[commit: ghc] master: users-guide/rel-notes: Note broken-ness of ImpredicativeTypes (ae6a56e)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ae6a56ee467d675776be75dc982f5efb2135d3d4/ghc

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

commit ae6a56ee467d675776be75dc982f5efb2135d3d4
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Tue Mar 29 16:30:29 2016 +0200

    users-guide/rel-notes: Note broken-ness of ImpredicativeTypes


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

ae6a56ee467d675776be75dc982f5efb2135d3d4
 docs/users_guide/8.0.1-notes.rst | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/docs/users_guide/8.0.1-notes.rst b/docs/users_guide/8.0.1-notes.rst
index 2ebba6e..fe77bf4 100644
--- a/docs/users_guide/8.0.1-notes.rst
+++ b/docs/users_guide/8.0.1-notes.rst
@@ -205,6 +205,14 @@ Language
    result, :ghc-flag:`-XDeriveTraversable` now works on datatypes that contain
    arguments which have unlifted types.
 
+-  Note that the :ghc-flag:`-XImpredicativeTypes` extension, which has been
+   :ghc-wiki:`known <ImpredicativePolymorphism>` to be broken for many years, is even more
+   broken than usual in this release (see :ghc-ticket:`11319`,
+   :ghc-ticket:`11675`, and others). During pre-release testing we encountered
+   a number of projects that broke with confusing type errors due to (often
+   unnecessary) use of :ghc-flag:`-XImpredicativeTypes`. Users of
+   :ghc-flag:`-XImpredicativeTypes` do so at their own risk!
+
 Compiler
 ~~~~~~~~