RankNTypes composition now requires ImpredicativeTypes

{-# LANGUAGE RankNTypes #-}

instantiateId :: (forall a. a -> a) -> Int -> Int
instantiateId f = f

passes :: [Int] -> [Int]
passes = map (instantiateId id)

-- Couldn't match type ‘a0 -> a0’ with ‘forall a. a -> a’
fails :: [Int] -> [Int]
fails = (map . instantiateId) id

works with ghc 7.6, fails with ghc 7.8, works with ghc 7.8 + ImpredicativeTypes.

Trac metadata

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