A constraint is both redundant and not deducible.

Summary

GHC says that a constraint is redundant but then says the constraint is not deducible if I remove it.

Steps to reproduce

From the redundant-but-not-deducible package:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -Werror=redundant-constraints #-}

module EventThing where

class Monad m => Event m where
  thingsForEvent :: m [Int]

things :: Event m => m [Int]
things = thingsForEvent

-- class Thingy m where
class Monad m => Thingy m where
  thingies :: m [Int]

-- instance Event m => Thingy m where
instance (Monad m, Event m) => Thingy m where
  thingies = things

With ghc-9.2.3 .. ghc-9.4.4 (but not in ghc-9.6.1) this module fails to compile with the following error:

   src/EventThing.hs error: [-Wredundant-constraints, -Werror=redundant-constraints]
     • Redundant constraint: Monad m
     • In the instance declaration for ‘Thingy m’
     |
  __ | instance (Monad m, Event m) => Thingy m where
     |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Removing the redundant constraint, the module then also fails:

    src/EventThing.hs: error:
      • Could not deduce (Monad m)
          arising from the superclasses of an instance declaration
        from the context: Event m
          bound by the instance declaration at src/EventThing.hs
        Possible fix:
          add (Monad m) to the context of the instance declaration
      • In the instance declaration for ‘Thingy m’
     |
  __ | instance Event m => Thingy m where
     |          ^^^^^^^^^^^^^^^^^^^

Using the commented out class and instance lines, the module compiles without error.

Expected behavior

If a constraint is redundant then removing it should be no problem.

Environment

The module EventThing compiles with ghc-8.10.7 .. ghc-9.2.2 fine but from ghc-9.2.3 .. ghc-9.4.4 it doesn't. With ghc-9.6.1 it is good again.

• Operating System: ubuntu-22.04
• System Architecture: x86_64