[GHC] #8644: 'Untouchable' error with constraint variable in rank-2 type

[GHC]

#8644: 'Untouchable' error with constraint variable in rank-2 type
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        Reporter:  sbarclay                   |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.7
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"cf68a8e798fea44bea2804439732e534e096b0d1/testsuite"]:
 {{{
 #!CommitTicketReference repository="testsuite"
 revision="cf68a8e798fea44bea2804439732e534e096b0d1"
 Update T7594 as a result of fixing #8644

 The fix to #8644 makes the original T7594 pass (rightly).
 I've added a variant that shouuld and does fail
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8644#comment:3>
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