users-guide/rel-notes: Note broken-ness of ImpredicativeTypes
authorBen Gamari <ben@smart-cactus.org>
Tue, 29 Mar 2016 14:30:29 +0000 (16:30 +0200)
committerBen Gamari <ben@smart-cactus.org>
Tue, 29 Mar 2016 17:01:14 +0000 (19:01 +0200)
(cherry picked from commit ae6a56ee467d675776be75dc982f5efb2135d3d4)

docs/users_guide/8.0.1-notes.rst

index 288141b..ea82fc1 100644 (file)
@@ -197,6 +197,14 @@ 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
 ~~~~~~~~