Regression in resolving quantified constraint from GHC 8.10.7 to GHC 9.0.2/9.2.2
The following compiles on GHC 8.10.7, but fails on GHC 9.0.2 and GHC 9.2.2:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
module Main where
import Data.Function
import Data.Kind
import GHC.Generics
import GHC.TypeLits
-- inlined generic-data imports:
from' :: Generic a => a -> Rep a ()
from' = from
geq :: (Generic a, Eq (Rep a ())) => a -> a -> Bool
geq = (==) `on` from'
gcompare :: (Generic a, Ord (Rep a ())) => a -> a -> Ordering
gcompare = compare `on` from'
-- test case:
data A (v :: Symbol -> Type -> Type) a b deriving (Generic,Generic1)
instance (Eq a , (forall w z . Eq z => Eq (v w z)) , Eq b) => Eq (A v a b) where
{-# INLINE (==) #-}
(==) = geq
instance (Ord a , (forall w z . Eq z => Eq (v w z)) , (forall w z . Ord z => Ord (v w z)) , Ord b) => Ord (A v a b) where
{-# INLINE compare #-}
compare = gcompare
main :: IO ()
main = pure ()