> TidyEnv
    ve_tidy_env ValidityEnv
    ve

    checkVdqOK :: ValidityEnv -> [TyVarBinder] -> Type -> TcM ()
    checkVdqOK :: ValidityEnv -> [TyVarBinder] -> Type -> TcM ()
    checkVdqOK ValidityEnv
    ve [TyVarBinder]
    tvbs Type
    ty = do
      required_type_arguments <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.RequiredTypeArguments
      checkTcM (required_type_arguments || vdqAllowed ctxt || no_vdq)
               (env, TcRnVDQInTermType (Just (tidyType env ty)))
      where
        no_vdq :: Bool
    no_vdq = (TyVarBinder -> Bool) -> [TyVarBinder] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all (ForAllTyFlag -> Bool
    isInvisibleForAllTyFlag (ForAllTyFlag -> Bool)
    -> (TyVarBinder -> ForAllTyFlag) -> TyVarBinder -> Bool
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TyVarBinder -> ForAllTyFlag
    forall tv argf. VarBndr tv argf -> argf
    binderFlag) [TyVarBinder]
    tvbs
        ValidityEnv{ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env, ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt} = ValidityEnv
    ve

    -- | Check for a DataKinds violation in a kind context.
    -- See @Note [Checking for DataKinds]@.
    --
    -- Note that emitting DataKinds errors from the typechecker is a fairly recent
    -- addition to GHC (introduced in GHC 9.10), and in order to prevent these new
    -- errors from breaking users' code, we temporarily downgrade these errors to
    -- warnings. (This is why we use 'diagnosticTcM' below.) See
    -- @Note [Checking for DataKinds] (Wrinkle: Migration story for DataKinds
    -- typechecker errors)@.
    checkDataKinds :: ValidityEnv -> Type -> TcM ()
    checkDataKinds :: ValidityEnv -> Type -> TcM ()
    checkDataKinds (ValidityEnv{ ve_ctxt :: ValidityEnv -> UserTypeCtxt
    ve_ctxt = UserTypeCtxt
    ctxt, ve_tidy_env :: ValidityEnv -> TidyEnv
    ve_tidy_env = TidyEnv
    env }) Type
    ty = do
      data_kinds <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.DataKinds
      diagnosticTcM
        (not (data_kinds || typeLevelUserTypeCtxt ctxt)) $
        (env, TcRnDataKindsError KindLevel (Right (tidyType env ty)))

    {- Note [No constraints in kinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    GHC does not allow constraints in kinds. Equality constraints
    in kinds were allowed from GHC 8.0, but this "feature" was removed
    as part of Proposal #547 (https://github.com/ghc-proposals/ghc-proposals/pull/547),
    which contains further context and motivation for the removal.

    The lack of constraints in kinds is enforced by checkConstraintsOK, which
    uses the UserTypeCtxt to determine if we are unambiguously checking a kind.
    There are two ambiguous contexts (constructor BothTypeAndKindCtxt of TypeOrKindCtxt)
    as written in typeOfKindCtxt:
      - TySynCtxt: this is the RHS of a type synonym. We check the expansion of type
        synonyms for constraints, so this is handled at the usage site of the synonym.
      - GhciCtxt: This is the type in a :kind command. A constraint here does not cause
        any trouble, because the type cannot be used to classify a type.

    Beyond these two cases, we also promote data constructors. We check for constraints
    in data constructor types in GHC.Tc.Gen.HsType.tcTyVar.

    Note [Liberal type synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If -XLiberalTypeSynonyms is on, expand closed type synonyms *before*
    doing validity checking.  This allows us to instantiate a synonym defn
    with a for-all type, or with a partially-applied type synonym.
            e.g.   type T a b = a
                   type S m   = m ()
                   f :: S (T Int)
    Here, T is partially applied, so it's illegal in H98.  But if you
    expand S first, then T we get just
                   f :: Int
    which is fine.

    IMPORTANT: suppose T is a type synonym.  Then we must do validity
    checking on an application (T ty1 ty2)

            *either* before expansion (i.e. check ty1, ty2)
            *or* after expansion (i.e. expand T ty1 ty2, and then check)
            BUT NOT BOTH

    If we do both, we get exponential behaviour!!

      data TIACons1 i r c = c i ::: r c
      type TIACons2 t x = TIACons1 t (TIACons1 t x)
      type TIACons3 t x = TIACons2 t (TIACons1 t x)
      type TIACons4 t x = TIACons2 t (TIACons2 t x)
      type TIACons7 t x = TIACons4 t (TIACons3 t x)

    The order in which you do validity checking is also somewhat delicate. Consider
    the `check_type` function, which drives the validity checking for unsaturated
    uses of type synonyms. There is a special case for rank-n types, such as
    (forall x. x -> x) or (Show x => x), since those require at least one language
    extension to use. It used to be the case that this case came before every other
    case, but this can lead to bugs. Imagine you have this scenario (from #15954):

      type A a = Int
      type B (a :: Type -> Type) = forall x. x -> x
      type C = B A

    If the rank-n case came first, then in the process of checking for `forall`s
    or contexts, we would expand away `B A` to `forall x. x -> x`. This is because
    the functions that split apart `forall`s/contexts
    (tcSplitForAllTyVarBinders/tcSplitPhiTy) expand type synonyms! If `B A` is expanded
    away to `forall x. x -> x` before the actually validity checks occur, we will
    have completely obfuscated the fact that we had an unsaturated application of
    the `A` type synonym.

    We have since learned from our mistakes and now put this rank-n case /after/
    the case for TyConApp, which ensures that an unsaturated `A` TyConApp will be
    caught properly. But be careful! We can't make the rank-n case /last/ either,
    as the FunTy case must came after the rank-n case. Otherwise, something like
    (Eq a => Int) would be treated as a function type (FunTy), which just
    wouldn't do.

    Note [Checking for DataKinds]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Checking whether a piece of code requires -XDataKinds or not is surprisingly
    complicated, so here is a specification (adapted from #22141) for what
    -XDataKinds does and does not allow. First, some definitions:

    * A user-written type (i.e. part of the source text of a program) is in a
      /kind context/ if it follows a `::` in:
      * A standalone kind signature, e.g.,
          type T :: Nat -> Type
      * A kind signature in a type, e.g.:
        - forall (a :: Nat -> Type). blah
        - type F = G :: Nat -> Type
        - etc.
      * A result kind signature in a type declaration, e.g.:
        - data T a :: Nat -> Type where ...
        - type family Fam :: Nat -> Type
        - etc.

    * All other contexts where types can appear are referred to as /type contexts/.

    * The /kind type constructors/ are (see GHC.Core.TyCon.isKindTyCon):
      * TYPE and Type
      * CONSTRAINT and Constraint
      * LiftedRep
      * RuntimeRep, Levity, and their data constructors
      * Multiplicity and its data construtors
      * VecCount, VecElem, and their data constructors

    * A `type data` type constructor is defined using the -XTypeData extension, such
      as the T in `type data T = A | B`.

    * The following are rejected in type contexts unless -XDataKinds is enabled:
      * Promoted data constructors (e.g., 'Just), except for those data constructors
        listed under /kind type constructors/
      * Promoted list or tuple syntax (e.g., '[Int, Bool] or '(Int, Bool))
      * Type-level literals (e.g., 42, "hello", or 'a' at the type level)

    * The following are rejected in kind contexts unless -XDataKinds is enabled:
      * Everything that is rejected in a type context.
      * Any type constructor that is not a kind type constructor or a `type data`
        type constructor (e.g., Maybe, [], Char, Nat, Symbol, etc.)

        Note that this includes rejecting occurrences of non-kind type construtors
        in type synomym (or type family) applications, even it the expansion would
        be legal. For example:

          type T a = Type
          f :: forall (x :: T Int). blah

        Here the `Int` in `T Int` is rejected even though the expansion is just
        `Type`. This is consistent with, for example, rejecting `T (forall a. a->a)`
        without -XImpredicativeTypes.

        This check only occurs in kind contexts. It is always permissible to mention
        type synonyms in a type context without enabling -XDataKinds, even if the
        type synonym expands to something that would otherwise require -XDataKinds.

    Because checking for DataKinds in a kind context requires looking beneath type
    synonyms, it is natural to implement these checks in checkValidType, which has
    the necessary machinery to check for language extensions in the presence of
    type synonyms. For the exact same reason, checkValidType is *not* a good place
    to check for DataKinds in a type context, since we deliberately do not want to
    look beneath type synonyms there. As a result, we check for DataKinds in two
    different places in the code:

    * We check for DataKinds violations in kind contexts in the typechecker. See
      checkDataKinds in this module.
    * We check for DataKinds violations in type contexts in the renamer. See
      checkDataKinds in GHC.Rename.HsType and check_data_kinds in GHC.Rename.Pat.

      Note that the renamer can also catch "obvious" kind-level violations such as
      `data Dat :: Proxy 42 -> Type` (where 42 is not hidden beneath a type
      synonym), so we also catch a subset of kind-level violations in the renamer
      to allow for earlier reporting of these errors.

    -----
    -- Wrinkle: Migration story for DataKinds typechecker errors
    -----

    As mentioned above, DataKinds is checked in two different places: the renamer
    and the typechecker. The checks in the renamer have been around since DataKinds
    was introduced. The checks in the typechecker, on the other hand, are a fairly
    recent addition, having been introduced in GHC 9.10. As such, it is possible
    that there are some programs in the wild that (1) do not enable DataKinds, and
    (2) were accepted by a previous GHC version, but would now be rejected by the
    new DataKinds checks in the typechecker.

    To prevent the new DataKinds checks in the typechecker from breaking users'
    code, we temporarily allow programs to compile if they violate a DataKinds
    check in the typechecker, but GHC will emit a warning if such a violation
    occurs. Users can then silence the warning by enabling DataKinds in the module
    where the affected code lives. It is fairly straightforward to distinguish
    between DataKinds violations arising from the renamer versus the typechecker,
    as TcRnDataKindsError (the error message type classifying all DataKinds errors)
    stores an Either field that is Left when the error comes from the renamer and
    Right when the error comes from the typechecker.

    ************************************************************************
    *                                                                      *
    \subsection{Checking a theta or source type}
    *                                                                      *
    ************************************************************************

    Note [Implicit parameters in instance decls]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Implicit parameters _only_ allowed in type signatures; not in instance
    decls, superclasses etc. The reason for not allowing implicit params in
    instances is a bit subtle.  If we allowed
      instance (?x::Int, Eq a) => Foo [a] where ...
    then when we saw
         (e :: (?x::Int) => t)
    it would be unclear how to discharge all the potential uses of the ?x
    in e.  For example, a constraint Foo [Int] might come out of e, and
    applying the instance decl would show up two uses of ?x.  #8912.
    -}

    checkValidTheta :: UserTypeCtxt -> ThetaType -> TcM ()
    -- Assumes argument is fully zonked
    checkValidTheta :: UserTypeCtxt -> [Type] -> TcM ()
    checkValidTheta UserTypeCtxt
    ctxt [Type]
    theta
      = (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM () -> TcM ()
    forall a. (TidyEnv -> ZonkM (TidyEnv, SDoc)) -> TcM a -> TcM a
    addErrCtxtM (UserTypeCtxt -> [Type] -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    checkThetaCtxt UserTypeCtxt
    ctxt [Type]
    theta) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
        do { env <- ZonkM TidyEnv -> TcM TidyEnv
    forall a. ZonkM a -> TcM a
    liftZonkM (ZonkM TidyEnv -> TcM TidyEnv) -> ZonkM TidyEnv -> TcM TidyEnv
    forall a b. (a -> b) -> a -> b
    $ [Var] -> ZonkM TidyEnv
    tcInitOpenTidyEnv ([Type] -> [Var]
    tyCoVarsOfTypesList [Type]
    theta)
           ; expand <- initialExpandMode
           ; check_valid_theta env ctxt expand theta }

    -------------------------
    check_valid_theta :: TidyEnv -> UserTypeCtxt -> ExpandMode
                      -> [PredType] -> TcM ()
    check_valid_theta :: TidyEnv -> UserTypeCtxt -> ExpandMode -> [Type] -> TcM ()
    check_valid_theta TidyEnv
    _ UserTypeCtxt
    _ ExpandMode
    _ []
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
    check_valid_theta TidyEnv
    env UserTypeCtxt
    ctxt ExpandMode
    expand [Type]
    theta
      = do { dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; traceTc "check_valid_theta" (ppr theta)
           ; mapM_ (check_pred_ty env dflags ctxt expand) theta }

    -------------------------
    {- Note [Validity checking for constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We look through constraint synonyms so that we can see the underlying
    constraint(s).  For example
       type Foo = ?x::Int
       instance Foo => C T
    We should reject the instance because it has an implicit parameter in
    the context.

    But we record, in 'under_syn', whether we have looked under a synonym
    to avoid requiring language extensions at the use site.  Main example
    (#9838):

       {-# LANGUAGE ConstraintKinds #-}
       module A where
          type EqShow a = (Eq a, Show a)

       module B where
          import A
          foo :: EqShow a => a -> String

    We don't want to require ConstraintKinds in module B.
    -}

    check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> ExpandMode
                  -> PredType -> TcM ()
    -- Check the validity of a predicate in a signature
    -- See Note [Validity checking for constraints]
    check_pred_ty :: TidyEnv -> DynFlags -> UserTypeCtxt -> ExpandMode -> Type -> TcM ()
    check_pred_ty TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt ExpandMode
    expand Type
    pred
      = do { ValidityEnv -> Type -> TcM ()
    check_type ValidityEnv
    ve Type
    pred
           ; Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> TcM ()
    check_pred_help Bool
    False TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred }
      where
        rank :: Rank
    rank | Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.QuantifiedConstraints DynFlags
    dflags
             = Rank
    ArbitraryRank
             | Bool
    otherwise
             = Rank
    MonoTypeConstraint

        ve :: ValidityEnv
        ve :: ValidityEnv
    ve = ValidityEnv{ ve_tidy_env :: TidyEnv
    ve_tidy_env = TidyEnv
    env
                        , ve_ctxt :: UserTypeCtxt
    ve_ctxt     = UserTypeCtxt
    SigmaCtxt
                        , ve_rank :: Rank
    ve_rank     = Rank
    rank
                        , ve_expand :: ExpandMode
    ve_expand   = ExpandMode
    expand }

    check_pred_help :: Bool    -- True <=> under a type synonym
                    -> TidyEnv
                    -> DynFlags -> UserTypeCtxt
                    -> PredType -> TcM ()
    check_pred_help :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> TcM ()
    check_pred_help Bool
    under_syn TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred
      | Just Type
    pred' <- Type -> Maybe Type
    coreView Type
    pred  -- Switch on under_syn when going under a
                                     -- synonym (#9838, yuk)
      = Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> TcM ()
    check_pred_help Bool
    True TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred'

      | Bool
    otherwise  -- A bit like classifyPredType, but not the same
                   -- E.g. we treat (~) like (~#); and we look inside tuples
      = case Type -> Pred
    classifyPredType Type
    pred of
          ClassPred Class
    cls [Type]
    tys
            | Class -> Bool
    isCTupleClass Class
    cls   -> Bool
    -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> [Type] -> TcM ()
    check_tuple_pred Bool
    under_syn TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred [Type]
    tys
            | Bool
    otherwise           -> TidyEnv
    -> DynFlags -> UserTypeCtxt -> Type -> Class -> [Type] -> TcM ()
    check_class_pred TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred Class
    cls [Type]
    tys

          EqPred EqRel
    _ Type
    _ Type
    _      -> String -> SDoc -> TcM ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "check_pred_help" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    pred)
                  -- EqPreds, such as (t1 ~# t2) or (t1 ~R# t2), don't even have kind Constraint
                  -- and should never appear before the '=>' of a type.  Thus
                  --     f :: (a ~# b) => blah
                  -- is wrong.  For user written signatures, it'll be rejected by kind-checking
                  -- well before we get to validity checking.  For inferred types we are careful
                  -- to box such constraints in GHC.Tc.Utils.TcType.pickQuantifiablePreds, as described
                  -- in Note [Lift equality constraints when quantifying] in GHC.Tc.Solver

          ForAllPred [Var]
    _ [Type]
    theta Type
    head -> TidyEnv
    -> DynFlags -> UserTypeCtxt -> Type -> [Type] -> Type -> TcM ()
    check_quant_pred TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred [Type]
    theta Type
    head
          Pred
    _                       -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    check_quant_pred :: TidyEnv -> DynFlags -> UserTypeCtxt
                     -> PredType -> ThetaType -> PredType -> TcM ()
    check_quant_pred :: TidyEnv
    -> DynFlags -> UserTypeCtxt -> Type -> [Type] -> Type -> TcM ()
    check_quant_pred TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred [Type]
    theta Type
    head_pred
      = SDoc -> TcM () -> TcM ()
    forall a. SDoc -> TcM a -> TcM a
    addErrCtxt (String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "In the quantified constraint" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> SDoc -> SDoc
    quotes (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    pred)) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
        do { -- Check the instance head
             case Type -> Pred
    classifyPredType Type
    head_pred of
                                     -- SigmaCtxt tells checkValidInstHead that
                                     -- this is the head of a quantified constraint
                ClassPred Class
    cls [Type]
    tys -> do { UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkValidInstHead UserTypeCtxt
    SigmaCtxt Class
    cls [Type]
    tys
                                        ; Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> TcM ()
    check_pred_help Bool
    False TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    head_pred }
                                   -- need check_pred_help to do extra pred-only validity
                                   -- checks, such as for (~). Otherwise, we get #17563
                                   -- NB: checks for the context are covered by the check_type
                                   -- in check_pred_ty
                IrredPred {}      | Type -> Bool
    hasTyVarHead Type
    head_pred
                                  -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
                Pred
    _                 -> (TidyEnv, TcRnMessage) -> TcM ()
    forall a. (TidyEnv, TcRnMessage) -> TcM a
    failWithTcM (TidyEnv
    env, Type -> TcRnMessage
    TcRnBadQuantPredHead (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    pred))

             -- Check for termination
           ; Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    unless (Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.UndecidableInstances DynFlags
    dflags) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             [Type] -> Type -> TcM ()
    checkInstTermination [Type]
    theta Type
    head_pred
        }

    check_tuple_pred :: Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> PredType -> [PredType] -> TcM ()
    check_tuple_pred :: Bool
    -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> [Type] -> TcM ()
    check_tuple_pred Bool
    under_syn TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred [Type]
    ts
      = do { -- See Note [ConstraintKinds in predicates]
             Bool -> (TidyEnv, TcRnMessage) -> TcM ()
    checkTcM (Bool
    under_syn Bool -> Bool -> Bool
    || Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.ConstraintKinds DynFlags
    dflags)
                      (TidyEnv
    env, Type -> TcRnMessage
    TcRnIllegalTupleConstraint (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    pred))
           ; (Type -> TcM ()) -> [Type] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (Bool -> TidyEnv -> DynFlags -> UserTypeCtxt -> Type -> TcM ()
    check_pred_help Bool
    under_syn TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt) [Type]
    ts }
        -- This case will not normally be executed because without
        -- -XConstraintKinds tuple types are only kind-checked as *

    {- Note [ConstraintKinds in predicates]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Don't check for -XConstraintKinds under a type synonym, because that
    was done at the type synonym definition site; see #9838
    e.g.   module A where
              type C a = (Eq a, Ix a)   -- Needs -XConstraintKinds
           module B where
              import A
              f :: C a => a -> a        -- Does *not* need -XConstraintKinds
    -}

    -------------------------
    check_class_pred :: TidyEnv -> DynFlags -> UserTypeCtxt
                     -> PredType -> Class -> [TcType] -> TcM ()
    check_class_pred :: TidyEnv
    -> DynFlags -> UserTypeCtxt -> Type -> Class -> [Type] -> TcM ()
    check_class_pred TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Type
    pred Class
    cls [Type]
    tys
      | Class -> Bool
    isEqualityClass Class
    cls  -- (~) and (~~) and Coercible are classified as classes,
                             -- but here we want to treat them as equalities
      = -- Equational constraints are valid in all contexts, and
        -- we do not need to check e.g. for FlexibleContexts here, so just do nothing
        -- We used to require TypeFamilies/GADTs for equality constraints,
        -- but not anymore (GHC Proposal #371)
       () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

      | Class -> Bool
    isIPClass Class
    cls
      = do { TcM ()
    check_arity
           ; Bool -> (TidyEnv, TcRnMessage) -> TcM ()
    checkTcM (UserTypeCtxt -> Bool
    okIPCtxt UserTypeCtxt
    ctxt) (TidyEnv
    env, Type -> TcRnMessage
    TcRnIllegalImplicitParam (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    pred)) }

      | Bool
    otherwise     -- Includes Coercible
      = do { TcM ()
    check_arity
           ; TidyEnv -> DynFlags -> UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkSimplifiableClassConstraint TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Class
    cls [Type]
    tys
           ; Bool -> (TidyEnv, TcRnMessage) -> TcM ()
    checkTcM Bool
    arg_tys_ok (TidyEnv
    env, Type -> TcRnMessage
    TcRnNonTypeVarArgInConstraint (TidyEnv -> Type -> Type
    tidyType TidyEnv
    env Type
    pred)) }
      where
        check_arity :: TcM ()
    check_arity = Bool -> TcRnMessage -> TcM ()
    checkTc ([Type]
    tys [Type] -> Int -> Bool
    forall a. [a] -> Int -> Bool
    `lengthIs` Class -> Int
    classArity Class
    cls)
                              (TyCon -> [Type] -> TcRnMessage
    tyConArityErr (Class -> TyCon
    classTyCon Class
    cls) [Type]
    tys)

        -- Check the arguments of a class constraint
        flexible_contexts :: Bool
    flexible_contexts = Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.FlexibleContexts     DynFlags
    dflags
        arg_tys_ok :: Bool
    arg_tys_ok = case UserTypeCtxt
    ctxt of
            UserTypeCtxt
    SpecInstCtxt -> Bool
    True    -- {-# SPECIALISE instance Eq (T Int) #-} is fine
            InstDeclCtxt {} -> Bool -> Class -> [Type] -> Bool
    checkValidClsArgs Bool
    flexible_contexts Class
    cls [Type]
    tys
                                    -- Further checks on head and theta
                                    -- in checkInstTermination
            UserTypeCtxt
    _               -> Bool -> Class -> [Type] -> Bool
    checkValidClsArgs Bool
    flexible_contexts Class
    cls [Type]
    tys

    checkSimplifiableClassConstraint :: TidyEnv -> DynFlags -> UserTypeCtxt
                                     -> Class -> [TcType] -> TcM ()
    -- See Note [Simplifiable given constraints]
    checkSimplifiableClassConstraint :: TidyEnv -> DynFlags -> UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkSimplifiableClassConstraint TidyEnv
    env DynFlags
    dflags UserTypeCtxt
    ctxt Class
    cls [Type]
    tys
      | Bool -> Bool
    not (WarningFlag -> DynFlags -> Bool
    wopt WarningFlag
    Opt_WarnSimplifiableClassConstraints DynFlags
    dflags)
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      | Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.MonoLocalBinds DynFlags
    dflags
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

      | DataTyCtxt {} <- UserTypeCtxt
    ctxt   -- Don't do this check for the "stupid theta"
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()               -- of a data type declaration

      | Class
    cls Class -> Unique -> Bool
    forall a. Uniquable a => a -> Unique -> Bool
    `hasKey` Unique
    coercibleTyConKey
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()   -- Oddly, we treat (Coercible t1 t2) as unconditionally OK
                    -- matchGlobalInst will reply "yes" because we can reduce
                    -- (Coercible a b) to (a ~R# b)

      | Bool
    otherwise
      = do { result <- DynFlags -> Bool -> Class -> [Type] -> TcM ClsInstResult
    matchGlobalInst DynFlags
    dflags Bool
    False Class
    cls [Type]
    tys
           ; case result of
               OneInst { cir_what :: ClsInstResult -> InstanceWhat
    cir_what = InstanceWhat
    what }
                  -> TcRnMessage -> TcM ()
    addDiagnosticTc (Type -> InstanceWhat -> TcRnMessage
    TcRnSimplifiableConstraint Type
    pred InstanceWhat
    what)
               ClsInstResult
    _  -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return () }
      where
        pred :: Type
    pred = TidyEnv -> Type -> Type
    tidyType TidyEnv
    env (Class -> [Type] -> Type
    mkClassPred Class
    cls [Type]
    tys)

    {- Note [Simplifiable given constraints]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A type signature like
       f :: Eq [(a,b)] => a -> b
    is very fragile, for reasons described at length in GHC.Tc.Solver.Dict
    Note [Instance and Given overlap].  As that Note discusses, for the
    most part the clever stuff in GHC.Tc.Solver.Dict means that we don't use a
    top-level instance if a local Given might fire, so there is no
    fragility. But if we /infer/ the type of a local let-binding, things
    can go wrong (#11948 is an example, discussed in the Note).

    So this warning is switched on only if we have NoMonoLocalBinds; in
    that case the warning discourages users from writing simplifiable
    class constraints.

    The warning only fires if the constraint in the signature
    matches the top-level instances in only one way, and with no
    unifiers -- that is, under the same circumstances that
    GHC.Tc.Instance.Class.matchInstEnv fires an interaction with the top
    level instances.  For example (#13526), consider

      instance {-# OVERLAPPABLE #-} Eq (T a) where ...
      instance                   Eq (T Char) where ..
      f :: Eq (T a) => ...

    We don't want to complain about this, even though the context
    (Eq (T a)) matches an instance, because the user may be
    deliberately deferring the choice so that the Eq (T Char)
    has a chance to fire when 'f' is called.  And the fragility
    only matters when there's a risk that the instance might
    fire instead of the local 'given'; and there is no such
    risk in this case.  Just use the same rules as for instance
    firing!
    -}

    -------------------------
    okIPCtxt :: UserTypeCtxt -> Bool
      -- See Note [Implicit parameters in instance decls]
    okIPCtxt :: UserTypeCtxt -> Bool
    okIPCtxt (FunSigCtxt {})        = Bool
    True
    okIPCtxt (InfSigCtxt {})        = Bool
    True
    okIPCtxt (ExprSigCtxt {})       = Bool
    True
    okIPCtxt UserTypeCtxt
    TypeAppCtxt            = Bool
    True
    okIPCtxt UserTypeCtxt
    PatSigCtxt             = Bool
    True
    okIPCtxt UserTypeCtxt
    GenSigCtxt             = Bool
    True
    okIPCtxt (ConArgCtxt {})        = Bool
    True
    okIPCtxt (ForSigCtxt {})        = Bool
    True  -- ??
    okIPCtxt (GhciCtxt {})          = Bool
    True
    okIPCtxt UserTypeCtxt
    SigmaCtxt              = Bool
    True
    okIPCtxt (DataTyCtxt {})        = Bool
    True
    okIPCtxt (PatSynCtxt {})        = Bool
    True
    okIPCtxt (TySynCtxt {})         = Bool
    True   -- e.g.   type Blah = ?x::Int
                                             -- #11466

    okIPCtxt (KindSigCtxt {})       = Bool
    False
    okIPCtxt (StandaloneKindSigCtxt {}) = Bool
    False
    okIPCtxt (ClassSCCtxt {})       = Bool
    False
    okIPCtxt (InstDeclCtxt {})      = Bool
    False
    okIPCtxt (SpecInstCtxt {})      = Bool
    False
    okIPCtxt (RuleSigCtxt {})       = Bool
    False
    okIPCtxt UserTypeCtxt
    DefaultDeclCtxt        = Bool
    False
    okIPCtxt UserTypeCtxt
    DerivClauseCtxt        = Bool
    False
    okIPCtxt (TyVarBndrKindCtxt {}) = Bool
    False
    okIPCtxt (DataKindCtxt {})      = Bool
    False
    okIPCtxt (TySynKindCtxt {})     = Bool
    False
    okIPCtxt (TyFamResKindCtxt {})  = Bool
    False

    {-
    Note [Kind polymorphic type classes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    MultiParam check:

        class C f where...   -- C :: forall k. k -> Constraint
        instance C Maybe where...

      The dictionary gets type [C * Maybe] even if it's not a MultiParam
      type class.

    Flexibility check:

        class C f where...   -- C :: forall k. k -> Constraint
        data D a = D a
        instance C D where

      The dictionary gets type [C * (D *)]. IA0_TODO it should be
      generalized actually.
    -}

    checkThetaCtxt :: UserTypeCtxt -> ThetaType -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    checkThetaCtxt :: UserTypeCtxt -> [Type] -> TidyEnv -> ZonkM (TidyEnv, SDoc)
    checkThetaCtxt UserTypeCtxt
    ctxt [Type]
    theta TidyEnv
    env
      = (TidyEnv, SDoc) -> ZonkM (TidyEnv, SDoc)
    forall a. a -> ZonkM a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ( TidyEnv
    env
               , [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "In the context:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [Type] -> SDoc
    pprTheta (TidyEnv -> [Type] -> [Type]
    tidyTypes TidyEnv
    env [Type]
    theta)
                      , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "While checking" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> UserTypeCtxt -> SDoc
    pprUserTypeCtxt UserTypeCtxt
    ctxt ] )

    tyConArityErr :: TyCon -> [TcType] -> TcRnMessage
    -- For type-constructor arity errors, be careful to report
    -- the number of /visible/ arguments required and supplied,
    -- ignoring the /invisible/ arguments, which the user does not see.
    -- (e.g. #10516)
    tyConArityErr :: TyCon -> [Type] -> TcRnMessage
    tyConArityErr TyCon
    tc [Type]
    tks
      = TyThing -> Int -> Int -> TcRnMessage
    TcRnArityMismatch (TyCon -> TyThing
    ATyCon TyCon
    tc) Int
    tc_type_arity Int
    tc_type_args
      where
        vis_tks :: [Type]
    vis_tks = TyCon -> [Type] -> [Type]
    filterOutInvisibleTypes TyCon
    tc [Type]
    tks

        -- tc_type_arity = number of *type* args expected
        -- tc_type_args  = number of *type* args encountered
        tc_type_arity :: Int
    tc_type_arity = (VarBndr Var TyConBndrVis -> Bool)
    -> [VarBndr Var TyConBndrVis] -> Int
    forall a. (a -> Bool) -> [a] -> Int
    count VarBndr Var TyConBndrVis -> Bool
    forall tv. VarBndr tv TyConBndrVis -> Bool
    isVisibleTyConBinder (TyCon -> [VarBndr Var TyConBndrVis]
    tyConBinders TyCon
    tc)
        tc_type_args :: Int
    tc_type_args  = [Type] -> Int
    forall a. [a] -> Int
    forall (t :: * -> *) a. Foldable t => t a -> Int
    length [Type]
    vis_tks

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Checking for a decent instance head type}
    *                                                                      *
    ************************************************************************

    @checkValidInstHead@ checks the type {\em and} its syntactic constraints:
    it must normally look like: @instance Foo (Tycon a b c ...) ...@

    The exceptions to this syntactic checking: (1)~if the @GlasgowExts@
    flag is on, or (2)~the instance is imported (they must have been
    compiled elsewhere). In these cases, we let them go through anyway.

    We can also have instances for functions: @instance Foo (a -> b) ...@.
    -}

    checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkValidInstHead :: UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkValidInstHead UserTypeCtxt
    ctxt Class
    clas [Type]
    cls_args
      = do { dflags   <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
           ; hsc_src  <- tcHscSource
           ; check_special_inst_head dflags hsc_src ctxt clas cls_args
           ; checkValidTypePats (classTyCon clas) cls_args
           }

    {-

    Note [Instances of built-in classes in signature files]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    User defined instances for KnownNat, KnownSymbol, KnownChar,
    and Typeable are disallowed
      -- they are generated when needed by GHC itself, on-the-fly.

    However, if they occur in a Backpack signature file, they have an
    entirely different meaning. To illustrate, suppose in M.hsig we see

      signature M where
        data T :: Nat
        instance KnownNat T

    That says that any module satisfying M.hsig must provide a KnownNat
    instance for T.  We absolutely need that instance when compiling a
    module that imports M.hsig: see #15379 and
    Note [Fabricating Evidence for Literals in Backpack] in GHC.Tc.Instance.Class.

    Hence, checkValidInstHead accepts a user-written instance declaration
    in hsig files, where `is_sig` is True.

    -}

    check_special_inst_head :: DynFlags -> HscSource -> UserTypeCtxt
                            -> Class -> [Type] -> TcM ()
    -- Wow!  There are a surprising number of ad-hoc special cases here.
    -- TODO: common up the logic for special typeclasses (see GHC ticket #20441).
    check_special_inst_head :: DynFlags -> HscSource -> UserTypeCtxt -> Class -> [Type] -> TcM ()
    check_special_inst_head DynFlags
    dflags HscSource
    hs_src UserTypeCtxt
    ctxt Class
    clas [Type]
    cls_args

      -- Abstract classes cannot have instances, except in hs-boot or signature files.
      | Class -> Bool
    isAbstractClass Class
    clas
      , HscSource
    hs_src HscSource -> HscSource -> Bool
    forall a. Eq a => a -> a -> Bool
    == HscSource
    HsSrcFile
      = IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceHeadReason -> IllegalClassInstanceReason
    IllegalInstanceHead
                           (IllegalInstanceHeadReason -> IllegalClassInstanceReason)
    -> IllegalInstanceHeadReason -> IllegalClassInstanceReason
    forall a b. (a -> b) -> a -> b
    $ Class -> IllegalInstanceHeadReason
    InstHeadAbstractClass Class
    clas

      -- Complain about hand-written instances of built-in classes
      -- Typeable, KnownNat, KnownSymbol, Coercible, HasField.

      -- Disallow hand-written Typeable instances, except that we
      -- allow a standalone deriving declaration: they are no-ops,
      -- and we warn about them in GHC.Tc.Deriv.deriveStandalone.
      | Name
    clas_nm Name -> Name -> Bool
    forall a. Eq a => a -> a -> Bool
    == Name
    typeableClassName
      , Bool -> Bool
    not (HscSource
    hs_src HscSource -> HscSource -> Bool
    forall a. Eq a => a -> a -> Bool
    == HscSource
    HsigFile)
        -- Note [Instances of built-in classes in signature files]
      , Bool
    hand_written_bindings
      = IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ Class -> Bool -> IllegalClassInstanceReason
    IllegalSpecialClassInstance Class
    clas Bool
    False

      -- Handwritten instances of KnownNat/KnownChar/KnownSymbol
      -- are forbidden outside of signature files (#12837).
      -- Derived instances are forbidden completely (#21087).
         -- FIXME: DataToTag instances in signature files don't actually work yet
      | Name
    clas_nm Name -> [Name] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [ Name
    knownNatClassName, Name
    knownSymbolClassName
                       , Name
    knownCharClassName, Name
    dataToTagClassName ]
      , (Bool -> Bool
    not (HscSource
    hs_src HscSource -> HscSource -> Bool
    forall a. Eq a => a -> a -> Bool
    == HscSource
    HsigFile) Bool -> Bool -> Bool
    && Bool
    hand_written_bindings) Bool -> Bool -> Bool
    || Bool
    derived_instance
        -- Note [Instances of built-in classes in signature files]
      = IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ Class -> Bool -> IllegalClassInstanceReason
    IllegalSpecialClassInstance Class
    clas Bool
    False

      -- For the most part we don't allow
      -- instances for (~), (~~), or Coercible;
      -- but we DO want to allow them in quantified constraints:
      --   f :: (forall a b. Coercible a b => Coercible (m a) (m b)) => ...m...
      | Name
    clas_nm Name -> [Name] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem`
        [ Name
    heqTyConName, Name
    eqTyConName, Name
    coercibleTyConName
        , Name
    withDictClassName, Name
    unsatisfiableClassName ]
      , Bool -> Bool
    not Bool
    quantified_constraint
      = IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ Class -> Bool -> IllegalClassInstanceReason
    IllegalSpecialClassInstance Class
    clas Bool
    False

      -- Check for hand-written Generic instances (disallowed in Safe Haskell)
      | Name
    clas_nm Name -> [Name] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Name]
    genericClassNames
      , Bool
    hand_written_bindings
      =  do { Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when (DynFlags -> Bool
    safeLanguageOn DynFlags
    dflags) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
               IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ Class -> Bool -> IllegalClassInstanceReason
    IllegalSpecialClassInstance Class
    clas Bool
    True
            ; Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when (DynFlags -> Bool
    safeInferOn DynFlags
    dflags) (Messages TcRnMessage -> TcM ()
    recordUnsafeInfer Messages TcRnMessage
    forall e. Messages e
    emptyMessages) }

      | Name
    clas_nm Name -> Name -> Bool
    forall a. Eq a => a -> a -> Bool
    == Name
    hasFieldClassName
      , Bool -> Bool
    not Bool
    quantified_constraint
      -- Don't do any validity checking for HasField contexts
      -- inside quantified constraints (#20989): the validity checks
      -- only apply to user-written instances.
      = Class -> [Type] -> TcM ()
    checkHasFieldInst Class
    clas [Type]
    cls_args

      | Class -> Bool
    isCTupleClass Class
    clas
      = do
        -- Since we're now declaring instances for constraint tuples in
        -- GHC.Classes, this check must exclude that file.
        this_mod <- (TcGblEnv -> Module)
    -> IOEnv (Env TcGblEnv TcLclEnv) TcGblEnv
    -> IOEnv (Env TcGblEnv TcLclEnv) Module
    forall a b.
    (a -> b)
    -> IOEnv (Env TcGblEnv TcLclEnv) a
    -> IOEnv (Env TcGblEnv TcLclEnv) b
    forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
    fmap TcGblEnv -> Module
    tcg_mod IOEnv (Env TcGblEnv TcLclEnv) TcGblEnv
    forall gbl lcl. TcRnIf gbl lcl gbl
    getGblEnv
        when (this_mod /= gHC_CLASSES) (failWithTc (TcRnTupleConstraintInst clas))

      -- Check language restrictions on the args to the class
      | Bool
    check_h98_arg_shape
      , Just IllegalInstanceHeadReason
    illegal_head <- Maybe IllegalInstanceHeadReason
    mb_ty_args_type
      = IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err (IllegalClassInstanceReason -> TcM ())
    -> IllegalClassInstanceReason -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceHeadReason -> IllegalClassInstanceReason
    IllegalInstanceHead IllegalInstanceHeadReason
    illegal_head

      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure ()
      where

        fail_with_inst_err :: IllegalClassInstanceReason -> TcM ()
    fail_with_inst_err IllegalClassInstanceReason
    err =
          TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance
                     (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ TypedThing -> IllegalClassInstanceReason -> IllegalInstanceReason
    IllegalClassInstance
                        (Type -> TypedThing
    TypeThing (Type -> TypedThing) -> Type -> TypedThing
    forall a b. (a -> b) -> a -> b
    $ Class -> [Type] -> Type
    mkClassPred Class
    clas [Type]
    cls_args)
                     (IllegalClassInstanceReason -> IllegalInstanceReason)
    -> IllegalClassInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $ IllegalClassInstanceReason
    err

        clas_nm :: Name
    clas_nm = Class -> Name
    forall a. NamedThing a => a -> Name
    getName Class
    clas
        ty_args :: [Type]
    ty_args = TyCon -> [Type] -> [Type]
    filterOutInvisibleTypes (Class -> TyCon
    classTyCon Class
    clas) [Type]
    cls_args

        hand_written_bindings :: Bool
    hand_written_bindings
          = case UserTypeCtxt
    ctxt of
              InstDeclCtxt Bool
    standalone -> Bool -> Bool
    not Bool
    standalone
              UserTypeCtxt
    SpecInstCtxt            -> Bool
    False
              UserTypeCtxt
    DerivClauseCtxt         -> Bool
    False
              UserTypeCtxt
    SigmaCtxt               -> Bool
    False
              UserTypeCtxt
    _                       -> Bool
    True

        derived_instance :: Bool
    derived_instance
          = case UserTypeCtxt
    ctxt of
                InstDeclCtxt Bool
    standalone -> Bool
    standalone
                UserTypeCtxt
    DerivClauseCtxt         -> Bool
    True
                UserTypeCtxt
    _                       -> Bool
    False

        check_h98_arg_shape :: Bool
    check_h98_arg_shape = case UserTypeCtxt
    ctxt of
                                UserTypeCtxt
    SpecInstCtxt    -> Bool
    False
                                UserTypeCtxt
    DerivClauseCtxt -> Bool
    False
                                UserTypeCtxt
    SigmaCtxt       -> Bool
    False
                                UserTypeCtxt
    _               -> Bool
    True
            -- SigmaCtxt: once we are in quantified-constraint land, we
            -- aren't so picky about enforcing H98-language restrictions
            -- E.g. we want to allow a head like Coercible (m a) (m b)

        -- When we are looking at the head of a quantified constraint,
        -- check_quant_pred sets ctxt to SigmaCtxt
        quantified_constraint :: Bool
    quantified_constraint = case UserTypeCtxt
    ctxt of
                                  UserTypeCtxt
    SigmaCtxt -> Bool
    True
                                  UserTypeCtxt
    _         -> Bool
    False

        mb_ty_args_type :: Maybe IllegalInstanceHeadReason
    mb_ty_args_type
          | Bool -> Bool
    not (Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.TypeSynonymInstances DynFlags
    dflags)
          , Bool -> Bool
    not ((Type -> Bool) -> [Type] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all Type -> Bool
    tcInstHeadTyNotSynonym [Type]
    ty_args)
          = IllegalInstanceHeadReason -> Maybe IllegalInstanceHeadReason
    forall a. a -> Maybe a
    Just IllegalInstanceHeadReason
    InstHeadTySynArgs

          | Bool -> Bool
    not (Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.FlexibleInstances DynFlags
    dflags)
          , Bool -> Bool
    not ((Type -> Bool) -> [Type] -> Bool
    forall (t :: * -> *) a. Foldable t => (a -> Bool) -> t a -> Bool
    all Type -> Bool
    tcInstHeadTyAppAllTyVars [Type]
    ty_args)
          = IllegalInstanceHeadReason -> Maybe IllegalInstanceHeadReason
    forall a. a -> Maybe a
    Just IllegalInstanceHeadReason
    InstHeadNonTyVarArgs

          | [Type] -> Int
    forall a. [a] -> Int
    forall (t :: * -> *) a. Foldable t => t a -> Int
    length [Type]
    ty_args Int -> Int -> Bool
    forall a. Eq a => a -> a -> Bool
    /= Int
    1
          , Bool -> Bool
    not (Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.MultiParamTypeClasses DynFlags
    dflags)
          , Bool -> Bool
    not (Extension -> DynFlags -> Bool
    xopt Extension
    LangExt.NullaryTypeClasses DynFlags
    dflags Bool -> Bool -> Bool
    && [Type] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null [Type]
    ty_args)
          = IllegalInstanceHeadReason -> Maybe IllegalInstanceHeadReason
    forall a. a -> Maybe a
    Just IllegalInstanceHeadReason
    InstHeadMultiParam

          | Bool
    otherwise
          = Maybe IllegalInstanceHeadReason
    forall a. Maybe a
    Nothing

    tcInstHeadTyNotSynonym :: Type -> Bool
    -- Used in Haskell-98 mode, for the argument types of an instance head
    -- These must not be type synonyms, but everywhere else type synonyms
    -- are transparent, so we need a special function here
    tcInstHeadTyNotSynonym :: Type -> Bool
    tcInstHeadTyNotSynonym Type
    ty
      = case Type
    ty of  -- Do not use splitTyConApp,
                    -- because that expands synonyms!
            TyConApp TyCon
    tc [Type]
    _ -> Bool -> Bool
    not (TyCon -> Bool
    isTypeSynonymTyCon TyCon
    tc) Bool -> Bool -> Bool
    || TyCon
    tc TyCon -> TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon
    unrestrictedFunTyCon
                    -- Allow (->), e.g. instance Category (->),
                    -- even though it's a type synonym for FUN 'Many
            Type
    _ -> Bool
    True

    tcInstHeadTyAppAllTyVars :: Type -> Bool
    -- Used in Haskell-98 mode, for the argument types of an instance head
    -- These must be a constructor applied to type variable arguments
    -- or a type-level literal.
    -- But we allow
    -- 1) kind instantiations
    -- 2) the type (->) = FUN 'Many, even though it's not in this form.
    tcInstHeadTyAppAllTyVars :: Type -> Bool
    tcInstHeadTyAppAllTyVars Type
    ty
      | Just (TyCon
    tc, [Type]
    tys) <- HasCallStack => Type -> Maybe (TyCon, [Type])
    Type -> Maybe (TyCon, [Type])
    tcSplitTyConApp_maybe (Type -> Type
    dropCasts Type
    ty)
      = let tys' :: [Type]
    tys' = TyCon -> [Type] -> [Type]
    filterOutInvisibleTypes TyCon
    tc [Type]
    tys  -- avoid kinds
            tys'' :: [Type]
    tys'' | TyCon
    tc TyCon -> Unique -> Bool
    forall a. Uniquable a => a -> Unique -> Bool
    `hasKey` Unique
    fUNTyConKey
                  , Type
    ManyTy : [Type]
    tys_t <- [Type]
    tys'
                  = [Type]
    tys_t
                  | Bool
    otherwise = [Type]
    tys'
        in [Type] -> Bool
    ok [Type]
    tys''
      | LitTy TyLit
    _ <- Type
    ty = Bool
    True  -- accept type literals (#13833)
      | Bool
    otherwise
      = Bool
    False
      where
            -- Check that all the types are type variables,
            -- and that each is distinct
        ok :: [Type] -> Bool
    ok [Type]
    tys = [Var] -> [Type] -> Bool
    forall a b. [a] -> [b] -> Bool
    equalLength [Var]
    tvs [Type]
    tys Bool -> Bool -> Bool
    && [Var] -> Bool
    forall a. Eq a => [a] -> Bool
    hasNoDups [Var]
    tvs
               where
                 tvs :: [Var]
    tvs = (Type -> Maybe Var) -> [Type] -> [Var]
    forall a b. (a -> Maybe b) -> [a] -> [b]
    mapMaybe Type -> Maybe Var
    getTyVar_maybe [Type]
    tys

    dropCasts :: Type -> Type
    -- See Note [Casts during validity checking]
    -- This function can turn a well-kinded type into an ill-kinded
    -- one, so I've kept it local to this module
    -- To consider: drop only HoleCo casts
    dropCasts :: Type -> Type
    dropCasts (CastTy Type
    ty KindCoercion
    _)       = Type -> Type
    dropCasts Type
    ty
    dropCasts (AppTy Type
    t1 Type
    t2)       = Type -> Type -> Type
    mkAppTy (Type -> Type
    dropCasts Type
    t1) (Type -> Type
    dropCasts Type
    t2)
    dropCasts ty :: Type
    ty@(FunTy FunTyFlag
    _ Type
    w Type
    t1 Type
    t2)  = Type
    ty { ft_mult = dropCasts w, ft_arg = dropCasts t1, ft_res = dropCasts t2 }
    dropCasts (TyConApp TyCon
    tc [Type]
    tys)   = TyCon -> [Type] -> Type
    mkTyConApp TyCon
    tc ((Type -> Type) -> [Type] -> [Type]
    forall a b. (a -> b) -> [a] -> [b]
    map Type -> Type
    dropCasts [Type]
    tys)
    dropCasts (ForAllTy TyVarBinder
    b Type
    ty)     = TyVarBinder -> Type -> Type
    ForAllTy (TyVarBinder -> TyVarBinder
    dropCastsB TyVarBinder
    b) (Type -> Type
    dropCasts Type
    ty)
    dropCasts Type
    ty                  = Type
    ty  -- LitTy, TyVarTy, CoercionTy

    dropCastsB :: TyVarBinder -> TyVarBinder
    dropCastsB :: TyVarBinder -> TyVarBinder
    dropCastsB TyVarBinder
    b = TyVarBinder
    b   -- Don't bother in the kind of a forall

    -- | See Note [Validity checking of HasField instances]
    checkHasFieldInst :: Class -> [Type] -> TcM ()
    checkHasFieldInst :: Class -> [Type] -> TcM ()
    checkHasFieldInst Class
    cls tys :: [Type]
    tys@[Type
    _k_ty, Type
    lbl_ty, Type
    r_ty, Type
    _a_ty] =
      case HasDebugCallStack => Type -> Maybe (TyCon, [Type])
    Type -> Maybe (TyCon, [Type])
    splitTyConApp_maybe Type
    r_ty of
        Maybe (TyCon, [Type])
    Nothing -> IllegalHasFieldInstance -> TcM ()
    add_err IllegalHasFieldInstance
    IllegalHasFieldInstanceNotATyCon
        Just (TyCon
    tc, [Type]
    _)
          | TyCon -> Bool
    isFamilyTyCon TyCon
    tc
                      -> IllegalHasFieldInstance -> TcM ()
    add_err IllegalHasFieldInstance
    IllegalHasFieldInstanceFamilyTyCon
          | Bool
    otherwise -> case Type -> Maybe FastString
    isStrLitTy Type
    lbl_ty of
           Just FastString
    lbl
             | let lbl_str :: FieldLabelString
    lbl_str = FastString -> FieldLabelString
    FieldLabelString FastString
    lbl
             , Maybe FieldLabel -> Bool
    forall a. Maybe a -> Bool
    isJust (FieldLabelString -> TyCon -> Maybe FieldLabel
    lookupTyConFieldLabel FieldLabelString
    lbl_str TyCon
    tc)
             -> IllegalHasFieldInstance -> TcM ()
    add_err (IllegalHasFieldInstance -> TcM ())
    -> IllegalHasFieldInstance -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ TyCon -> FieldLabelString -> IllegalHasFieldInstance
    IllegalHasFieldInstanceTyConHasField TyCon
    tc FieldLabelString
    lbl_str
             | Bool
    otherwise
             -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
           Maybe FastString
    Nothing
             -- If the label is not a literal, we need to ensure it can't unify
             -- with any of the field labels of the TyCon.
             | [FieldLabel] -> Bool
    forall a. [a] -> Bool
    forall (t :: * -> *) a. Foldable t => t a -> Bool
    null (TyCon -> [FieldLabel]
    tyConFieldLabels TyCon
    tc)
               -- No field labels at all: nothing to check.
             Bool -> Bool -> Bool
    || Type -> Type -> Bool
    typesAreApart (HasDebugCallStack => Type -> Type
    Type -> Type
    typeKind Type
    lbl_ty) Type
    typeSymbolKind
               -- If the label has a type whose kind can't unify with Symbol,
               -- then it definitely can't unify with any of the field labels.
             -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
             | Bool
    otherwise
             -> IllegalHasFieldInstance -> TcM ()
    add_err (IllegalHasFieldInstance -> TcM ())
    -> IllegalHasFieldInstance -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ TyCon -> Type -> IllegalHasFieldInstance
    IllegalHasFieldInstanceTyConHasFields TyCon
    tc Type
    lbl_ty
      where
        add_err :: IllegalHasFieldInstance -> TcM ()
    add_err IllegalHasFieldInstance
    err = TcRnMessage -> TcM ()
    addErrTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance
                               (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ TypedThing -> IllegalClassInstanceReason -> IllegalInstanceReason
    IllegalClassInstance
                                   (Type -> TypedThing
    TypeThing (Type -> TypedThing) -> Type -> TypedThing
    forall a b. (a -> b) -> a -> b
    $ Class -> [Type] -> Type
    mkClassPred Class
    cls [Type]
    tys)
                               (IllegalClassInstanceReason -> IllegalInstanceReason)
    -> IllegalClassInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $ IllegalHasFieldInstance -> IllegalClassInstanceReason
    IllegalHasFieldInstance IllegalHasFieldInstance
    err

    checkHasFieldInst Class
    _ [Type]
    tys = String -> SDoc -> TcM ()
    forall a. HasCallStack => String -> SDoc -> a
    pprPanic String
    "checkHasFieldInst" ([Type] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Type]
    tys)

    {- Note [Casts during validity checking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the (bogus)
         instance Eq Char#
    We elaborate to  'Eq (Char# |> UnivCo(hole))'  where the hole is an
    insoluble equality constraint for Type ~ TYPE WordRep.  We'll report the insoluble
    constraint separately, but we don't want to *also* complain that Eq is
    not applied to a type constructor.  So we look gaily look through
    CastTys here.

    Another example:  Eq (Either a).  Then we actually get a cast in
    the middle:
       Eq ((Either |> g) a)

    Note [Validity checking of HasField instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The HasField class has magic constraint solving behaviour (see Note
    [HasField instances] in GHC.Tc.Instance.Class).  However, we permit users to
    declare their own instances, provided they do not clash with the
    built-in behaviour.  In particular, we forbid:

      1. `HasField _ r _` where r is a variable

      2. `HasField _ (T ...) _` if T is a data family
         (because it might have fields introduced later)

      3. `HasField x (T ...) _` where x is a variable,
          if T has any fields at all

      4. `HasField "foo" (T ...) _` if T has a "foo" field

    The usual functional dependency checks also apply.

    Note [Valid 'deriving' predicate]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a 'derived' instance declaration, we *infer* the context. It's a bit unclear
    what rules we should apply for this; the Haskell report is silent. Obviously,
    constraints like `Eq a` are fine, but what about

      data T f a = MkT (f a) deriving Eq

    where we'd get an `Eq (f a)` constraint. That's probably fine too.

    On the other hand, we don't want to allow *every* inferred constraint, as there
    are some particularly complex constraints that are tricky to handle. If GHC
    encounters a constraint that is too complex, it will reject it, and you will
    have to use StandaloneDeriving to manually specify the instance context that
    you want.

    There are two criteria for a constraint inferred by a `deriving` clause to be
    considered valid, which are described below as (VD1) and (VD2). (Here, "VD"
    stands for "valid deriving".) `validDerivPred` implements these checks. While
    `validDerivPred` is similar to other things defined in GHC.Tc.Deriv.Infer, we
    define it here in GHC.Tc.Validity because it is quite similar to
    `checkInstTermination`.

    -----------------------------------
    -- (VD1) The Paterson conditions --
    -----------------------------------

    Constraints must satisfy the Paterson conditions (see Note [Paterson
    conditions]) to be valid. Not only does this check for termination (of course),
    but it also deals with a nasty corner case:

      instance C a b => D (T a) where ...

    Note that `b` isn't a parameter of T.  This gives rise to all sorts of
    problems; in particular, it's hard to compare solutions for equality when
    finding the fixpoint, and that means the
    GHC.Tc.Deriv.Infer.simplifyInstanceContexts loop does not converge.
    See #5287 and #21302.

    Another common situation in which a derived instance's context fails to meet
    the Paterson conditions is when a constraint mentions a type variable more
    often than the instance head, e.g.,

      data Fix f = In (f (Fix f)) deriving Eq

    This would result in the following derived `Eq` instance:

      instance Eq (f (Fix f)) => Eq (Fix f)

    Because `f` is mentioned more often in the `Eq (f (Fix f))` constraint than in
    the instance head `Eq (Fix f)`, GHC rejects this instance.

    This is a somewhat contentious restriction, and some have suggested that
    instances like this one should be accepted if UndecidableInstances is enabled
    (see #15868 for one such discussion). If we *do* lift this restriction in the
    future, we should make sure to still meet the termination conditions. If not,
    the deriving mechanism generates larger and larger constraints. Example:

      data Succ a = S a
      data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show

    Note the lack of a Show instance for Succ.  First we'll generate:

      instance (Show (Succ a), Show a) => Show (Seq a)

    and then:

      instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)

    and so on. Instead we want to complain of no instance for (Show (Succ a)).

    ---------------------------------
    -- (VD2) No exotic constraints --
    ---------------------------------

    A constraint must satisfy one of the following properties in order to be valid:

    * It is a `ClassPred` of the form `C a_1 ... a_n`, where C is a type class
      constructor and a_1, ..., a_n are either raw type variables or applications
      of type variables (e.g., `f a`).
    * It is an `IrredPred` of the form `c a_1 ... a_n`, where `c` is a raw type
      variable and a_1, ..., a_n are either raw type variables or applications of
      type variables (e.g., `f a`).

    If a constraint does not meet either of these properties, it is considered
    *exotic*. A constraint will be exotic if it contains:

    * Other type constructors (besides the class in a `ClassPred`),
    * Foralls, or
    * Equalities

    A common form of exotic constraint is one that mentions another type
    constructor. For example, given the following:

      data NotAShowInstance
      data Foo = MkFoo Int NotAShowInstance deriving Show

    GHC would attempt to generate the following derived `Show` instance:

      instance (Show Int, Show NotAShowInstance) => Show Foo

    Note that because there is a top-level `Show Int` instance, GHC is able to
    simplify away the inferred `Show Int` constraint. However, it cannot do the
    same for the `Show NotAShowInstance` constraint. One possibility would be to
    generate this instance:

      instance Show NotAShowInstance => Show Foo

    But this is almost surely not what we want most of the time. For this reason,
    we reject the constraint above as being exotic.

    Here are some other interesting examples:

    * Derived instances whose instance context would mention TypeError, such as the
      code from the deriving/should_fail/T14339 test case, are exotic. For example:

        newtype Foo = Foo Int

        class Bar a where
          bar :: a

        instance (TypeError (Text "Boo")) => Bar Foo where
          bar = undefined

        newtype Baz = Baz Foo
          deriving Bar

      The `deriving Bar` clause would generate this instance:

        instance TypeError (Text "Boo") => Bar Baz

      The instance context is exotic, as `TypeError` is not a type constructor, and
      `Text "Boo"` is not an application of type variables. As such, GHC rejects
      it. This has the desirable side effect of causing the TypeError to fire in
      the resulting error message.

    * The following `IrredPred`s are not exotic:

        instance c => C (T c a)
        instance c a => C (T c a)

      This `IrredPred`, however, *is* exotic:

        instance c NotAShowInstance => C (T c)

      This is rejected for the same reasons that we do not permit a `ClassPred`
      with a type constructor argument, such as the `Show NotAShowInstance` example
      above.

    As part of implementing this check, GHC calls `tyConsOfType` on the arguments
    of the constraint, ensuring that there are no other type constructors.
    Wrinkle: for `ClassPred`s, we look only at the /visible/ arguments of the class
    type constructor. Including the non-visible arguments can cause the following,
    perfectly valid instance to be rejected:

      class Category (cat :: k -> k -> Type) where ...
      newtype T (c :: Type -> Type -> Type) a b = MkT (c a b)
      instance Category c => Category (T c) where ...

    since the first argument to `Category` is a non-visible `Type`, which has a type
    constructor! See #11833.

    Note [Equality class instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We can't have users writing instances for the equality classes. But we
    still need to be able to write instances for them ourselves. So we allow
    instances only in the defining module.
    -}

    validDerivPred :: PatersonSize -> PredType -> Bool
    -- See Note [Valid 'deriving' predicate]
    validDerivPred :: PatersonSize -> Type -> Bool
    validDerivPred PatersonSize
    head_size Type
    pred
      = case Type -> Pred
    classifyPredType Type
    pred of
                EqPred {}     -> Bool
    False  -- Reject equality constraints (VD2)
                ForAllPred {} -> Bool
    False  -- Rejects quantified predicates (VD2)

                ClassPred Class
    cls [Type]
    tys -> PatersonSize -> Bool
    check_size (Class -> [Type] -> PatersonSize
    pSizeClassPred Class
    cls [Type]
    tys)        -- (VD1)
                                  Bool -> Bool -> Bool
    && UniqSet TyCon -> Bool
    forall a. UniqSet a -> Bool
    isEmptyUniqSet ([Type] -> UniqSet TyCon
    tyConsOfTypes [Type]
    visible_tys) -- (VD2)
                    where
                      -- See the wrinkle about visible arguments in (VD2)
                      visible_tys :: [Type]
    visible_tys = TyCon -> [Type] -> [Type]
    filterOutInvisibleTypes (Class -> TyCon
    classTyCon Class
    cls) [Type]
    tys

                IrredPred {} -> PatersonSize -> Bool
    check_size (Type -> PatersonSize
    pSizeType Type
    pred)        -- (VD1)
                             Bool -> Bool -> Bool
    && UniqSet TyCon -> Bool
    forall a. UniqSet a -> Bool
    isEmptyUniqSet (Type -> UniqSet TyCon
    tyConsOfType Type
    pred) -- (VD2)

      where
        check_size :: PatersonSize -> Bool
    check_size PatersonSize
    pred_size = Maybe PatersonCondFailure -> Bool
    forall a. Maybe a -> Bool
    isNothing (PatersonSize
    pred_size PatersonSize -> PatersonSize -> Maybe PatersonCondFailure
    `ltPatersonSize` PatersonSize
    head_size)
            -- Check (VD1)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Checking instance for termination}
    *                                                                      *
    ************************************************************************
    -}

    {- Note [Paterson conditions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The Paterson Conditions ensure termination of instance resolution.
    Given an instance declaration
       instance (..., C t1.. tn, ...) => D s1 .. sm

    we check that each constraint in the context of the instance is
    "Paterson-smaller" than the instance head.  The underlying idea of
    Paterson-smaller is that

        For any ground substitution S, for each constraint P in the
        context, S(P) has fewer type constructors, counting repetitions,
        than the head S(H)

    We implement this check by checking the following syntactic conditions:

    (PC1) No type variable has more (shallow) occurrences in P than in H.

          (If not, a substitution that replaces that variable with a big type
          would make P have many more type constructors than H. Side note: we
          could in principle skip this test for a variable of kind Bool,
          since there are no big ground types we can substitute for it.)

    (PC2) The constraint P has fewer constructors and variables (taken
          together and counting repetitions) than the head H.  This size
          metric is computed by sizeType.

          (A substitution that replaces each variable with Int demonstrates
          the need.)

    (PC3) The constraint P mentions no type functions.

          (A type function application can in principle expand to a type of
          arbitrary size, and so are rejected out of hand.  See #15172.)

    (See Section 5 of "Understanding functional dependencies via Constraint
    Handling Rules", JFP Jan 2007; and the user manual section "Instance
    termination rules".)

    We measure "size" with the data type PatersonSize, in GHC.Tc.Utils.TcType.
      data PatersonSize
        = PS_TyFam TyCon
        | PS_Vanilla { ps_tvs :: [TyVar]  -- Free tyvars, including repetitions;
                     , ps_size :: Int}    -- Number of type constructors and variables

    * ps_tvs deals with (PC1)
    * ps_size deals with (PC2)
    * PS_TyFam deals with (PC3)

    Note [Tuples in checkInstTermination]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider these two ways of giving the same instance decl (#8359):

       instance (Eq a, Num a) => C (T a)

       type X a = (Eq a, Num a)
       instance X a => C (T a)

    In the former, `checkInstTermination` will check the size of two predicates:
    (Eq a) and (Num a). In the latter, it will see only one: (X a). But we want
    to treat the latter like the former.

    So the `check` function in `checkInstTermination`, we simply recurse
    if we find a constraint tuple.
    -}

    checkValidInstance :: UserTypeCtxt -> LHsSigType GhcRn -> Type -> TcM ()
    checkValidInstance :: UserTypeCtxt -> LHsSigType GhcRn -> Type -> TcM ()
    checkValidInstance UserTypeCtxt
    ctxt LHsSigType GhcRn
    hs_type Type
    ty = case Type
    tau of
      -- See Note [Instances and constraint synonyms]
      TyConApp TyCon
    tc [Type]
    inst_tys
        | Just Class
    clas <- TyCon -> Maybe Class
    tyConClass_maybe TyCon
    tc
        -> do
            { EpAnn AnnListItem -> TcM () -> TcM ()
    forall ann a. EpAnn ann -> TcRn a -> TcRn a
    setSrcSpanA EpAnn AnnListItem
    head_loc (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
              UserTypeCtxt -> Class -> [Type] -> TcM ()
    checkValidInstHead UserTypeCtxt
    ctxt Class
    clas [Type]
    inst_tys

            ; String -> SDoc -> TcM ()
    traceTc String
    "checkValidInstance {" (Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    ty)

            ; env0 <- ZonkM TidyEnv -> TcM TidyEnv
    forall a. ZonkM a -> TcM a
    liftZonkM (ZonkM TidyEnv -> TcM TidyEnv) -> ZonkM TidyEnv -> TcM TidyEnv
    forall a b. (a -> b) -> a -> b
    $ ZonkM TidyEnv
    tcInitTidyEnv
            ; expand <- initialExpandMode
            ; check_valid_theta env0 ctxt expand theta

            -- The Termination and Coverage Conditions
            -- Check that instance inference will terminate (if we care)
            -- For Haskell 98 this will already have been done by checkValidTheta,
            -- but as we may be using other extensions we need to check.
            --
            -- Note that the Termination Condition is *more conservative* than
            -- the checkAmbiguity test we do on other type signatures
            --   e.g.  Bar a => Bar Int is ambiguous, but it also fails
            --   the termination condition, because 'a' appears more often
            --   in the constraint than in the head
            ; undecidable_ok <- xoptM LangExt.UndecidableInstances
            ; if undecidable_ok
              then checkAmbiguity ctxt ty
              else checkInstTermination theta tau

            ; traceTc "cvi 2" (ppr ty)

            ; case checkInstCoverage undecidable_ok clas theta inst_tys of
                Validity' CoverageProblem
    IsValid
                  -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()   -- Check succeeded
                NotValid CoverageProblem
    coverageInstErr
                  -> TcRnMessage -> TcM ()
    addErrTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalClassInstanceReason -> TcRnMessage
    mk_err
                              (IllegalClassInstanceReason -> TcRnMessage)
    -> IllegalClassInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ Class -> CoverageProblem -> IllegalClassInstanceReason
    IllegalInstanceFailsCoverageCondition Class
    clas CoverageProblem
    coverageInstErr

            ; traceTc "End checkValidInstance }" empty }
        | Bool
    otherwise
        -> TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalClassInstanceReason -> TcRnMessage
    mk_err (IllegalClassInstanceReason -> TcRnMessage)
    -> IllegalClassInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceHeadReason -> IllegalClassInstanceReason
    IllegalInstanceHead
                               (IllegalInstanceHeadReason -> IllegalClassInstanceReason)
    -> IllegalInstanceHeadReason -> IllegalClassInstanceReason
    forall a b. (a -> b) -> a -> b
    $ Maybe TyCon -> IllegalInstanceHeadReason
    InstHeadNonClass (TyCon -> Maybe TyCon
    forall a. a -> Maybe a
    Just TyCon
    tc)
      Type
    _ -> TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalClassInstanceReason -> TcRnMessage
    mk_err (IllegalClassInstanceReason -> TcRnMessage)
    -> IllegalClassInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceHeadReason -> IllegalClassInstanceReason
    IllegalInstanceHead
                               (IllegalInstanceHeadReason -> IllegalClassInstanceReason)
    -> IllegalInstanceHeadReason -> IllegalClassInstanceReason
    forall a b. (a -> b) -> a -> b
    $ Maybe TyCon -> IllegalInstanceHeadReason
    InstHeadNonClass Maybe TyCon
    forall a. Maybe a
    Nothing
      where
        ([Type]
    theta, Type
    tau) = Type -> ([Type], Type)
    splitInstTyForValidity Type
    ty

        mk_err :: IllegalClassInstanceReason -> TcRnMessage
    mk_err IllegalClassInstanceReason
    err = IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance
                   (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ TypedThing -> IllegalClassInstanceReason -> IllegalInstanceReason
    IllegalClassInstance (Type -> TypedThing
    TypeThing Type
    tau) IllegalClassInstanceReason
    err

            -- The location of the "head" of the instance
        head_loc :: EpAnn AnnListItem
    head_loc = GenLocated (EpAnn AnnListItem) (HsType GhcRn) -> EpAnn AnnListItem
    forall l e. GenLocated l e -> l
    getLoc (LHsSigType GhcRn -> LHsType GhcRn
    forall (p :: Pass). LHsSigType (GhcPass p) -> LHsType (GhcPass p)
    getLHsInstDeclHead LHsSigType GhcRn
    hs_type)

    -- | Split an instance type of the form @forall tvbs. inst_ctxt => inst_head@
    -- and return @(inst_ctxt, inst_head)@. This function makes no attempt to look
    -- through type synonyms. See @Note [Instances and constraint synonyms]@.
    splitInstTyForValidity :: Type -> (ThetaType, Type)
    splitInstTyForValidity :: Type -> ([Type], Type)
    splitInstTyForValidity = [Type] -> Type -> ([Type], Type)
    split_context [] (Type -> ([Type], Type))
    -> (Type -> Type) -> Type -> ([Type], Type)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Type -> Type
    drop_foralls
      where
        -- This is like 'dropForAlls', except that it does not look through type
        -- synonyms.
        drop_foralls :: Type -> Type
        drop_foralls :: Type -> Type
    drop_foralls (ForAllTy (Bndr Var
    _tv ForAllTyFlag
    argf) Type
    ty)
          | ForAllTyFlag -> Bool
    isInvisibleForAllTyFlag ForAllTyFlag
    argf = Type -> Type
    drop_foralls Type
    ty
        drop_foralls Type
    ty = Type
    ty

        -- This is like 'tcSplitPhiTy', except that it does not look through type
        -- synonyms.
        split_context :: ThetaType -> Type -> (ThetaType, Type)
        split_context :: [Type] -> Type -> ([Type], Type)
    split_context [Type]
    preds (FunTy { ft_af :: Type -> FunTyFlag
    ft_af = FunTyFlag
    af, ft_arg :: Type -> Type
    ft_arg = Type
    pred, ft_res :: Type -> Type
    ft_res = Type
    tau })
          | FunTyFlag -> Bool
    isInvisibleFunArg FunTyFlag
    af = [Type] -> Type -> ([Type], Type)
    split_context (Type
    predType -> [Type] -> [Type]
    forall a. a -> [a] -> [a]
    :[Type]
    preds) Type
    tau
        split_context [Type]
    preds Type
    ty = ([Type] -> [Type]
    forall a. [a] -> [a]
    reverse [Type]
    preds, Type
    ty)

    checkInstTermination :: ThetaType -> TcPredType -> TcM ()
    -- See Note [Paterson conditions]
    checkInstTermination :: [Type] -> Type -> TcM ()
    checkInstTermination [Type]
    theta Type
    head_pred
      = VarSet -> [Type] -> TcM ()
    check_preds VarSet
    emptyVarSet [Type]
    theta
      where
       head_size :: PatersonSize
    head_size = Type -> PatersonSize
    pSizeType Type
    head_pred
       -- This is inconsistent and probably wrong.  pSizeType does not filter out
       -- invisible type args (making the instance head look big), whereas the use of
       -- pSizeClassPredX below /does/ filter them out (making the tested constraints
       -- look smaller). I'm sure there is non termination lurking here, but see #15177
       -- for why I didn't change it. See Note [Invisible arguments and termination]
       -- in GHC.Tc.Utils.TcType

       check_preds :: VarSet -> [PredType] -> TcM ()
       check_preds :: VarSet -> [Type] -> TcM ()
    check_preds VarSet
    foralld_tvs [Type]
    preds = (Type -> TcM ()) -> [Type] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (VarSet -> Type -> TcM ()
    check VarSet
    foralld_tvs) [Type]
    preds

       check :: VarSet -> PredType -> TcM ()
       check :: VarSet -> Type -> TcM ()
    check VarSet
    foralld_tvs Type
    pred
         = case Type -> Pred
    classifyPredType Type
    pred of
             EqPred {}      -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()  -- See #4200.
             IrredPred {}   -> PatersonSize -> TcM ()
    check2 (VarSet -> Type -> PatersonSize
    pSizeTypeX VarSet
    foralld_tvs Type
    pred)

             ClassPred Class
    cls [Type]
    tys
               | Class -> Bool
    isCTupleClass Class
    cls  -- See Note [Tuples in checkInstTermination]
               -> VarSet -> [Type] -> TcM ()
    check_preds VarSet
    foralld_tvs [Type]
    tys

               | Bool
    otherwise          -- Other ClassPreds
               -> PatersonSize -> TcM ()
    check2 (VarSet -> Class -> [Type] -> PatersonSize
    pSizeClassPredX VarSet
    foralld_tvs Class
    cls [Type]
    tys)

             ForAllPred [Var]
    tvs [Type]
    _ Type
    head_pred'
               -> VarSet -> Type -> TcM ()
    check (VarSet
    foralld_tvs VarSet -> [Var] -> VarSet
    `extendVarSetList` [Var]
    tvs) Type
    head_pred'
                  -- Termination of the quantified predicate itself is checked
                  -- when the predicates are individually checked for validity

          where
            check2 :: PatersonSize -> TcM ()
    check2 PatersonSize
    pred_size
              = case PatersonSize
    pred_size PatersonSize -> PatersonSize -> Maybe PatersonCondFailure
    `ltPatersonSize` PatersonSize
    head_size of
                  Just PatersonCondFailure
    pc_failure -> TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ PatersonCondFailure
    -> PatersonCondFailureContext -> Type -> Type -> TcRnMessage
    TcRnPatersonCondFailure PatersonCondFailure
    pc_failure PatersonCondFailureContext
    InInstanceDecl Type
    pred Type
    head_pred
                  Maybe PatersonCondFailure
    Nothing         -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    {- Note [Instances and constraint synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Currently, we don't allow instances for constraint synonyms at all.
    Consider these (#13267):
      type C1 a = Show (a -> Bool)
      instance C1 Int where    -- I1
        show _ = "ur"

    This elicits "show is not a (visible) method of class C1", which isn't
    a great message. But it comes from the renamer, so it's hard to improve.

    This needs a bit more care:
      type C2 a = (Show a, Show Int)
      instance C2 Int           -- I2

    If we use (splitTyConApp_maybe tau) in checkValidInstance to decompose
    the instance head, we'll expand the synonym on fly, and it'll look like
      instance (%,%) (Show Int, Show Int)
    and we /really/ don't want that.  So we carefully do /not/ expand
    synonyms, by matching on TyConApp directly.

    For similar reasons, we do not use tcSplitSigmaTy when decomposing the instance
    context, as the looks through type synonyms. If we looked through type
    synonyms, then it could be possible to write an instance for a type synonym
    involving a quantified constraint (see #22570). Instead, we define
    splitInstTyForValidity, a specialized version of tcSplitSigmaTy (local to
    GHC.Tc.Validity) that does not expand type synonyms.
    -}

    {-
    ************************************************************************
    *                                                                      *
            Checking type instance well-formedness and termination
    *                                                                      *
    ************************************************************************
    -}

    checkValidCoAxiom :: CoAxiom Branched -> TcM ()
    checkValidCoAxiom :: CoAxiom Branched -> TcM ()
    checkValidCoAxiom ax :: CoAxiom Branched
    ax@(CoAxiom { co_ax_tc :: forall (br :: BranchFlag). CoAxiom br -> TyCon
    co_ax_tc = TyCon
    fam_tc, co_ax_branches :: forall (br :: BranchFlag). CoAxiom br -> Branches br
    co_ax_branches = Branches Branched
    branches })
      = do { (CoAxBranch -> TcM ()) -> [CoAxBranch] -> TcM ()
    forall (t :: * -> *) (m :: * -> *) a b.
    (Foldable t, Monad m) =>
    (a -> m b) -> t a -> m ()
    mapM_ (TyCon -> CoAxBranch -> TcM ()
    checkValidCoAxBranch TyCon
    fam_tc) [CoAxBranch]
    branch_list
           ; ([CoAxBranch]
     -> CoAxBranch -> IOEnv (Env TcGblEnv TcLclEnv) [CoAxBranch])
    -> [CoAxBranch] -> [CoAxBranch] -> TcM ()
    forall (m :: * -> *) (t :: * -> *) a b.
    (Monad m, Foldable t) =>
    (a -> b -> m a) -> a -> t b -> m ()
    foldlM_ [CoAxBranch]
    -> CoAxBranch -> IOEnv (Env TcGblEnv TcLclEnv) [CoAxBranch]
    check_branch_compat [] [CoAxBranch]
    branch_list }
      where
        branch_list :: [CoAxBranch]
    branch_list = Branches Branched -> [CoAxBranch]
    forall (br :: BranchFlag). Branches br -> [CoAxBranch]
    fromBranches Branches Branched
    branches
        injectivity :: Injectivity
    injectivity = TyCon -> Injectivity
    tyConInjectivityInfo TyCon
    fam_tc

        check_branch_compat :: [CoAxBranch]    -- previous branches in reverse order
                            -> CoAxBranch      -- current branch
                            -> TcM [CoAxBranch]-- current branch : previous branches
        -- Check for
        --   (a) this branch is dominated by previous ones
        --   (b) failure of injectivity
        check_branch_compat :: [CoAxBranch]
    -> CoAxBranch -> IOEnv (Env TcGblEnv TcLclEnv) [CoAxBranch]
    check_branch_compat [CoAxBranch]
    prev_branches CoAxBranch
    cur_branch
          | CoAxBranch
    cur_branch CoAxBranch -> [CoAxBranch] -> Bool
    `isDominatedBy` [CoAxBranch]
    prev_branches
          = do { let dia :: TcRnMessage
    dia = TyCon -> CoAxBranch -> TcRnMessage
    TcRnInaccessibleCoAxBranch TyCon
    fam_tc CoAxBranch
    cur_branch
               ; SrcSpan -> TcRnMessage -> TcM ()
    addDiagnosticAt (CoAxBranch -> SrcSpan
    coAxBranchSpan CoAxBranch
    cur_branch) TcRnMessage
    dia
               ; [CoAxBranch] -> IOEnv (Env TcGblEnv TcLclEnv) [CoAxBranch]
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return [CoAxBranch]
    prev_branches }
          | Bool
    otherwise
          = do { [CoAxBranch] -> CoAxBranch -> TcM ()
    check_injectivity [CoAxBranch]
    prev_branches CoAxBranch
    cur_branch
               ; [CoAxBranch] -> IOEnv (Env TcGblEnv TcLclEnv) [CoAxBranch]
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (CoAxBranch
    cur_branch CoAxBranch -> [CoAxBranch] -> [CoAxBranch]
    forall a. a -> [a] -> [a]
    : [CoAxBranch]
    prev_branches) }

         -- Injectivity check: check whether a new (CoAxBranch) can extend
         -- already checked equations without violating injectivity
         -- annotation supplied by the user.
         -- See Note [Verifying injectivity annotation] in GHC.Core.FamInstEnv
        check_injectivity :: [CoAxBranch] -> CoAxBranch -> TcM ()
    check_injectivity [CoAxBranch]
    prev_branches CoAxBranch
    cur_branch
          | Injective [Bool]
    inj <- Injectivity
    injectivity
          = do { dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags
    forall (m :: * -> *). HasDynFlags m => m DynFlags
    getDynFlags
               ; let conflicts =
                         ([CoAxBranch], Int) -> [CoAxBranch]
    forall a b. (a, b) -> a
    fst (([CoAxBranch], Int) -> [CoAxBranch])
    -> ([CoAxBranch], Int) -> [CoAxBranch]
    forall a b. (a -> b) -> a -> b
    $ (([CoAxBranch], Int) -> CoAxBranch -> ([CoAxBranch], Int))
    -> ([CoAxBranch], Int) -> [CoAxBranch] -> ([CoAxBranch], Int)
    forall b a. (b -> a -> b) -> b -> [a] -> b
    forall (t :: * -> *) b a.
    Foldable t =>
    (b -> a -> b) -> b -> t a -> b
    foldl' ([Bool]
    -> [CoAxBranch]
    -> CoAxBranch
    -> ([CoAxBranch], Int)
    -> CoAxBranch
    -> ([CoAxBranch], Int)
    gather_conflicts [Bool]
    inj [CoAxBranch]
    prev_branches CoAxBranch
    cur_branch)
                                     ([], Int
    0) [CoAxBranch]
    prev_branches
               ; reportConflictingInjectivityErrs fam_tc conflicts cur_branch
               ; reportInjectivityErrors dflags ax cur_branch inj }
          | Bool
    otherwise
          = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

        gather_conflicts :: [Bool]
    -> [CoAxBranch]
    -> CoAxBranch
    -> ([CoAxBranch], Int)
    -> CoAxBranch
    -> ([CoAxBranch], Int)
    gather_conflicts [Bool]
    inj [CoAxBranch]
    prev_branches CoAxBranch
    cur_branch ([CoAxBranch]
    acc, Int
    n) CoAxBranch
    branch
                   -- n is 0-based index of branch in prev_branches
          = case [Bool] -> CoAxBranch -> CoAxBranch -> InjectivityCheckResult
    injectiveBranches [Bool]
    inj CoAxBranch
    cur_branch CoAxBranch
    branch of
               -- Case 1B2 in Note [Verifying injectivity annotation] in GHC.Core.FamInstEnv
              InjectivityUnified CoAxBranch
    ax1 CoAxBranch
    ax2
                | CoAxBranch
    ax1 CoAxBranch -> [CoAxBranch] -> Bool
    `isDominatedBy` ([CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
    replace_br [CoAxBranch]
    prev_branches Int
    n CoAxBranch
    ax2)
                    -> ([CoAxBranch]
    acc, Int
    n Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    1)
                | Bool
    otherwise
                    -> (CoAxBranch
    branch CoAxBranch -> [CoAxBranch] -> [CoAxBranch]
    forall a. a -> [a] -> [a]
    : [CoAxBranch]
    acc, Int
    n Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    1)
              InjectivityCheckResult
    InjectivityAccepted -> ([CoAxBranch]
    acc, Int
    n Int -> Int -> Int
    forall a. Num a => a -> a -> a
    + Int
    1)

        -- Replace n-th element in the list. Assumes 0-based indexing.
        replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
        replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
    replace_br [CoAxBranch]
    brs Int
    n CoAxBranch
    br = Int -> [CoAxBranch] -> [CoAxBranch]
    forall a. Int -> [a] -> [a]
    take Int
    n [CoAxBranch]
    brs [CoAxBranch] -> [CoAxBranch] -> [CoAxBranch]
    forall a. [a] -> [a] -> [a]
    ++ [CoAxBranch
    br] [CoAxBranch] -> [CoAxBranch] -> [CoAxBranch]
    forall a. [a] -> [a] -> [a]
    ++ Int -> [CoAxBranch] -> [CoAxBranch]
    forall a. Int -> [a] -> [a]
    drop (Int
    nInt -> Int -> Int
    forall a. Num a => a -> a -> a
    +Int
    1) [CoAxBranch]
    brs

    -- Check that a "type instance" is well-formed (which includes decidability
    -- unless -XUndecidableInstances is given).
    --
    -- See also the separate 'checkFamPatBinders' which performs scoping checks
    -- on a type family equation.
    -- (It's separate because it expects 'TyFamEqnValidityInfo', which comes from a
    -- separate place e.g. for associated type defaults.)
    checkValidCoAxBranch :: TyCon -> CoAxBranch -> TcM ()
    checkValidCoAxBranch :: TyCon -> CoAxBranch -> TcM ()
    checkValidCoAxBranch TyCon
    fam_tc
                        (CoAxBranch { cab_lhs :: CoAxBranch -> [Type]
    cab_lhs = [Type]
    typats
                                    , cab_rhs :: CoAxBranch -> Type
    cab_rhs = Type
    rhs, cab_loc :: CoAxBranch -> SrcSpan
    cab_loc = SrcSpan
    loc })
      = SrcSpan -> TcM () -> TcM ()
    forall a. SrcSpan -> TcRn a -> TcRn a
    setSrcSpan SrcSpan
    loc (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
        TyCon -> [Type] -> Type -> TcM ()
    checkValidTyFamEqn TyCon
    fam_tc [Type]
    typats Type
    rhs

    -- | Do validity checks on a type family equation, including consistency
    -- with any enclosing class instance head, termination, and lack of
    -- polytypes.
    --
    -- See also the separate 'checkFamPatBinders' which performs scoping checks
    -- on a type family equation.
    -- (It's separate because it expects 'TyFamEqnValidityInfo', which comes from a
    -- separate place e.g. for associated type defaults.)
    checkValidTyFamEqn :: TyCon    -- ^ of the type family
                       -> [Type]   -- ^ Type patterns
                       -> Type     -- ^ Rhs
                       -> TcM ()
    checkValidTyFamEqn :: TyCon -> [Type] -> Type -> TcM ()
    checkValidTyFamEqn TyCon
    fam_tc [Type]
    typats Type
    rhs
      = do { TyCon -> [Type] -> TcM ()
    checkValidTypePats TyCon
    fam_tc [Type]
    typats

             -- Check for oversaturated visible kind arguments in a type family
             -- equation.
             -- See Note [Oversaturated type family equations]
           ; Bool -> TcM () -> TcM ()
    forall (f :: * -> *). Applicative f => Bool -> f () -> f ()
    when (TyCon -> Bool
    isTypeFamilyTyCon TyCon
    fam_tc) (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
               case Int -> [Type] -> [Type]
    forall a. Int -> [a] -> [a]
    drop (TyCon -> Int
    tyConArity TyCon
    fam_tc) [Type]
    typats of
                 [] -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure ()
                 Type
    spec_arg:[Type]
    _ ->
                   TcRnMessage -> TcM ()
    addErr (Type -> TcRnMessage
    TcRnOversaturatedVisibleKindArg Type
    spec_arg)

             -- The argument patterns, and RHS, are all boxed tau types
             -- E.g  Reject type family F (a :: k1) :: k2
             --             type instance F (forall a. a->a) = ...
             --             type instance F Int#             = ...
             --             type instance F Int              = forall a. a->a
             --             type instance F Int              = Int#
             -- See #9357
           ; Type -> TcM ()
    checkValidMonoType Type
    rhs

             -- We have a decidable instance unless otherwise permitted
           ; undecidable_ok <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool
    forall gbl lcl. Extension -> TcRnIf gbl lcl Bool
    xoptM Extension
    LangExt.UndecidableInstances
           ; traceTc "checkVTFE" (ppr fam_tc $$ ppr rhs $$ ppr (tcTyFamInsts rhs))
           ; unless undecidable_ok $
             mapM_ addErrTc (checkFamInstRhs fam_tc typats (tcTyFamInsts rhs)) }

    -- | Checks that an associated type family default:
    --
    -- 1. Only consists of arguments that are bare type variables, and
    --
    -- 2. Has a distinct type variable in each argument.
    --
    -- See @Note [Type-checking default assoc decls]@ in "GHC.Tc.TyCl".
    checkValidAssocTyFamDeflt :: TyCon  -- ^ of the type family
                              -> [Type] -- ^ Type patterns
                              -> TcM ()
    checkValidAssocTyFamDeflt :: TyCon -> [Type] -> TcM ()
    checkValidAssocTyFamDeflt TyCon
    fam_tc [Type]
    pats =
      do { cpt_tvs <- (Type -> ForAllTyFlag -> IOEnv (Env TcGblEnv TcLclEnv) Var)
    -> [Type] -> [ForAllTyFlag] -> IOEnv (Env TcGblEnv TcLclEnv) [Var]
    forall (m :: * -> *) a b c.
    Applicative m =>
    (a -> b -> m c) -> [a] -> [b] -> m [c]
    zipWithM Type -> ForAllTyFlag -> IOEnv (Env TcGblEnv TcLclEnv) Var
    extract_tv [Type]
    pats [ForAllTyFlag]
    pats_vis
         ; check_all_distinct_tvs $ zip cpt_tvs pats_vis }
      where
        pats_vis :: [ForAllTyFlag]
        pats_vis :: [ForAllTyFlag]
    pats_vis = TyCon -> [Type] -> [ForAllTyFlag]
    tyConForAllTyFlags TyCon
    fam_tc [Type]
    pats

        -- Checks that a pattern on the LHS of a default is a type
        -- variable. If so, return the underlying type variable, and if
        -- not, throw an error.
        -- See Note [Type-checking default assoc decls]
        extract_tv :: Type          -- The particular type pattern from which to
                                    -- extrace its underlying type variable
                   -> ForAllTyFlag  -- The visibility of the type pattern
                                    -- (only used for error message purposes)
                   -> TcM TyVar
        extract_tv :: Type -> ForAllTyFlag -> IOEnv (Env TcGblEnv TcLclEnv) Var
    extract_tv Type
    pat ForAllTyFlag
    pat_vis =
          case Type -> Maybe Var
    getTyVar_maybe Type
    pat of
            Just Var
    tv -> Var -> IOEnv (Env TcGblEnv TcLclEnv) Var
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure Var
    tv
            Maybe Var
    Nothing -> TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Var
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Var)
    -> TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Var
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance
                                  (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalFamilyInstanceReason -> IllegalInstanceReason
    IllegalFamilyInstance
                                  (IllegalFamilyInstanceReason -> IllegalInstanceReason)
    -> IllegalFamilyInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssoc -> IllegalFamilyInstanceReason
    InvalidAssoc (InvalidAssoc -> IllegalFamilyInstanceReason)
    -> InvalidAssoc -> IllegalFamilyInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssocDefault -> InvalidAssoc
    InvalidAssocDefault
                                  (InvalidAssocDefault -> InvalidAssoc)
    -> InvalidAssocDefault -> InvalidAssoc
    forall a b. (a -> b) -> a -> b
    $ TyCon -> [Type] -> AssocDefaultBadArgs -> InvalidAssocDefault
    AssocDefaultBadArgs TyCon
    fam_tc [Type]
    pats
                                  (AssocDefaultBadArgs -> InvalidAssocDefault)
    -> AssocDefaultBadArgs -> InvalidAssocDefault
    forall a b. (a -> b) -> a -> b
    $ (Type, ForAllTyFlag) -> AssocDefaultBadArgs
    AssocDefaultNonTyVarArg (Type
    pat, ForAllTyFlag
    pat_vis)

        -- Checks that no type variables in an associated default declaration are
        -- duplicated. If that is the case, throw an error.
        -- See Note [Type-checking default assoc decls]
        check_all_distinct_tvs ::
             [(TyVar, ForAllTyFlag)] -- The type variable arguments in the associated
                                -- default declaration, along with their respective
                                -- visibilities (the latter are only used for error
                                -- message purposes)
          -> TcM ()
        check_all_distinct_tvs :: [(Var, ForAllTyFlag)] -> TcM ()
    check_all_distinct_tvs [(Var, ForAllTyFlag)]
    cpt_tvs_vis =
          let dups :: [NonEmpty (Var, ForAllTyFlag)]
    dups = ((Var, ForAllTyFlag) -> (Var, ForAllTyFlag) -> Bool)
    -> [(Var, ForAllTyFlag)] -> [NonEmpty (Var, ForAllTyFlag)]
    forall a. (a -> a -> Bool) -> [a] -> [NonEmpty a]
    findDupsEq (Var -> Var -> Bool
    forall a. Eq a => a -> a -> Bool
    (==) (Var -> Var -> Bool)
    -> ((Var, ForAllTyFlag) -> Var)
    -> (Var, ForAllTyFlag)
    -> (Var, ForAllTyFlag)
    -> Bool
    forall b c a. (b -> b -> c) -> (a -> b) -> a -> a -> c
    `on` (Var, ForAllTyFlag) -> Var
    forall a b. (a, b) -> a
    fst) [(Var, ForAllTyFlag)]
    cpt_tvs_vis in
          (NonEmpty (Var, ForAllTyFlag) -> IOEnv (Env TcGblEnv TcLclEnv) Any)
    -> [NonEmpty (Var, ForAllTyFlag)] -> TcM ()
    forall (t :: * -> *) (f :: * -> *) a b.
    (Foldable t, Applicative f) =>
    (a -> f b) -> t a -> f ()
    traverse_
            (\NonEmpty (Var, ForAllTyFlag)
    d -> TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Any
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Any)
    -> TcRnMessage -> IOEnv (Env TcGblEnv TcLclEnv) Any
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance
                              (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalFamilyInstanceReason -> IllegalInstanceReason
    IllegalFamilyInstance
                              (IllegalFamilyInstanceReason -> IllegalInstanceReason)
    -> IllegalFamilyInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssoc -> IllegalFamilyInstanceReason
    InvalidAssoc (InvalidAssoc -> IllegalFamilyInstanceReason)
    -> InvalidAssoc -> IllegalFamilyInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssocDefault -> InvalidAssoc
    InvalidAssocDefault
                              (InvalidAssocDefault -> InvalidAssoc)
    -> InvalidAssocDefault -> InvalidAssoc
    forall a b. (a -> b) -> a -> b
    $ TyCon -> [Type] -> AssocDefaultBadArgs -> InvalidAssocDefault
    AssocDefaultBadArgs TyCon
    fam_tc [Type]
    pats
                              (AssocDefaultBadArgs -> InvalidAssocDefault)
    -> AssocDefaultBadArgs -> InvalidAssocDefault
    forall a b. (a -> b) -> a -> b
    $ NonEmpty (Var, ForAllTyFlag) -> AssocDefaultBadArgs
    AssocDefaultDuplicateTyVars NonEmpty (Var, ForAllTyFlag)
    d)
            [NonEmpty (Var, ForAllTyFlag)]
    dups

    checkFamInstRhs :: TyCon -> [Type]         -- LHS
                    -> [(TyCon, [Type])]       -- type family calls in RHS
                    -> [TcRnMessage]
    -- Ensure that the type family instance terminates. Specifically:
    -- ensure that each type family application in the RHS is
    --    (TF1) a call to a stuck family like (TypeError ...) or Any
    --          See Note [Stuck type families] in GHC.Tc.Utils.TcType
    -- or (TF2) obeys the Paterson conditions, namely:
    --          - strictly smaller than the lhs,
    --          - mentions no type variable more often than the lhs, and
    --          - does not contain any further type family applications
    checkFamInstRhs :: TyCon -> [Type] -> [(TyCon, [Type])] -> [TcRnMessage]
    checkFamInstRhs TyCon
    lhs_tc [Type]
    lhs_tys [(TyCon, [Type])]
    famInsts
      = ((TyCon, [Type]) -> Maybe TcRnMessage)
    -> [(TyCon, [Type])] -> [TcRnMessage]
    forall a b. (a -> Maybe b) -> [a] -> [b]
    mapMaybe (TyCon, [Type]) -> Maybe TcRnMessage
    check [(TyCon, [Type])]
    famInsts
      where
       lhs_size :: PatersonSize
    lhs_size = [Type] -> PatersonSize
    pSizeTypes [Type]
    lhs_tys
       check :: (TyCon, [Type]) -> Maybe TcRnMessage
    check (TyCon
    tc, [Type]
    tys)
          | Bool -> Bool
    not (TyCon -> Bool
    isStuckTypeFamily TyCon
    tc)                                   -- (TF1)
          , Just PatersonCondFailure
    pc_failure <- [Type] -> PatersonSize
    pSizeTypes [Type]
    tys PatersonSize -> PatersonSize -> Maybe PatersonCondFailure
    `ltPatersonSize` PatersonSize
    lhs_size  -- (TF2)
          = TcRnMessage -> Maybe TcRnMessage
    forall a. a -> Maybe a
    Just (TcRnMessage -> Maybe TcRnMessage)
    -> TcRnMessage -> Maybe TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ PatersonCondFailure
    -> PatersonCondFailureContext -> Type -> Type -> TcRnMessage
    TcRnPatersonCondFailure PatersonCondFailure
    pc_failure PatersonCondFailureContext
    InTyFamEquation (TyCon -> [Type] -> Type
    TyConApp TyCon
    lhs_tc [Type]
    lhs_tys) (TyCon -> [Type] -> Type
    TyConApp TyCon
    tc [Type]
    tys)
          | Bool
    otherwise
          = Maybe TcRnMessage
    forall a. Maybe a
    Nothing

    -----------------

    -- | Perform scoping check on a type family equation.
    --
    -- See 'TyFamEqnValidityInfo'.
    checkTyFamEqnValidityInfo :: TyCon -> TyFamEqnValidityInfo -> TcM ()
    checkTyFamEqnValidityInfo :: TyCon -> TyFamEqnValidityInfo -> TcM ()
    checkTyFamEqnValidityInfo TyCon
    fam_tc = \ case
      TyFamEqnValidityInfo
    NoVI -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      VI { vi_loc :: TyFamEqnValidityInfo -> SrcSpan
    vi_loc          = SrcSpan
    loc
         , vi_qtvs :: TyFamEqnValidityInfo -> [Var]
    vi_qtvs         = [Var]
    qtvs
         , vi_non_user_tvs :: TyFamEqnValidityInfo -> VarSet
    vi_non_user_tvs = VarSet
    non_user_tvs
         , vi_pats :: TyFamEqnValidityInfo -> [Type]
    vi_pats         = [Type]
    pats
         , vi_rhs :: TyFamEqnValidityInfo -> Type
    vi_rhs          = Type
    rhs } ->
        SrcSpan -> TcM () -> TcM ()
    forall a. SrcSpan -> TcRn a -> TcRn a
    setSrcSpan SrcSpan
    loc (TcM () -> TcM ()) -> TcM () -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
        TyCon -> [Var] -> VarSet -> [Type] -> Type -> TcM ()
    checkFamPatBinders TyCon
    fam_tc [Var]
    qtvs VarSet
    non_user_tvs [Type]
    pats Type
    rhs

    -- | Check binders for a type or data family declaration.
    --
    -- Specifically, this function checks for:
    --
    --  - type variables used on the RHS but not bound (explicitly or implicitly)
    --    in the LHS,
    --  - variables bound by a forall in the LHS but not used in the RHS.
    --
    -- See Note [Check type family instance binders].
    checkFamPatBinders :: TyCon
                       -> [TcTyVar]   -- ^ Bound on LHS of family instance
                       -> TyVarSet    -- ^ non-user-written tyvars
                       -> [TcType]    -- ^ LHS patterns
                       -> TcType      -- ^ RHS
                       -> TcM ()
    checkFamPatBinders :: TyCon -> [Var] -> VarSet -> [Type] -> Type -> TcM ()
    checkFamPatBinders TyCon
    fam_tc [Var]
    qtvs VarSet
    non_user_tvs [Type]
    pats Type
    rhs
      = do { String -> SDoc -> TcM ()
    traceTc String
    "checkFamPatBinders" (SDoc -> TcM ()) -> SDoc -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
             [SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ Type -> SDoc
    debugPprType (TyCon -> [Type] -> Type
    mkTyConApp TyCon
    fam_tc [Type]
    pats)
                  , Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (TyCon -> [Type] -> Type
    mkTyConApp TyCon
    fam_tc [Type]
    pats)
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "rhs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr Type
    rhs
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "qtvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [Var] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Var]
    qtvs
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "rhs_fvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> VarSet -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr (FV -> VarSet
    fvVarSet FV
    rhs_fvs)
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "cpt_tvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> VarSet -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr VarSet
    cpt_tvs
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "inj_cpt_tvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> VarSet -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr VarSet
    inj_cpt_tvs
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "bad_rhs_tvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [Var] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Var]
    bad_rhs_tvs
                  , String -> SDoc
    forall doc. IsLine doc => String -> doc
    text String
    "bad_qtvs:" SDoc -> SDoc -> SDoc
    forall doc. IsLine doc => doc -> doc -> doc
    <+> [Var] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr ((InvalidFamInstQTv -> Var) -> [InvalidFamInstQTv] -> [Var]
    forall a b. (a -> b) -> [a] -> [b]
    map InvalidFamInstQTv -> Var
    ifiqtv [InvalidFamInstQTv]
    bad_qtvs) ]

             -- Check for implicitly-bound tyvars, mentioned on the
             -- RHS but not bound on the LHS
             --    data T            = MkT (forall (a::k). blah)
             --    data family D Int = MkD (forall (a::k). blah)
             -- In both cases, 'k' is not bound on the LHS, but is used on the RHS
             -- We catch the former in kcDeclHeader, and the latter right here
             -- See Note [Check type family instance binders]
           ; (NonEmpty Name -> IllegalFamilyInstanceReason) -> [Name] -> TcM ()
    forall {a}.
    NamedThing a =>
    (NonEmpty a -> IllegalFamilyInstanceReason) -> [a] -> TcM ()
    check_tvs (Maybe (TyCon, [Type], VarSet)
    -> NonEmpty Name -> IllegalFamilyInstanceReason
    FamInstRHSOutOfScopeTyVars ((TyCon, [Type], VarSet) -> Maybe (TyCon, [Type], VarSet)
    forall a. a -> Maybe a
    Just (TyCon
    fam_tc, [Type]
    pats, VarSet
    dodgy_tvs)))
                       ((Var -> Name) -> [Var] -> [Name]
    forall a b. (a -> b) -> [a] -> [b]
    map Var -> Name
    tyVarName [Var]
    bad_rhs_tvs)

             -- Check for explicitly forall'd variable that is not bound on LHS
             --    data instance forall a.  T Int = MkT Int
             -- See Note [Unused explicitly bound variables in a family pattern]
             -- See Note [Check type family instance binders]
           ; (NonEmpty InvalidFamInstQTv -> IllegalFamilyInstanceReason)
    -> [InvalidFamInstQTv] -> TcM ()
    forall {a}.
    NamedThing a =>
    (NonEmpty a -> IllegalFamilyInstanceReason) -> [a] -> TcM ()
    check_tvs NonEmpty InvalidFamInstQTv -> IllegalFamilyInstanceReason
    FamInstLHSUnusedBoundTyVars [InvalidFamInstQTv]
    bad_qtvs
           }
      where
        rhs_fvs :: FV
    rhs_fvs = Type -> FV
    tyCoFVsOfType Type
    rhs

        cpt_tvs :: VarSet
    cpt_tvs     = [Type] -> VarSet
    tyCoVarsOfTypes [Type]
    pats
        inj_cpt_tvs :: VarSet
    inj_cpt_tvs = FV -> VarSet
    fvVarSet (FV -> VarSet) -> FV -> VarSet
    forall a b. (a -> b) -> a -> b
    $ Bool -> [Type] -> FV
    injectiveVarsOfTypes Bool
    False [Type]
    pats
          -- The type variables that are in injective positions.
          -- See Note [Dodgy binding sites in type family instances]
          -- NB: The False above is irrelevant, as we never have type families in
          -- patterns.
          --
          -- NB: It's OK to use the nondeterministic `fvVarSet` function here,
          -- since the order of `inj_cpt_tvs` is never revealed in an error
          -- message.

        -- Bound but not used at all
        bad_qtvs :: [InvalidFamInstQTv]
    bad_qtvs    = (Var -> Maybe InvalidFamInstQTv) -> [Var] -> [InvalidFamInstQTv]
    forall a b. (a -> Maybe b) -> [a] -> [b]
    mapMaybe Var -> Maybe InvalidFamInstQTv
    bad_qtv_maybe [Var]
    qtvs

        bad_qtv_maybe :: Var -> Maybe InvalidFamInstQTv
    bad_qtv_maybe Var
    qtv
          | Bool
    not_bound_in_pats
          = let reason :: InvalidFamInstQTvReason
    reason
                  | Bool
    dodgy
                  = InvalidFamInstQTvReason
    InvalidFamInstQTvDodgy
                  | Bool
    used_in_rhs
                  = InvalidFamInstQTvReason
    InvalidFamInstQTvNotBoundInPats
                  | Bool
    otherwise
                  = InvalidFamInstQTvReason
    InvalidFamInstQTvNotUsedInRHS
            in InvalidFamInstQTv -> Maybe InvalidFamInstQTv
    forall a. a -> Maybe a
    Just (InvalidFamInstQTv -> Maybe InvalidFamInstQTv)
    -> InvalidFamInstQTv -> Maybe InvalidFamInstQTv
    forall a b. (a -> b) -> a -> b
    $ InvalidFamInstQTv
                        { ifiqtv :: Var
    ifiqtv = Var
    qtv
                        , ifiqtv_user_written :: Bool
    ifiqtv_user_written = Bool -> Bool
    not (Bool -> Bool) -> Bool -> Bool
    forall a b. (a -> b) -> a -> b
    $ Var
    qtv Var -> VarSet -> Bool
    `elemVarSet` VarSet
    non_user_tvs
                        , ifiqtv_reason :: InvalidFamInstQTvReason
    ifiqtv_reason = InvalidFamInstQTvReason
    reason
                        }
          | Bool
    otherwise
          = Maybe InvalidFamInstQTv
    forall a. Maybe a
    Nothing
            where
              not_bound_in_pats :: Bool
    not_bound_in_pats = Bool -> Bool
    not (Bool -> Bool) -> Bool -> Bool
    forall a b. (a -> b) -> a -> b
    $ Var
    qtv Var -> VarSet -> Bool
    `elemVarSet` VarSet
    inj_cpt_tvs
              dodgy :: Bool
    dodgy             = Bool
    not_bound_in_pats Bool -> Bool -> Bool
    && Var
    qtv Var -> VarSet -> Bool
    `elemVarSet` VarSet
    cpt_tvs
              used_in_rhs :: Bool
    used_in_rhs       = Var
    qtv Var -> VarSet -> Bool
    `elemVarSet` FV -> VarSet
    fvVarSet FV
    rhs_fvs

        -- Used on RHS but not bound on LHS
        bad_rhs_tvs :: [Var]
    bad_rhs_tvs = (Var -> Bool) -> [Var] -> [Var]
    forall a. (a -> Bool) -> [a] -> [a]
    filterOut ((Var -> VarSet -> Bool
    `elemVarSet` VarSet
    inj_cpt_tvs) (Var -> Bool) -> (Var -> Bool) -> Var -> Bool
    forall (f :: * -> *). Applicative f => f Bool -> f Bool -> f Bool
    <||> (Var -> [Var] -> Bool
    forall a. Eq a => a -> [a] -> Bool
    forall (t :: * -> *) a. (Foldable t, Eq a) => a -> t a -> Bool
    `elem` [Var]
    qtvs)) (FV -> [Var]
    fvVarList FV
    rhs_fvs)

        dodgy_tvs :: VarSet
    dodgy_tvs   = VarSet
    cpt_tvs VarSet -> VarSet -> VarSet
    `minusVarSet` VarSet
    inj_cpt_tvs

        check_tvs :: (NonEmpty a -> IllegalFamilyInstanceReason) -> [a] -> TcM ()
    check_tvs NonEmpty a -> IllegalFamilyInstanceReason
    mk_err [a]
    tvs
          = Maybe (NonEmpty a) -> (NonEmpty a -> TcM ()) -> TcM ()
    forall (t :: * -> *) (f :: * -> *) a b.
    (Foldable t, Applicative f) =>
    t a -> (a -> f b) -> f ()
    for_ ([a] -> Maybe (NonEmpty a)
    forall a. [a] -> Maybe (NonEmpty a)
    NE.nonEmpty [a]
    tvs) ((NonEmpty a -> TcM ()) -> TcM ())
    -> (NonEmpty a -> TcM ()) -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ \ ne_tvs :: NonEmpty a
    ne_tvs@(a
    tv0 :| [a]
    _) ->
            SrcSpan -> TcRnMessage -> TcM ()
    addErrAt (a -> SrcSpan
    forall a. NamedThing a => a -> SrcSpan
    getSrcSpan a
    tv0) (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
            IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalFamilyInstanceReason -> IllegalInstanceReason
    IllegalFamilyInstance (IllegalFamilyInstanceReason -> IllegalInstanceReason)
    -> IllegalFamilyInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $
            NonEmpty a -> IllegalFamilyInstanceReason
    mk_err NonEmpty a
    ne_tvs

    -- | Checks that a list of type patterns is valid in a matching (LHS)
    -- position of a class instances or type/data family instance.
    --
    -- Specifically:
    --    * All monotypes
    --    * No type-family applications
    checkValidTypePats :: TyCon -> [Type] -> TcM ()
    checkValidTypePats :: TyCon -> [Type] -> TcM ()
    checkValidTypePats TyCon
    tc [Type]
    pat_ty_args
      = do { -- Check that each of pat_ty_args is a monotype.
             -- One could imagine generalising to allow
             --      instance C (forall a. a->a)
             -- but we don't know what all the consequences might be.
             (Type -> TcM ()) -> [Type] -> TcM ()
    forall (t :: * -> *) (f :: * -> *) a b.
    (Foldable t, Applicative f) =>
    (a -> f b) -> t a -> f ()
    traverse_ Type -> TcM ()
    checkValidMonoType [Type]
    pat_ty_args

           -- Ensure that no type family applications occur a type pattern
           ; case TyCon -> [Type] -> [(Bool, TyCon, [Type])]
    tcTyConAppTyFamInstsAndVis TyCon
    tc [Type]
    pat_ty_args of
                [] -> () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (f :: * -> *) a. Applicative f => a -> f a
    pure ()
                ((Bool
    tf_is_invis_arg, TyCon
    tf_tc, [Type]
    tf_args):[(Bool, TyCon, [Type])]
    _) ->
                  TcRnMessage -> TcM ()
    forall a. TcRnMessage -> TcM a
    failWithTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $
                    Type -> Bool -> TyCon -> [Type] -> IllegalInstanceReason
    IllegalFamilyApplicationInInstance Type
    inst_ty
                      Bool
    tf_is_invis_arg TyCon
    tf_tc [Type]
    tf_args }
      where
        inst_ty :: Type
    inst_ty = TyCon -> [Type] -> Type
    mkTyConApp TyCon
    tc [Type]
    pat_ty_args

    -------------------------
    checkConsistentFamInst :: AssocInstInfo
                           -> TyCon     -- ^ Family tycon
                           -> CoAxBranch
                           -> TcM ()
    -- See Note [Checking consistent instantiation]

    checkConsistentFamInst :: AssocInstInfo -> TyCon -> CoAxBranch -> TcM ()
    checkConsistentFamInst AssocInstInfo
    NotAssociated TyCon
    _ CoAxBranch
    _
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()

    checkConsistentFamInst (InClsInst { ai_class :: AssocInstInfo -> Class
    ai_class = Class
    clas
                                      , ai_tyvars :: AssocInstInfo -> [Var]
    ai_tyvars = [Var]
    inst_tvs
                                      , ai_inst_env :: AssocInstInfo -> VarEnv Type
    ai_inst_env = VarEnv Type
    mini_env })
                           TyCon
    fam_tc CoAxBranch
    branch
      = do { String -> SDoc -> TcM ()
    traceTc String
    "checkConsistentFamInst" ([SDoc] -> SDoc
    forall doc. IsDoc doc => [doc] -> doc
    vcat [ [Var] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Var]
    inst_tvs
                                                    , [(Type, Type, ForAllTyFlag)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(Type, Type, ForAllTyFlag)]
    arg_triples
                                                    , VarEnv Type -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr VarEnv Type
    mini_env
                                                    , [Var] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Var]
    ax_tvs
                                                    , [Type] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [Type]
    ax_arg_tys
                                                    , [(Type, Type, ForAllTyFlag)] -> SDoc
    forall a. Outputable a => a -> SDoc
    ppr [(Type, Type, ForAllTyFlag)]
    arg_triples ])
           -- Check that the associated type indeed comes from this class
           -- See [Mismatched class methods and associated type families]
           -- in TcInstDecls.
           ; Bool -> TcRnMessage -> TcM ()
    checkTc (TyCon -> Maybe TyCon
    forall a. a -> Maybe a
    Just (Class -> TyCon
    classTyCon Class
    clas) Maybe TyCon -> Maybe TyCon -> Bool
    forall a. Eq a => a -> a -> Bool
    == TyCon -> Maybe TyCon
    tyConAssoc_maybe TyCon
    fam_tc) (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $
              IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalFamilyInstanceReason -> IllegalInstanceReason
    IllegalFamilyInstance (IllegalFamilyInstanceReason -> IllegalInstanceReason)
    -> IllegalFamilyInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $
                InvalidAssoc -> IllegalFamilyInstanceReason
    InvalidAssoc (InvalidAssoc -> IllegalFamilyInstanceReason)
    -> InvalidAssoc -> IllegalFamilyInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssocInstance -> InvalidAssoc
    InvalidAssocInstance (InvalidAssocInstance -> InvalidAssoc)
    -> InvalidAssocInstance -> InvalidAssoc
    forall a b. (a -> b) -> a -> b
    $
                Class -> TyCon -> InvalidAssocInstance
    AssocNotInThisClass Class
    clas TyCon
    fam_tc

           ; [(Type, Type, ForAllTyFlag)] -> TcM ()
    check_match [(Type, Type, ForAllTyFlag)]
    arg_triples
           }
      where
        ([Var]
    ax_tvs, [Type]
    ax_arg_tys, Type
    _) = CoAxBranch -> ([Var], [Type], Type)
    etaExpandCoAxBranch CoAxBranch
    branch

        arg_triples :: [(Type,Type, ForAllTyFlag)]
        arg_triples :: [(Type, Type, ForAllTyFlag)]
    arg_triples = [ (Type
    cls_arg_ty, Type
    at_arg_ty, ForAllTyFlag
    vis)
                      | (Var
    fam_tc_tv, ForAllTyFlag
    vis, Type
    at_arg_ty)
                           <- [Var] -> [ForAllTyFlag] -> [Type] -> [(Var, ForAllTyFlag, Type)]
    forall a b c. [a] -> [b] -> [c] -> [(a, b, c)]
    zip3 (TyCon -> [Var]
    tyConTyVars TyCon
    fam_tc)
                                   (TyCon -> [Type] -> [ForAllTyFlag]
    tyConForAllTyFlags TyCon
    fam_tc [Type]
    ax_arg_tys)
                                   [Type]
    ax_arg_tys
                      , Just Type
    cls_arg_ty <- [VarEnv Type -> Var -> Maybe Type
    forall a. VarEnv a -> Var -> Maybe a
    lookupVarEnv VarEnv Type
    mini_env Var
    fam_tc_tv] ]

        -- Fiddling around to arrange that wildcards unconditionally print as "_"
        -- We only need to print the LHS, not the RHS at all
        -- See Note [Printing conflicts with class header]
        (TidyEnv
    tidy_env1, [Var]
    _) = TidyEnv -> [Var] -> (TidyEnv, [Var])
    tidyVarBndrs TidyEnv
    emptyTidyEnv [Var]
    inst_tvs
        (TidyEnv
    tidy_env2, [Var]
    _) = TidyEnv -> [Var] -> (TidyEnv, [Var])
    tidyCoAxBndrsForUser TidyEnv
    tidy_env1 ([Var]
    ax_tvs [Var] -> [Var] -> [Var]
    forall a. Eq a => [a] -> [a] -> [a]
    \\ [Var]
    inst_tvs)

        mk_wildcard :: Var -> Type
    mk_wildcard Var
    at_tv = Var -> Type
    mkTyVarTy (Name -> Type -> Var
    mkTyVar Name
    tv_name (Var -> Type
    tyVarKind Var
    at_tv))
        tv_name :: Name
    tv_name = Unique -> OccName -> SrcSpan -> Name
    mkInternalName (Int -> Unique
    mkAlphaTyVarUnique Int
    1) (FastString -> OccName
    mkTyVarOccFS (String -> FastString
    fsLit String
    "_")) SrcSpan
    noSrcSpan

        -- For check_match, bind_me, see
        -- Note [Matching in the consistent-instantiation check]
        check_match :: [(Type,Type,ForAllTyFlag)] -> TcM ()
        check_match :: [(Type, Type, ForAllTyFlag)] -> TcM ()
    check_match [(Type, Type, ForAllTyFlag)]
    triples = Subst -> Subst -> [(Type, Type, ForAllTyFlag)] -> TcM ()
    go Subst
    emptySubst Subst
    emptySubst [(Type, Type, ForAllTyFlag)]
    triples

        go :: Subst -> Subst -> [(Type, Type, ForAllTyFlag)] -> TcM ()
    go Subst
    _ Subst
    _ [] = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
        go Subst
    lr_subst Subst
    rl_subst ((Type
    ty1,Type
    ty2,ForAllTyFlag
    vis):[(Type, Type, ForAllTyFlag)]
    triples)
          | Just Subst
    lr_subst1 <- BindFun -> Subst -> Type -> Type -> Maybe Subst
    tcMatchTyX_BM BindFun
    bind_me Subst
    lr_subst Type
    ty1 Type
    ty2
          , Just Subst
    rl_subst1 <- BindFun -> Subst -> Type -> Type -> Maybe Subst
    tcMatchTyX_BM BindFun
    bind_me Subst
    rl_subst Type
    ty2 Type
    ty1
          = Subst -> Subst -> [(Type, Type, ForAllTyFlag)] -> TcM ()
    go Subst
    lr_subst1 Subst
    rl_subst1 [(Type, Type, ForAllTyFlag)]
    triples
          | Bool
    otherwise
          = TcRnMessage -> TcM ()
    addErrTc (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ IllegalInstanceReason -> TcRnMessage
    TcRnIllegalInstance (IllegalInstanceReason -> TcRnMessage)
    -> IllegalInstanceReason -> TcRnMessage
    forall a b. (a -> b) -> a -> b
    $ IllegalFamilyInstanceReason -> IllegalInstanceReason
    IllegalFamilyInstance
                     (IllegalFamilyInstanceReason -> IllegalInstanceReason)
    -> IllegalFamilyInstanceReason -> IllegalInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssoc -> IllegalFamilyInstanceReason
    InvalidAssoc (InvalidAssoc -> IllegalFamilyInstanceReason)
    -> InvalidAssoc -> IllegalFamilyInstanceReason
    forall a b. (a -> b) -> a -> b
    $ InvalidAssocInstance -> InvalidAssoc
    InvalidAssocInstance
                     (InvalidAssocInstance -> InvalidAssoc)
    -> InvalidAssocInstance -> InvalidAssoc
    forall a b. (a -> b) -> a -> b
    $ ForAllTyFlag -> TyCon -> [Type] -> [Type] -> InvalidAssocInstance
    AssocTyVarsDontMatch ForAllTyFlag
    vis TyCon
    fam_tc [Type]
    exp_tys [Type]
    act_tys

        -- Expected/actual family argument types (for error messages)
        exp_tys :: [Type]
    exp_tys =
          [ case VarEnv Type -> Var -> Maybe Type
    forall a. VarEnv a -> Var -> Maybe a
    lookupVarEnv VarEnv Type
    mini_env Var
    at_tv of
              Just Type
    cls_arg_ty -> TidyEnv -> Type -> Type
    tidyType TidyEnv
    tidy_env2 Type
    cls_arg_ty
              Maybe Type
    Nothing         -> Var -> Type
    mk_wildcard Var
    at_tv
          | Var
    at_tv <- TyCon -> [Var]
    tyConTyVars TyCon
    fam_tc ]
        act_tys :: [Type]
    act_tys = TidyEnv -> [Type] -> [Type]
    tidyTypes TidyEnv
    tidy_env2 [Type]
    ax_arg_tys

        -- The /scoped/ type variables from the class-instance header
        -- should not be alpha-renamed.  Inferred ones can be.
        no_bind_set :: VarSet
    no_bind_set = [Var] -> VarSet
    mkVarSet [Var]
    inst_tvs
        bind_me :: BindFun
    bind_me Var
    tv Type
    _ty | Var
    tv Var -> VarSet -> Bool
    `elemVarSet` VarSet
    no_bind_set = BindFlag
    Apart
                       | Bool
    otherwise                   = BindFlag
    BindMe

    {- Note [Check type family instance binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In a type family instance, we require (of course), type variables
    used on the RHS are matched on the LHS. This is checked by
    checkFamPatBinders.  Here is an interesting example:

        type family   T :: k
        type instance T = (Nothing :: Maybe a)

    Upon a cursory glance, it may appear that the kind variable `a` is unbound
    since there are no (visible) LHS patterns in `T`. However, there is an
    *invisible* pattern due to the return kind, so inside of GHC, the instance
    looks closer to this:

        type family T @k :: k
        type instance T @(Maybe a) = (Nothing :: Maybe a)

    Here, we can see that `a` really is bound by a LHS type pattern, so `a` is in
    fact not unbound. Contrast that with this example (#13985)

        type instance T = Proxy (Nothing :: Maybe a)

    This would looks like this inside of GHC:

        type instance T @(*) = Proxy (Nothing :: Maybe a)

    So this time, `a` is neither bound by a visible nor invisible type pattern on
    the LHS, so `a` would be reported as not in scope.

    Finally, here's one more brain-teaser (from #9574). In the example below:

        class Funct f where
          type Codomain f :: *
        instance Funct ('KProxy :: KProxy o) where
          type Codomain 'KProxy = NatTr (Proxy :: o -> *)

    As it turns out, `o` is in scope in this example. That is because `o` is
    bound by the kind signature of the LHS type pattern 'KProxy. To make this more
    obvious, one can also write the instance like so:

        instance Funct ('KProxy :: KProxy o) where
          type Codomain ('KProxy :: KProxy o) = NatTr (Proxy :: o -> *)

    Note [Dodgy binding sites in type family instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider the following example (from #7536):

      type T a = Int
      type instance F (T a) = a

    This `F` instance is extremely fishy, since the RHS, `a`, purports to be
    "bound" by the LHS pattern `T a`. "Bound" has scare quotes around it because
    `T a` expands to `Int`, which doesn't mention at all, so it's as if one had
    actually written:

      type instance F Int = a

    That is clearly bogus, so to reject this, we check that every type variable
    that is mentioned on the RHS is /actually/ bound on the LHS. In other words,
    we need to do something slightly more sophisticated that just compute the free
    variables of the LHS patterns.

    It's tempting to just expand all type synonyms on the LHS and then compute
    their free variables, but even that isn't sophisticated enough. After all,
    an impish user could write the following (#17008):

      type family ConstType (a :: Type) :: Type where
        ConstType _ = Type

      type family F (x :: ConstType a) :: Type where
        F (x :: ConstType a) = a

    Just like in the previous example, the `a` on the RHS isn't actually bound
    on the LHS, but this time a type family is responsible for the deception, not
    a type synonym.

    We avoid both issues by requiring that all RHS type variables are mentioned
    in injective positions on the left-hand side (by way of
    `injectiveVarsOfTypes`). For instance, the `a` in `T a` is not in an injective
    position, as `T` is not an injective type constructor, so we do not count that.
    Similarly for the `a` in `ConstType a`.

    Note [Matching in the consistent-instantiation check]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Matching the class-instance header to family-instance tyvars is
    tricker than it sounds.  Consider (#13972)
        class C (a :: k) where
          type T k :: Type
        instance C Left where
          type T (a -> Either a b) = Int

    Here there are no lexically-scoped variables from (C Left).
    Yet the real class-instance header is   C @(p -> Either @p @q)) (Left @p @q)
    while the type-family instance is       T (a -> Either @a @b)
    So we allow alpha-renaming of variables that don't come
    from the class-instance header.

    We track the lexically-scoped type variables from the
    class-instance header in ai_tyvars.

    Here's another example (#14045a)
        class C (a :: k) where
          data S (a :: k)
        instance C (z :: Bool) where
          data S :: Bool -> Type where

    Again, there is no lexical connection, but we will get
       class-instance header:   C @Bool (z::Bool)
       family instance          S @Bool (a::Bool)

    When looking for mis-matches, we check left-to-right,
    kinds first.  If we look at types first, we'll fail to
    suggest -fprint-explicit-kinds for a mis-match with
          T @k    vs    T @Type
    somewhere deep inside the type

    Note [Checking consistent instantiation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    See #11450 for background discussion on this check.

      class C a b where
        type T a x b

    With this class decl, if we have an instance decl
      instance C ty1 ty2 where ...
    then the type instance must look like
         type T ty1 v ty2 = ...
    with exactly 'ty1' for 'a', 'ty2' for 'b', and some type 'v' for 'x'.
    For example:

      instance C [p] Int
        type T [p] y Int = (p,y,y)

    Note that

    * We used to allow completely different bound variables in the
      associated type instance; e.g.
        instance C [p] Int
          type T [q] y Int = ...
      But from GHC 8.2 onwards, we don't.  It's much simpler this way.
      See #11450.

    * When the class variable isn't used on the RHS of the type instance,
      it's tempting to allow wildcards, thus
        instance C [p] Int
          type T [_] y Int = (y,y)
      But it's awkward to do the test, and it doesn't work if the
      variable is repeated:
        instance C (p,p) Int
          type T (_,_) y Int = (y,y)
      Even though 'p' is not used on the RHS, we still need to use 'p'
      on the LHS to establish the repeated pattern.  So to keep it simple
      we just require equality.

    * For variables in associated type families that are not bound by the class
      itself, we do _not_ check if they are over-specific. In other words,
      it's perfectly acceptable to have an instance like this:

        instance C [p] Int where
          type T [p] (Maybe x) Int = x

      While the first and third arguments to T are required to be exactly [p] and
      Int, respectively, since they are bound by C, the second argument is allowed
      to be more specific than just a type variable. Furthermore, it is permissible
      to define multiple equations for T that differ only in the non-class-bound
      argument:

        instance C [p] Int where
          type T [p] (Maybe x)    Int = x
          type T [p] (Either x y) Int = x -> y

      We once considered requiring that non-class-bound variables in associated
      type family instances be instantiated with distinct type variables. However,
      that requirement proved too restrictive in practice, as there were examples
      of extremely simple associated type family instances that this check would
      reject, and fixing them required tiresome boilerplate in the form of
      auxiliary type families. For instance, you would have to define the above
      example as:

        instance C [p] Int where
          type T [p] x Int = CAux x

        type family CAux x where
          CAux (Maybe x)    = x
          CAux (Either x y) = x -> y

      We decided that this restriction wasn't buying us much, so we opted not
      to pursue that design (see also GHC #13398).

    Implementation
      * Form the mini-envt from the class type variables a,b
        to the instance decl types [p],Int:   [a->[p], b->Int]

      * Look at the tyvars a,x,b of the type family constructor T
        (it shares tyvars with the class C)

      * Apply the mini-evnt to them, and check that the result is
        consistent with the instance types [p] y Int. (where y can be any type, as
        it is not scoped over the class type variables.

    We make all the instance type variables scope over the
    type instances, of course, which picks up non-obvious kinds.  Eg
       class Foo (a :: k) where
          type F a
       instance Foo (b :: k -> k) where
          type F b = Int
    Here the instance is kind-indexed and really looks like
          type F (k->k) (b::k->k) = Int
    But if the 'b' didn't scope, we would make F's instance too
    poly-kinded.

    Note [Printing conflicts with class header]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's remarkably painful to give a decent error message for conflicts
    with the class header.  Consider
       class C b where
         type F a b c
       instance C [b] where
         type F x Int _ _ = ...

    Here we want to report a conflict between
        Expected: F _ [b] _
        Actual:   F x Int _ _

    But if the type instance shadows the class variable like this
    (rename/should_fail/T15828):
       instance C [b] where
         type forall b. F x (Tree b) _ _ = ...

    then we must use a fresh variable name
        Expected: F _ [b] _
        Actual:   F x [b1] _ _

    Notice that:
      - We want to print an underscore in the "Expected" type in
        positions where the class header has no influence over the
        parameter.  Hence the fancy footwork in pp_expected_ty

      - Although the binders in the axiom are already tidy, we must
        re-tidy them to get a fresh variable name when we shadow

      - The (ax_tvs \\ inst_tvs) is to avoid tidying one of the
        class-instance variables a second time, from 'a' to 'a1' say.
        Remember, the ax_tvs of the axiom share identity with the
        class-instance variables, inst_tvs..

      - We use tidyCoAxBndrsForUser to get underscores rather than
        _1, _2, etc in the axiom tyvars; see the definition of
        tidyCoAxBndrsForUser

    This all seems absurdly complicated.

    Note [Unused explicitly bound variables in a family pattern]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Why is 'unusedExplicitForAllErr' not just a warning?

    Consider the following examples:

      type instance F a = Maybe b
      type instance forall b. F a = Bool
      type instance forall b. F a = Maybe b

    In every case, b is a type variable not determined by the LHS pattern. The
    first is caught by the renamer, but we catch the last two here. Perhaps one
    could argue that the second should be accepted, albeit with a warning, but
    consider the fact that in a type family instance, there is no way to interact
    with such a variable. At least with @x :: forall a. Int@ we can use visible
    type application, like @x \@Bool 1@. (Of course it does nothing, but it is
    permissible.) In the type family case, the only sensible explanation is that
    the user has made a mistake -- thus we throw an error.

    Note [Oversaturated type family equations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Type family tycons have very rigid arities. We want to reject something like
    this:

      type family Foo :: Type -> Type where
        Foo x = ...

    Because Foo has arity zero (i.e., it doesn't bind anything to the left of the
    double colon), we want to disallow any equation for Foo that has more than zero
    arguments, such as `Foo x = ...`. The algorithm here is pretty simple: if an
    equation has more arguments than the arity of the type family, reject.

    Things get trickier when visible kind application enters the picture. Consider
    the following example:

      type family Bar (x :: j) :: forall k. Either j k where
        Bar 5 @Symbol = ...

    The arity of Bar is two, since it binds two variables, `j` and `x`. But even
    though Bar's equation has two arguments, it's still invalid. Imagine the same
    equation in Core:

        Bar Nat 5 Symbol = ...

    Here, it becomes apparent that Bar is actually taking /three/ arguments! So
    we can't just rely on a simple counting argument to reject
    `Bar 5 @Symbol = ...`, since it only has two user-written arguments.
    Moreover, there's one explicit argument (5) and one visible kind argument
    (@Symbol), which matches up perfectly with the fact that Bar has one required
    binder (x) and one specified binder (j), so that's not a valid way to detect
    oversaturation either.

    To solve this problem in a robust way, we do the following:

    1. When kind-checking, we count the number of user-written *required*
       arguments and check if there is an equal number of required tycon binders.
       If not, reject. (See `wrongNumberOfParmsErr` in GHC.Tc.TyCl.)

       We perform this step during kind-checking, not during validity checking,
       since we can give better error messages if we catch it early.
    2. When validity checking, take all of the (Core) type patterns from on
       equation, drop the first n of them (where n is the arity of the type family
       tycon), and check if there are any types leftover. If so, reject.

       Why does this work? We know that after dropping the first n type patterns,
       none of the leftover types can be required arguments, since step (1) would
       have already caught that. Moreover, the only places where visible kind
       applications should be allowed are in the first n types, since those are the
       only arguments that can correspond to binding forms. Therefore, the
       remaining arguments must correspond to oversaturated uses of visible kind
       applications, which are precisely what we want to reject.

    Note that we only perform this check for type families, and not for data
    families. This is because it is perfectly acceptable to oversaturate data
    family instance equations: see Note [Arity of data families] in GHC.Core.FamInstEnv.

    ************************************************************************
    *                                                                      *
       Telescope checking
    *                                                                      *
    ************************************************************************

    Note [Bad TyCon telescopes]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Now that we can mix type and kind variables, there are an awful lot of
    ways to shoot yourself in the foot. Here are some.

      data SameKind :: k -> k -> *   -- just to force unification

    1.  data T1 a k (b :: k) (x :: SameKind a b)

    The problem here is that we discover that a and b should have the same
    kind. But this kind mentions k, which is bound *after* a.
    (Testcase: dependent/should_fail/BadTelescope)

    2.  data T2 a (c :: Proxy b) (d :: Proxy a) (x :: SameKind b d)

    Note that b is not bound. Yet its kind mentions a. Because we have
    a nice rule that all implicitly bound variables come before others,
    this is bogus.

    To catch these errors, we call checkTyConTelescope during kind-checking
    datatype declarations.  This checks for

    * Ill-scoped binders. From (1) and (2) above we can get putative
      kinds like
           T1 :: forall (a:k) (k:*) (b:k). SameKind a b -> *
      where 'k' is mentioned a's kind before k is bound

      This is easy to check for: just look for
      out-of-scope variables in the kind

    * We should arguably also check for ambiguous binders
      but we don't.  See Note [Ambiguous kind vars].

    See also
      * Note [Required, Specified, and Inferred for types] in GHC.Tc.TyCl.
      * Note [Checking telescopes] in GHC.Tc.Types.Constraint discusses how
        this check works for `forall x y z.` written in a type.

    Note [Ambiguous kind vars]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    We used to be concerned about ambiguous binders. Suppose we have the kind
         S1 :: forall k -> * -> *
         S2 :: forall k. * -> *
    Here S1 is OK, because k is Required, and at a use of S1 we will
    see (S1 *) or (S1 (*->*)) or whatever.

    But S2 is /not/ OK because 'k' is Specfied (and hence invisible) and
    we have no way (ever) to figure out how 'k' should be instantiated.
    For example if we see (S2 Int), that tells us nothing about k's
    instantiation.  (In this case we'll instantiate it to Any, but that
    seems wrong.)  This is really the same test as we make for ambiguous
    type in term type signatures.

    Now, it's impossible for a Specified variable not to occur
    at all in the kind -- after all, it is Specified so it must have
    occurred.  (It /used/ to be possible; see tests T13983 and T7873.  But
    with the advent of the forall-or-nothing rule for kind variables,
    those strange cases went away. See Note [forall-or-nothing rule] in
    GHC.Hs.Type.)

    But one might worry about
        type v k = *
        S3 :: forall k. V k -> *
    which appears to mention 'k' but doesn't really.  Or
        S4 :: forall k. F k -> *
    where F is a type function.  But we simply don't check for
    those cases of ambiguity, yet anyway.  The worst that can happen
    is ambiguity at the call sites.

    Historical note: this test used to be called reportFloatingKvs.
    -}

    -- | Check a list of binders to see if they make a valid telescope.
    -- See Note [Bad TyCon telescopes]
    type TelescopeAcc
          = ( TyVarSet   -- Bound earlier in the telescope
            , Bool       -- At least one binder occurred (in a kind) before
                         -- it was bound in the telescope.  E.g.
            )            --    T :: forall (a::k) k. blah

    checkTyConTelescope :: TyCon -> TcM ()
    checkTyConTelescope :: TyCon -> TcM ()
    checkTyConTelescope TyCon
    tc
      | Bool
    bad_scope
      = -- See "Ill-scoped binders" in Note [Bad TyCon telescopes]
        TcRnMessage -> TcM ()
    addErr (TcRnMessage -> TcM ()) -> TcRnMessage -> TcM ()
    forall a b. (a -> b) -> a -> b
    $ TyCon -> TcRnMessage
    TcRnBadTyConTelescope TyCon
    tc

      | Bool
    otherwise
      = () -> TcM ()
    forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a
    forall (m :: * -> *) a. Monad m => a -> m a
    return ()
      where
        tcbs :: [VarBndr Var TyConBndrVis]
    tcbs = TyCon -> [VarBndr Var TyConBndrVis]
    tyConBinders TyCon
    tc

        (VarSet
    _, Bool
    bad_scope) = ((VarSet, Bool) -> VarBndr Var TyConBndrVis -> (VarSet, Bool))
    -> (VarSet, Bool) -> [VarBndr Var TyConBndrVis] -> (VarSet, Bool)
    forall b a. (b -> a -> b) -> b -> [a] -> b
    forall (t :: * -> *) b a.
    Foldable t =>
    (b -> a -> b) -> b -> t a -> b
    foldl (VarSet, Bool) -> VarBndr Var TyConBndrVis -> (VarSet, Bool)
    add_one (VarSet
    emptyVarSet, Bool
    False) [VarBndr Var TyConBndrVis]
    tcbs

        add_one :: TelescopeAcc -> TyConBinder -> TelescopeAcc
        add_one :: (VarSet, Bool) -> VarBndr Var TyConBndrVis -> (VarSet, Bool)
    add_one (VarSet
    bound, Bool
    bad_scope) VarBndr Var TyConBndrVis
    tcb
          = ( VarSet
    bound VarSet -> Var -> VarSet
    `extendVarSet` Var
    tv
            , Bool
    bad_scope Bool -> Bool -> Bool
    || Bool -> Bool
    not (VarSet -> Bool
    isEmptyVarSet (VarSet
    fkvs VarSet -> VarSet -> VarSet
    `minusVarSet` VarSet
    bound)) )
          where
            tv :: Var
    tv = VarBndr Var TyConBndrVis -> Var
    forall tv argf. VarBndr tv argf -> tv
    binderVar VarBndr Var TyConBndrVis
    tcb
            fkvs :: VarSet
    fkvs = Type -> VarSet
    tyCoVarsOfType (Var -> Type
    tyVarKind Var
    tv)