Re-export data family when exporting a data instance without an export list

Whenever a data instance is exported, the corresponding data family
is exported, too. This allows one to write

```
     -- Foo.hs
     module Foo where

     data family T a

     -- Bar.hs
     module Bar where

     import Foo

     data instance T Int = MkT

     -- Baz.hs
     module Baz where

     import Bar (T(MkT))
```

In previous versions of GHC, this required a workaround
explicit export list in `Bar`.

Reviewers: bgamari, goldfire, austin

Reviewed By: bgamari, goldfire

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D1573

GHC Trac Issues: #11164

2 files changed, 27 insertions, 4 deletions

diff --git a/docs/users_guide/7.12.1-notes.rst b/docs/users_guide/7.12.1-notes.rst
index 2e0ae6f18c..21ec1d39b3 100644
--- a/docs/users_guide/7.12.1-notes.rst
+++ b/docs/users_guide/7.12.1-notes.rst
@@ -109,6 +109,29 @@ Language
       -- P is imported
       import Foo (T(..))
 
+-  Whenever a data instance is exported, the corresponding data family
+   is exported, too. This allows one to write ::
+
+     -- Foo.hs
+     module Foo where
+     
+     data family T a
+
+     -- Bar.hs
+     module Bar where
+     
+     import Foo
+     
+     data instance T Int = MkT
+
+     -- Baz.hs
+     module Baz where
+     
+     import Bar (T(MkT))
+
+   In previous versions of GHC, this required a workaround via an
+   explicit export list in Bar. 
+
 
 
 Compiler
diff --git a/docs/users_guide/glasgow_exts.rst b/docs/users_guide/glasgow_exts.rst
index 93261a2227..7e448beb9c 100644
--- a/docs/users_guide/glasgow_exts.rst
+++ b/docs/users_guide/glasgow_exts.rst
@@ -6702,10 +6702,10 @@ Two things to watch out for:
            data instance D Int = D1 | D2
 
    Module Y exports all the entities defined in Y, namely the data
-   constructors ``D1`` and ``D2``, *but not the data family* ``D``. That
-   (annoyingly) means that you cannot selectively import Y selectively,
-   thus "``import Y( D(D1,D2) )``", because Y does not export ``D``.
-   Instead you should list the exports explicitly, thus:
+   constructors ``D1`` and ``D2``, and *implicitly* the data family ``D``,
+   even though it's defined in X. 
+   This means you can write "``import Y( D(D1,D2) )``" *without* 
+   giving an explicit export list like this: 
 
    ::