[GHC] #7861: deferred type error with rankNTypes

[GHC]

#7861: deferred type error with rankNTypes
-------------------------------------+-------------------------------------
        Reporter:  guest             |         Owner:
            Type:  bug               |        Status:  closed
        Priority:  normal            |     Milestone:
       Component:  Compiler          |       Version:  7.6.2
      Resolution:  wontfix           |      Keywords:
Operating System:  Linux             |  Architecture:  x86_64 (amd64)
 Type of failure:  Compile-time      |     Test Case:
  crash                              |  typecheck/should_run/T7861
      Blocked By:                    |      Blocking:
 Related Tickets:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"4c96e7cf56fc67ad09efaf8c5de1c8d7a0f5cedd/ghc"
 4c96e7cf/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="4c96e7cf56fc67ad09efaf8c5de1c8d7a0f5cedd"
 Testsuite: add ImpredicativeTypes to T7861 (#7861)

 The test was failing with:

     T7861: T7861.hs:15:13:
         Cannot instantiate unification variable ‘t0’
         with a type involving foralls: A a0 -> a0
           GHC doesn't yet support impredicative polymorphism
         In the first argument of ‘seq’, namely ‘f’
         In a stmt of a 'do' block: f `seq` print "Hello 2"

 It requires ImpredicativeTypes, at least since 7.8, because we
 instantiate seq's type (c->d->d) with f's type (c:= (forall b. a) -> a),
 which is polymorphic (it has foralls).

 I simplified the test a bit by removing the type synonym, and verified
 that ghc-7.6.3 still panics on this test.

 Reviewers: simonpj, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1080

 GHC Trac Issues: #7861
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7861#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler