ConstrainedClassMethods can trigger un-actionable redundant constraint warnings together with GND
Summary
If you use GeneralizedNewtypeDeriving (or indeed, DerivingVia) to derive an instance of a type class whose methods have constraints that aren't in the type class head, it is possible to trigger redundant constraint warnings if the generated code would cause the constraints to be satisfied by whatever the type variables are set to. See the example given in 'steps to reproduce' for a demonstration. These warnings can't be actioned - the only option currently is to disable redundant constraint warnings completely.
Steps to reproduce
Attempt to compile the following code with -Wredundant-constraints enabled:
{-# LANGUAGE ConstrainedClassMethods #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE StandaloneDeriving #-}
module Baz where
class Foo a where
bar :: (Num a) => a -> a
bar = (+ 1)
instance Foo Int
newtype Quux a = Quux a
deriving newtype instance Foo (Quux Int)This will emit a redundant constraint warning, as Int is already an instance of Num.
Expected behavior
Should compile, emitting no warnings.
Environment
- GHC version used: 8.10.1
Optional:
- Operating System: GNU/Linux
- System Architecture: x86_64