Rank-n typechecking regression between GHC 8.4 and 8.6
The following program typechecks on GHC 7.0.4 through 8.4.4:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Bug where
f :: (forall a. a -> forall b. b -> c) -> ()
f (_ :: forall a. a -> forall b. b -> c) = ()
However, it does not typecheck on GHC 8.6.3:
$ /opt/ghc/8.6.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:7:4: error:
• Couldn't match type ‘b0 -> c1’ with ‘forall b. b -> c’
Expected type: a -> forall b. b -> c
Actual type: a -> b0 -> c1
• When checking that the pattern signature:
forall a. a -> forall b. b -> c
fits the type of its context: forall a. a -> forall b. b -> c1
In the pattern: _ :: forall a. a -> forall b. b -> c
In an equation for ‘f’:
f (_ :: forall a. a -> forall b. b -> c) = ()
• Relevant bindings include
f :: (forall a. a -> forall b. b -> c1) -> () (bound at Bug.hs:7:1)
|
7 | f (_ :: forall a. a -> forall b. b -> c) = ()
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^