[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:
        Priority:  normal                        |  closed
       Component:  Compiler (Type checker)       |        Milestone:
      Resolution:  fixed                         |          Version:  7.7
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  GHC rejects valid program     |     Architecture:
       Test Case:                                |  Unknown/Multiple
  typecheck/should_compile/T8644                 |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"5d2fb2ee94fea4cf62ba767eb1555e42f4f21f46/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5d2fb2ee94fea4cf62ba767eb1555e42f4f21f46"
 Further refine the test for 'given' equalities

 Trac #8651 revealed that my previous fix (itself in response to #8644)
 wasn't quite right.  The plan, using the CtOrigin to identify
 constraints arising from flattening, is described in TcSimplify,
 Note [When does an implication have given equalities?]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8644#comment:5>
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