Document -Wderiving-typeable

Tracking: #18641

docs/users_guide/expected-undocumented-flags.txt

@@ -4,7 +4,6 @@
 --show-packages
 -Onot
 -Walternative-layout-rule-transitional
--Wderiving-typeable
 -XAlternativeLayoutRule
 -XAlternativeLayoutRuleTransitional
 -XAutoDeriveTypeable

docs/users_guide/using-warnings.rst

@@ -1881,6 +1881,20 @@ of ``-W(no-)*``.
 
     Does nothing.
 
+.. ghc-flag:: -Wderiving-typeable
+    :shortdesc: warn when Typeable is derived
+    :type: dynamic
+    :reverse: -Wno-deriving-typeable
+    :category:
+
+    :since: 7.10
+
+    This flag warns when ``Typeable`` is listed in a deriving clause
+    or derived with :extension:`StandaloneDeriving`.
+
+    Since GHC 7.10, ``Typeable`` is automatically derived for all types.
+    Thus, deriving ``Typeable`` yourself is redundant.
+
 If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice.
 It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
 sanity, not yours.)