Regression with TypeErrors in 9.4.1-alpha1

Summary

I encountered this while trying to build servant with GHC-9.4.1-alpha1 and managed to extract the following independent reproducer.

Steps to reproduce

Try to compile the following program:

{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE PolyKinds            #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE Haskell2010 #-}

import Data.Kind
import GHC.TypeLits

type NoInstanceForSub (tycls :: k) (expr :: k') =
  Text "There is no instance for " :<>: ShowType tycls
  :<>: Text " (" :<>: ShowType expr :<>: Text " :> ...)"

class Foo a

instance TypeError (NoInstanceForSub Foo ty) => Foo ty

main :: IO ()
main = pure ()

You will get the following error:

Test.hs:19:10: error:
    • There is no instance for Foo (ty :> ...)
    • In the ambiguity check for an instance declaration
      In the instance declaration for ‘Foo ty’
   |
19 | instance TypeError (NoInstanceForSub Foo ty) => Foo ty
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Expected behavior

This should compile without error.

Environment

• GHC version used: GHC-9.4.1-alpha1

Observation

The following program compiles fine (we change k to * -> Constraint), but this change cannot be applied to servant as there we want to be polymorphic in the kind. Using type applications at use sites does work though:

{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE PolyKinds            #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE Haskell2010 #-}

import Data.Kind
import GHC.TypeLits

type NoInstanceForSub (tycls :: * -> Constraint) (expr :: k') =
  Text "There is no instance for " :<>: ShowType tycls
  :<>: Text " (" :<>: ShowType expr :<>: Text " :> ...)"

class Foo a

instance TypeError (NoInstanceForSub Foo ty) => Foo ty

main :: IO ()
main = pure ()