Unnecessary -XRank2Types requirement involving type alias containing "forall" from another module

The following builds with GHC 7.4, but not 7.6.

Bottom.hs

{-# LANGUAGE Rank2Types #-}
module Bottom where

type Bottom = forall a. a

data Pipe l i o u m r = Pipe
type GSource m o = forall l i u. Pipe l i o u m ()

Main.hs

import Bottom

myBottom :: Int -> Bottom
myBottom _ = error "Bottom"

main :: IO ()
main = myBottom 3

Note that Main.hs does not have the Rank2Types extension turned on.

myBottom does not have a rank-2 type. After expanding the type alias, you get:

myBottom :: Int -> forall a. a

This is logically equivalent to:

myBottom :: forall a. Int -> a

GHC 7.4.2 accepts this use of type alias, but GHC 7.6 does not.

Note that if we make something that does in fact expand to a rank-2 type:

foo :: Bottom -> Int
foo x = x

GHC 7.4.2 correctly rejects the program unless you enable Rank2Types.

Also, if we use a standalone Bottom type alias:

myBottom :: Bottom

Then GHC 7.6 accepts the program. It does not accept Int -> Bottom without Rank2Types, but it does accept Bottom by itself.

This issue is easy to work around: just add {-# LANGUAGE Rank2Types #-} or {-# LANGUAGE RankNTypes #-} to the top of the module.

Is this a bug, or have the rules for type aliases and forall been deliberately tightened?

P.S.: This ticket is for GHC 7.6.1-rc1. That version tag is not available, though.

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