[GHC] #10644: Option GADTs invalidates a rank-2 function.

15 Jul 2015

      #10644: Option GADTs invalidates a  rank-2 function.
-------------------------------------+-------------------------------------
              Reporter:  bales       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Linux
          Architecture:  x86_64      |   Type of failure:  GHC rejects
  (amd64)                            |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 This module typechecks WITHOUT the extension GADTs
 and doesn't typecheck  WITH the extension GADTs

 Since it has nothing to do with GADTs, the consequence of using
 the option is surprising.

 I witnessed this behaviour on GHC versions 7.6.3, 7.8.3, 7.8.4, 7.10.1

 {{{#!hs
 {-# LANGUAGE
      RankNTypes
    , GADTs
  #-}
 module GADTMonomorphism where

 problem :: (forall x . () -> x) -> (a,b)
 problem f = (p, p)
   where p = f ()
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10644
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler