GHC 9.2.1 typechecking regression with RankNTypes and TemplateHaskell
While porting some code over to use GHC 9.2.1, I noticed a typechecking regression that doesn't appear to be explained in the 9.2.1 release notes. Here is a minimal example:
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where
data Foo = MkFoo (forall a. a -> a)
worksOnAllGHCs1 :: Foo
worksOnAllGHCs1 = MkFoo (\x -> x)
worksOnAllGHCs2 :: Foo
worksOnAllGHCs2 = MkFoo $ \x -> x
worksOnAllGHCs3 :: Foo
worksOnAllGHCs3 = $([| MkFoo |]) (\x -> x)
doesn'tWorkOnGHC9'2 :: Foo
doesn'tWorkOnGHC9'2 = $([| MkFoo |]) $ \x -> x
All four of these top-level functions will typecheck on GHC 9.0.2 and earlier. On GHC 9.2.1 and HEAD, worksOnAllGHCs{1,2,3}
functions will typecheck, but the doesn'tWorkOnGHC9'2
function will not typecheck:
$ ghc-9.2.1 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o, Bug.dyn_o )
Bug.hs:17:24: error:
• Couldn't match type: forall a. a -> a
with: p0 -> p0
Expected: (p0 -> p0) -> Foo
Actual: (forall a. a -> a) -> Foo
• In the first argument of ‘($)’, namely ‘(MkFoo)’
In the expression: (MkFoo) $ \ x -> x
In an equation for ‘doesn'tWorkOnGHC9'2’:
doesn'tWorkOnGHC9'2 = (MkFoo) $ \ x -> x
|
17 | doesn'tWorkOnGHC9'2 = $([| MkFoo |]) $ \x -> x
| ^^^^^^^^^^^^^
I'm unclear why using a Template Haskell splice would affect typechecking like this, so I believe this is a bug.