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Consider this code:

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -Wall #-}
module Bug where

class C a where
  type T a
  data D a
  m :: a

instance C Int
deriving instance C Bool

Neither C instance implements any of its type families or methods. However, the manual C Int instance and the derived C Bool instance give different warnings:

GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:12:1: warning: [-Wmissing-methods]
    • No explicit associated type or default declaration for      ‘T’
    • In the instance declaration for ‘C Int’
   |
12 | instance C Int
   | ^^^^^^^^^^^^^^

Bug.hs:12:1: warning: [-Wmissing-methods]
    • No explicit associated type or default declaration for      ‘D’
    • In the instance declaration for ‘C Int’
   |
12 | instance C Int
   | ^^^^^^^^^^^^^^

Bug.hs:12:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘m’
    • In the instance declaration for ‘C Int’
   |
12 | instance C Int
   |          ^^^^^

Bug.hs:13:1: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘m’
    • In the instance declaration for ‘C Bool’
   |
13 | deriving instance C Bool
   | ^^^^^^^^^^^^^^^^^^^^^^^^

Notice that the C Int instance warns about the lack of an implementation for T and D, as expected. However, the derived C Bool instance does not. It only warns about m!

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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