Typechecker regression involving RankNTypes

The following code compiles on GHC 7.0.4 through 7.10.3, but not with GHC 8.0.1, 8.0.2, or HEAD:

{-# LANGUAGE RankNTypes #-}
module Bug where

type NatTrans f g = forall x. f x -> g x

-- NatTrans analog of id
applyNT :: NatTrans f g -> NatTrans f g
applyNT x = x

b :: Bool
b = True

eitherToMaybe :: NatTrans (Either a) Maybe
eitherToMaybe (Left _)  = Nothing
eitherToMaybe (Right x) = Just x

trans :: NatTrans (Either a) Maybe -> NatTrans (Either a) Maybe
trans x = x

applyNTCheck :: NatTrans (Either a) Maybe
applyNTCheck = (if b then trans else applyNT) eitherToMaybe

$ /opt/ghc/8.0.1/bin/ghc Bug.hs 
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:21:27: error:
    • Couldn't match type ‘NatTrans (Either a) Maybe’
                     with ‘Either a0 x0 -> Maybe x0’
      Expected type: (Either a0 x0 -> Maybe x0) -> Either a x -> Maybe x
        Actual type: NatTrans (Either a) Maybe -> Either a x -> Maybe x
    • In the expression: trans
      In the expression: if b then trans else applyNT
      In the expression: (if b then trans else applyNT) eitherToMaybe
    • Relevant bindings include
        applyNTCheck :: Either a x -> Maybe x (bound at Bug.hs:21:1)

(Originally spotted here.)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture