Typecheck mutually-recursive type decls like mutually-recursive term decls

We could get a bit more kind polymoprphism if we typechecked mutually-recursive type decls in the same way that we do mutually-recursive term decls. The latter is describbed in the user manual 16.1.1.5. Typechecking of recursive binding groups.

Original bug report

GHC incorrectly rejects this program:

{-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds, KindSignatures, GADTs #-}
module TestConstraintKinds where
    import GHC.Exts hiding (Any)

    data WrappedType = forall a. WrapType a

    data A :: WrappedType -> * where
        MkA :: forall (a :: *). AW a -> A (WrapType a)

    type AW  (a :: k) = A (WrapType a)
    type AW' (a :: k) = A (WrapType a)

    class C (a :: k) where
        aw :: AW a -- workaround: AW'

    instance C [] where
        aw = aw

GHC accepts the program when AW is replaced with AW' on that line.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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