Type variable capture in InstanceSigs message

If I load this file in GHCi:

{-# LANGUAGE RankNTypes, InstanceSigs #-}

class C a where
  f :: a -> b

instance C (a -> b) where
  f :: x
  f = undefined

I get the message

    Method signature does not match class; it should be
      f :: forall b. (a -> b) -> b
    In the instance declaration for `C (a -> b)'

However, this is not the correct type (and if I copy-paste it, GHCi would still complain). The b from the instance head got captured. The correct type would be forall b1. (a -> b) -> b1.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	roma@ro-che.info
Operating system
Architecture