Deriving `Typeable` has no effect [GHC-90584]

Flag: -Wderiving-typeable

This warning occurs when trying to derive an instance of the Typeable class. Deriving Typeable is not necessary as all types automatically derive Typeable.

Examples

Main.hs:9:12: warning: [-Wderiving-typeable] [GHC-90584]
    • Deriving ‘Typeable’ has no effect: all types now auto-derive Typeable
    • In the data declaration for ‘Foo’
  |
9 |   deriving Typeable
  |            ^^^^^^^^

Main.hs

{-# OPTIONS_GHC -Wderiving-typeable #-}
module Example where

import Data.Typeable

data Foo = Foo Int | Bar Char
  deriving Typeable

{-# OPTIONS_GHC -Wderiving-typeable #-}
module Example where

import Data.Typeable

data Foo = Foo Int | Bar Char