Bad Constructor used with deriving clause [GHC-16437]

The deriving mechanism expects constructors of a particular form. When the constructors do not comport to this form, this error is thrown:

• Can't make a derived instance of ‘Functor (T a)’:
    Constructor ‘Mk’ is a GADT
• In the data declaration for ‘T’
Suggested fix: Use a standalone deriving declaration instead

Examples

Attempt at using deriving clause with GADT.

Deriving_gadt.hs

Before

module Deriving_gadt where

data T a b where
     Mk :: Int -> b -> T Int b
     deriving (Functor)

After

{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}

module Deriving_gadt where

data T a b where
     Mk :: Int -> b -> T Int b

deriving instance Functor (T a)

Use of higher-rank types with a deriving clause.

Higher_rank.hs

Before

{-# LANGUAGE RankNTypes #-}

module Higher_rank where

data Bad = MkBad (forall a. a) deriving Eq

After

module Higher_rank where

-- unfortunately we just need a rank 1 type
data Bad a = MkBad a deriving Eq