GHC fails to infer implied superclass constraint

The following code should compile, but does not.

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

module Bug where

class (Monad m) => RequiresMonad m where

class (Monad m) => ImpliesMonad m where

instance (ImpliesMonad m) => RequiresMonad m where

The basic idea is that I put the constraint ImpliesMonad m on the RequiresMonad instance, which does in fact imply Monad m. However, GHC complains:

    • Could not deduce (Monad m)
        arising from the superclasses of an instance declaration
      from the context: ImpliesMonad m
        bound by the instance declaration
        at Bug.hs:9:10-44
      Possible fix:
        add (Monad m) to the context of the instance declaration
    • In the instance declaration for ‘RequiresMonad m’

Of course, GHC is perfectly happy if I replace the instance constraint ImpliesMonad m with Monad m.

Possibly related to #10338 or #11948 (closed).

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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