[GHC] #14162: Core Lint error

[GHC]

#14162: Core Lint error
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
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:"3a27e34f7a59a30f91fad9dd2ca194acdb1bcb1a/ghc" 3a27e34/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3a27e34f7a59a30f91fad9dd2ca194acdb1bcb1a"
 Fix subtle bug in TcTyClsDecls.mkGADTVars

 This bug was revealed by Trac #14162.  In a GADT-style data-family
 instance we ended up a data constructor whose type mentioned
 an out-of-scope variable.  (This variable was in the kind of
 a variable in the kind of a variable.)

 Only Lint complained about this (actually only when the
 data constructor was injected into the bindings by CorePrep).
 So it doesn't matter much -- but it's a solid bug and might
 bite us some day.

 It took me quite a while to unravel because the test case was itself
 quite tricky.  But the fix is easy; just add a missing binding to the
 substitution we are building up.  It's in the regrettably-subtle
 mkGADTVars function.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14162#comment:5>
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