Implement GHC-16437 (#504)

Author: doyougnu

message-index/messages/GHC-16437/deriving_gadt/after/Deriving_gadt.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE StandaloneDeriving #-}
+
+module Deriving_gadt where
+
+data T a b where
+     Mk :: Int -> b -> T Int b
+
+deriving instance Functor (T a)

message-index/messages/GHC-16437/deriving_gadt/before/Deriving_gadt.hs

@@ -0,0 +1,5 @@
+module Deriving_gadt where
+
+data T a b where
+     Mk :: Int -> b -> T Int b
+     deriving (Functor)

message-index/messages/GHC-16437/deriving_gadt/index.md

@@ -0,0 +1,3 @@
+---
+title: Attempt at using deriving clause with GADT.
+---

message-index/messages/GHC-16437/higher_rank/after/Higher_rank.hs

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+module Higher_rank where
+
+-- unfortunately we just need a rank 1 type
+data Bad a = MkBad a deriving Eq

message-index/messages/GHC-16437/higher_rank/before/Higher_rank.hs

@@ -0,0 +1,5 @@
+{-# LANGUAGE RankNTypes #-}
+
+module Higher_rank where
+
+data Bad = MkBad (forall a. a) deriving Eq

message-index/messages/GHC-16437/higher_rank/index.md

@@ -0,0 +1,3 @@
+---
+title: Use of higher-rank types with a deriving clause.
+---

message-index/messages/GHC-16437/index.md

@@ -0,0 +1,13 @@
+---
+title: Bad Constructor used with deriving clause
+summary: Standard deriving fails on GADTs and higher-rank types 
+severity: error
+introduced: 9.6.1
+---
+
+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