Implement deriving strategies

Allows users to explicitly request which approach to `deriving` to use
via keywords, e.g.,

```
newtype Foo = Foo Bar
  deriving Eq
  deriving stock    Ord
  deriving newtype Show
```

Fixes #10598. Updates haddock submodule.

Test Plan: ./validate

Reviewers: hvr, kosmikus, goldfire, alanz, bgamari, simonpj, austin,
erikd, simonmar

Reviewed By: alanz, bgamari, simonpj

Subscribers: thomie, mpickering, oerjan

Differential Revision: https://phabricator.haskell.org/D2280

GHC Trac Issues: #10598

1 files changed, 6 insertions, 1 deletions

diff --git a/docs/users_guide/safe_haskell.rst b/docs/users_guide/safe_haskell.rst
index 6ce8b8fc26..653b741339 100644
--- a/docs/users_guide/safe_haskell.rst
+++ b/docs/users_guide/safe_haskell.rst
@@ -284,7 +284,12 @@ Furthermore, we restrict the following features:
   the structure of the data type for which the instance is defined, and
   allowing manually implemented ``Generic`` instances would break that
   invariant. Derived instances (through the :ghc-flag:`-XDeriveGeneric`
-  extension) are still allowed. Refer to the
+  extension) are still allowed. Note that the only allowed
+  :ref:`deriving strategy <deriving-strategies>` for deriving ``Generic`` under
+  Safe Haskell is ``stock``, as another strategy (e.g., ``anyclass``) would
+  produce an instance that violates the invariant.
+
+  Refer to the
   :ref:`generic programming <generic-programming>` section for more details.
 
 .. _safe-overlapping-instances: