Impredicative type instantiation without -XImpredicativeTypes

Consider this module:

{-# LANGUAGE RankNTypes #-}

module Bug where

rank2 :: ((forall a. a -> a) -> b) -> ()
rank2 _ = ()

foo :: () -> ()
foo x = x

quux :: (forall a. a -> a) -> Int
quux _ = 5

bar = foo . rank2 $ quux

The (.) in the definition of bar requires an impredicative instantiation -- that is, one of its type variables is instantiated to a forall-type. Yet, the module compiles without -XImpredicativeTypes. To confirm this behavior, the following is an excerpt from -ddump-simpl:

Bug.bar =
  break<7>()
  (break<6>()
   GHC.Base..
     @ ()
     @ ()
     @ ((forall a_a1Tj. a_a1Tj -> a_a1Tj) -> GHC.Types.Int)
     Bug.foo
     (Bug.rank2 @ GHC.Types.Int))
    Bug.quux

Note the third type argument to (.).

Fixing this would be a breaking change that could affect users.

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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