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The following program compiles:

{-# LANGUAGE RankNTypes #-}
module TestRN1 where

type X = forall a . a

comp :: (X -> c) -> (a -> X) -> (a -> c)
comp = (.)

But this one fails:

{-# LANGUAGE RankNTypes #-}
module TestRN2 where

comp :: ((forall a . a) -> c) -> (a -> (forall a . a)) -> (a -> c)
comp = (.)

Error:

    Cannot instantiate unification variable ‘b0’
    with a type involving foralls: forall a1. a1
      Perhaps you want ImpredicativeTypes
    In the expression: (.)
    In an equation for ‘comp’: comp = (.)

I would expect both to fail. Both seem to rely on impredicative types. I wouldn't expect expansion of a type synonym to make a difference between a type-checking program and one that does not typecheck.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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