[GHC] #12557: Regression in type inference with ImpredicativeTypes (was: Regression in type inference with RankNTypes)

[GHC]

#12557: Regression in type inference with ImpredicativeTypes
-------------------------------------+-------------------------------------
        Reporter:  slindley          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  ImpredicativeTypes
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * keywords:   => ImpredicativeTypes
 * priority:  high => low


@@ -4,0 +4,3 @@
+ {-# LANGUAGE ImpredicativeTypes #-}
+ module Test where
+

New description:

 Consider the following code:

 {{{#!hs
 {-# LANGUAGE ImpredicativeTypes #-}
 module Test where

 foo :: ((forall a.f a) -> f r) -> f r
 foo g = undefined
 }}}

 In GHC 7.10.3:
 {{{
 *Main> :t \g -> foo g
 \g -> foo g :: ((forall a. f a) -> f r) -> f r
 }}}

 In GHC 8.0.1:
 {{{
 *Main> :t \g -> foo g

 <interactive>:1:11: error:
     • Couldn't match expected type ‘(forall a. f a) -> f r’
                   with actual type ‘t’
       ‘t’ is a rigid type variable bound by
         the inferred type of it :: t -> f r at <interactive>:1:1
     • In the first argument of ‘foo’, namely ‘g’
       In the expression: foo g
       In the expression: \ g -> foo g
     • Relevant bindings include g :: t (bound at <interactive>:1:2)
 }}}

--

Comment:

 Your example relies on `-XImpredicativeTypes`. Please note that this
 language extension is entirely
 [https://downloads.haskell.org/~ghc/8.0.1/docs/html/users_guide/glasgow_exts.html
 #impredicative-polymorphism unsupported].

 That said, you can make you example work by running `:set
 -XImpredicativeTypes` on the GHCi prompt.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12557#comment:1>
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