forall on non-* types

See main ticket #22063

This code

{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
x = Nothing :: (forall a. Maybe) Int

gives

    Couldn't match type `Maybe' with `forall a. Maybe'
    Expected type: forall a. Maybe Int
      Actual type: Maybe Int

which does not seem correct. There are two options:

Option A: We allow this.

Option B: We reject this as ill-kinded and allow forall a. T only when T has kind *. I prefer this option; in theory this can break existing code, but I am not aware of any code that uses forall on non-* type (please correct me if I'm wrong), and e.g. this means forall a. a inhabits every kind, for example Bool.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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