`-Wredundant-constraints` depends on the ordering of constraints with overlapping superclasses

Summary

The -Wredundant-constraints flag does not result in a warning when it should, due to the ordering of constraints.

Steps to reproduce

In the following code

{-# OPTIONS_GHC -Wredundant-constraints #-}
{-# OPTIONS_GHC -Werror #-}

module Test where

class A a where
  a :: a -> a

class A a => B a where
  b :: a

class A a => C a where

noWarning :: (C a, B a) => a
noWarning = a b

warning :: (B a, C a) => a
warning = a b

I get only the error

Test.hs:17:1: error: [-Wredundant-constraints, -Werror=redundant-constraints]
    • Redundant constraint: C a
    • In the type signature for:
           warning :: forall a. (B a, C a) => a
   |
17 | warning :: (B a, C a) => a
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

Expected behavior

There should be two errors, one for warning and one for noWarning. This would be in line with the behaviour of GHC with the constraints (Eq a, Ord a) and (Ord a, Eq a), both of which result in warnings in similar situations.

Environment

• GHC version used: 8.6.5