Higher-rank constraint treated as type instead

When I say

{-# LANGUAGE RankNTypes #-}

module Bug where

class P z
foo :: (forall b. P (a, b)) => a -> a
foo x = x

I get

Bug.hs:7:9:
    Couldn't match expected type ‘a -> a’ with actual type ‘P (a, b0)’
    Relevant bindings include
      x :: forall b. P (a, b) (bound at Bug.hs:7:5)
      foo :: (forall b. P (a, b)) -> a -> a (bound at Bug.hs:7:1)
    In the expression: x
    In an equation for ‘foo’: foo x = x

I expected my code to be rejected, but not with that error message. It seems that GHC thinks forall b. P (a, b)) has kind *, not kind Constraint.

Encountered while experimenting with #9195 (closed).

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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