[commit: ghc] ghc-8.0: Make note of #12881 in 8.0.2 release notes (80c26da)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-8.0
Link       : http://ghc.haskell.org/trac/ghc/changeset/80c26da81ff3764887f272845388248ba34cacde/ghc

>---------------------------------------------------------------

commit 80c26da81ff3764887f272845388248ba34cacde
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Tue Nov 29 08:43:46 2016 -0500

    Make note of #12881 in 8.0.2 release notes
    
    Summary: Resolves #12881.
    
    Test Plan: Read it, commit it, merge it, ship it
    
    Reviewers: hvr, simonpj, austin, bgamari
    
    Reviewed By: simonpj
    
    Subscribers: simonpj, thomie
    
    Differential Revision: https://phabricator.haskell.org/D2760
    
    GHC Trac Issues: #12881
    
    (cherry picked from commit abd4a4c13e5dbaac8f1c28d8c9d9446e383f6037)


>---------------------------------------------------------------

80c26da81ff3764887f272845388248ba34cacde
 docs/users_guide/8.0.2-notes.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/docs/users_guide/8.0.2-notes.rst b/docs/users_guide/8.0.2-notes.rst
index 81f9ce4..063d947 100644
--- a/docs/users_guide/8.0.2-notes.rst
+++ b/docs/users_guide/8.0.2-notes.rst
@@ -71,6 +71,21 @@ Language
    type-checked in GHC 8.0.1 are now rejected by GHC 8.0.2. See
    :ghc-ticket:`12784` for details.
 
+-  Some programs which combine default type class method implementations and
+   overlapping instances may now fail to type-check. Here is an example: ::
+
+      class Foo a where
+        foo :: a -> [a]
+        foo _ = []
+
+      instance Foo a
+      instance Foo Int
+
+   The problem is that the overlapping ``Foo Int`` instance is not explicitly
+   marked as overlapping. To fix this, simply add an ``OVERLAPPING`` pragma: ::
+
+      instance {-# OVERLAPPING #-} Foo Int
+
 Compiler
 ~~~~~~~~