GHC 9.0+ fails to combine levity-polymorphic newtype constructor with TypeApplications
GHC 8.10.4 has no problem typechecking this program:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE UnliftedNewtypes #-}
module Bug where
import GHC.Exts
type Id :: TYPE r -> TYPE r
newtype Id a where
MkId :: forall r (a :: TYPE r). a -> Id a
idBool :: Id Bool
idBool = MkId @LiftedRep @Bool True
However, GHC 9.0.1 and later fail with:
$ /opt/ghc/9.0.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:17:10: error:
• Cannot apply expression of type ‘a0 -> Id a0’
to a visible type argument ‘LiftedRep’
• In the expression: MkId @LiftedRep @Bool True
In an equation for ‘idBool’: idBool = MkId @LiftedRep @Bool True
|
17 | idBool = MkId @LiftedRep @Bool True
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
As far as I can tell, this program should be expected to typecheck, so I believe this is a regression.