[GHC] #13135: Typechecker "panic! the 'impossible' happened"

[GHC]

#13135: Typechecker "panic! the 'impossible' happened"
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        Reporter:  Saulzar           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"2b64e926a628fb2a3710b0360123ea73331166fe/ghc"
 2b64e926/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2b64e926a628fb2a3710b0360123ea73331166fe"
 Apply the right substitution in ty-fam improvement

 Trac #13135 showed that we were failing to apply the
 correct substitution to the un-substituted tyvars during
 type-family improvement using injectivity.  Specifically
 in TcInteractlinjImproveEqns we need to use instFlexiX.

 An outright bug, easy to fix.

 Slight refactoring along the way.  The quantified tyars of the axiom are
 readily to hand; we don't need to take the free tyvars of the LHS
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13135#comment:4>
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