Hi GHC-Devs, I recently came across a compilation problem on 8.0 that do not trigger on < 8.0. I've reduced the issue to this snippet with Rank2Types: type Action = (forall a . Show a => a) -> Int data Command = Command (Maybe Action) com1 = Command Nothing With GHC 7.0 to 7.10, this compile and works. However on GHC 8.0, I now have: Test.hs:19:16: error: • Cannot instantiate unification variable ‘a0’ with a type involving foralls: Action GHC doesn't yet support impredicative polymorphism • In the first argument of ‘Command’, namely ‘Nothing’ In the expression: Command Nothing In an equation for ‘com1’: com1 = Command Nothing I workarounded it by "inlining" the maybe and creating a proper wrapping type: data Action2 = Action2 ((forall a . Show a => a) -> Int) | NoAction2 data Command2 = Command2 Action2 com2 = Command2 NoAction2 Any idea why this is triggering this issue in GHC 8.0 ? Is this something that need fixing maybe ? Cheers, -- Vincent