Wredundant-constraints does not work when constraint synonym is used

{-# LANGUAGE ConstraintKinds #-}

{-# OPTIONS_GHC -Wredundant-constraints #-}

module Main where

import Data.List

main :: IO ()
main = return ()

withWarning :: (Show a, Ord a) => [a] -> [a]
withWarning = sort

type ConstraintSynonym a = (Show a, Ord a)

withoutWarning :: ConstraintSynonym a => [a] -> [a]
withoutWarning = sort

Main.hs:12:1: warning: [-Wredundant-constraints]
    • Redundant constraint: Show a
    • In the type signature for:
           withWarning :: forall a. (Show a, Ord a) => [a] -> [a]
   |
14 | withWarning :: (Show a, Ord a) => [a] -> [a]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The withoutWarning function does not need the (Show a) constraint but there is no warning for it.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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