HEAD (7.5.20120421) requires RankNTypes for a rank-2 type

While building syb with HEAD, Ian reported that he needed to use RankNTypes instead of Rank2Types. I minimized the problem to:

{-# LANGUAGE Rank2Types #-}

module Rank2 where

f :: (forall a. a -> (forall b. b)) -> c
f = undefined

g :: (forall a b. a -> b) -> c
g = undefined

1. 4 accepts both f and g. HEAD complains on the signature of f:

    Illegal polymorphic or qualified type: forall b. b
    Perhaps you intended to use -XRankNTypes or -XRank2Types
    In the type signature for `f':
      f :: (forall a. a -> (forall b. b)) -> c

It doesn't complain about g, but I think the signatures are equivalent. Ian further reports that the change in behavior seems to have occurred after 7.5.20120401.

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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