Add -Wderiving-defaults (#15839)

Enabling both DeriveAnyClass and GeneralizedNewtypeDeriving can cause
a warning when no explicit deriving strategy is in use. This change adds
an enable/suppress flag for it.

2 files changed, 28 insertions, 0 deletions

diff --git a/docs/users_guide/8.10.1-notes.rst b/docs/users_guide/8.10.1-notes.rst
index 2ca9ce5c08..e8316d7059 100644
--- a/docs/users_guide/8.10.1-notes.rst
+++ b/docs/users_guide/8.10.1-notes.rst
@@ -90,6 +90,16 @@ Language
   for a ``newtype``. This was proposed in
   `GHC proposal #13 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0013-unlifted-newtypes.rst>`__.
 
+- New flag :ghc-flag:`-Wderiving-defaults` that controls a warning
+  message when both :extension:`DeriveAnyClass` and
+  :extension:`GeneralizedNewtypeDeriving` are enabled and no explicit
+  deriving strategy is in use. The warning is enabled by default and
+  has been present in earlier GHC versions but without the option of
+  disabling it.  For example, this code would trigger the warning: ::
+
+    class C a
+    newtype T a = MkT a deriving C
+
 Compiler
 ~~~~~~~~
 
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index dda7bb656c..31060d701d 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -26,6 +26,7 @@ generally likely to indicate bugs in your program. These are:
     * :ghc-flag:`-Wdeprecated-flags`
     * :ghc-flag:`-Wunrecognised-pragmas`
     * :ghc-flag:`-Wduplicate-exports`
+    * :ghc-flag:`-Wderiving-defaults`
     * :ghc-flag:`-Woverflowed-literals`
     * :ghc-flag:`-Wempty-enumerations`
     * :ghc-flag:`-Wmissing-fields`
@@ -640,6 +641,23 @@ of ``-W(no-)*``.
     Causes a warning to be emitted if an enumeration is empty, e.g.
     ``[5 .. 3]``.
 
+.. ghc-flag:: -Wderiving-defaults
+    :shortdesc: warn about default deriving when using both
+        DeriveAnyClass and GeneralizedNewtypeDeriving
+    :type: dynamic
+    :reverse: -Wno-deriving-defaults
+    :category:
+
+    :since: 8.10
+
+    Causes a warning when both :ref:`DeriveAnyClass` and
+    :ref:`GeneralizedNewtypeDeriving` are enabled and no explicit
+    deriving strategy is in use.  For example, this would result a
+    warning: ::
+
+        class C a
+        newtype T a = MkT a deriving C
+
 .. ghc-flag:: -Wduplicate-constraints
     :shortdesc: warn when a constraint appears duplicated in a type signature
     :type: dynamic