Constraint is required and redundant simultaneously

Summary

I ran into a case where an instance constraint is both required and redundant at the same time.

Steps to reproduce

Run nix build on this commit: https://github.com/jumper149/deriving-trans/commit/0c72ef445ce6de75bead858fa51ca4b45945bfde.

This will produce an error:

src/Control/Monad/Turn.hs:20:10: error:
    • Could not deduce (Monad m)
        arising from the superclasses of an instance declaration
      from the context: (BaseMonadTurn b, MonadBaseControlIdentity b m)
        bound by the instance declaration
        at src/Control/Monad/Turn.hs:20:10-71
      Possible fix:
        add (Monad m) to the context of the instance declaration
    • In the instance declaration for ‘MonadTurn m’
   |
20 | instance (BaseMonadTurn b, MonadBaseControlIdentity b m) => MonadTurn m where
   |

Without this single commit GHC complains about the Monad m constraint being redundant via -Wredundant-constraints.

src/Control/Monad/Turn.hs:21:10: warning: [-Wredundant-constraints]
    • Redundant constraint: Monad m
    • In the instance declaration for ‘MonadTurn m’
   |
21 | instance (BaseMonadTurn b, MonadBaseControlIdentity b m, Monad m) => MonadTurn m where
   |

Expected behavior

I checked the classes manually and Monad m should be implied already here.

The order of implications:

MonadBaseControlIdentity b m => MonadBaseControl b m => MonadBase b m => Monad m

So the redundancy checker is correct here. GHC should be able to compile it without the Monad m constraint.

Environment

• GHC version used: 9.2.4
• There is a full environment via a nix flake: https://github.com/jumper149/deriving-trans/tree/0c72ef445ce6de75bead858fa51ca4b45945bfde

Optional:

• Operating System: NixOS
• System Architecture: x86_64