GHC 9.6 incorrect redundant constraint warning with generalized newtype deriving
## Summary
GHC 9.6 introduces a new `-Wredundant-constraints` warning in derived code.
This is not present in 9.4. It is also not present in a manually written instance.
I believe that the constraint is actually necessary.
## Steps to reproduce
Compile the following code with GHC 9.6, mtl-2.3.1 and transformers-0.6.1.0:
```haskell
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Bug where
import Control.Monad.Reader
import Control.Applicative
import Data.Coerce
newtype NullDbT m a = NullDbT {unNullDbT :: ReaderT Int m a}
deriving newtype
( Functor
, Applicative
, Alternative
, Monad
, MonadTrans -- GHC complains the monad constraint is redundant
)
class (forall m. Monad m => Monad (t m)) => C t where
myLift :: Monad m => m a -> t m a
instance C NullDbT where
-- Monad constraint is required (GHC errors if it is not present, with either implementation)
myLift :: forall m a . Monad m => m a -> NullDbT m a
--myLift x = NullDbT (lift @(ReaderT Int) x)
myLift = coerce @(m a -> ReaderT Int m a) @(m a -> NullDbT m a) (lift @(ReaderT Int))
```
## Expected behavior
I expect consistent warnings/errors between the derived code and a manually written copy.
In particular here, I believe that the constraint is necessary, since it is needed for the `lift @(ReaderT Int)` call.
## Environment
* GHC version used: 9.6
Optional:
* Operating System: NixOS
* System Architecture: x86_64
issue