Core Lint error with RequiredTypeArguments

File:

{-# LANGUAGE RequiredTypeArguments, ExplicitNamespaces #-}
module M where

visId :: forall a -> a -> a
visId (type a) x = x

f :: forall a -> a -> a
f (type x) = visId (type x)

$ ./stage1/bin/ghc -dcore-lint M
[1 of 1] Compiling M                ( M.hs, M.o )
*** Core Lint errors : in result of Desugar (after optimization) ***
M.hs:8:1: warning:
    From-type of Cast differs from type of enclosed expression
    From-type: forall a. a -> a
    Type of enclosed expression: forall a -> a -> a
    Actual enclosed expression: visId
    Coercion used in cast: forall (a[spec]~[req] :: <*>_N). <a -> a>_R
    In the RHS of f :: forall a -> a -> a
    Substitution: <InScope = {}
                   IdSubst   = []
                   TvSubst   = []
                   CvSubst   = []>

Using an invisible forall:

{-# LANGUAGE RequiredTypeArguments, ExplicitNamespaces #-}
module M where

visId :: forall a -> a -> a
visId (type a) x = x

g :: forall a. a -> a
g = visId (type a)

$ ./stage1/bin/ghc -dcore-lint M
[1 of 1] Compiling M                ( M.hs, M.o )
*** Core Lint errors : in result of Desugar (after optimization) ***
M.hs:8:1: warning:
    The type of this binder doesn't match the type of its RHS: g
    Binder's type: forall a. a -> a
    Rhs type: forall a -> a -> a
    In the RHS of g :: forall a. a -> a
    Substitution: <InScope = {}
                   IdSubst   = []
                   TvSubst   = []
                   CvSubst   = []>