[GHC] #12562: GHC panic on rebuild (idInfo r_XsTP)

[GHC]

#12562: GHC panic on rebuild (idInfo r_XsTP)
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        Reporter:  cocreature        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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:"692c8df03969ee6a0de5158f05907b16689945d0/ghc"
 692c8df0/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="692c8df03969ee6a0de5158f05907b16689945d0"
 Fix shadowing in mkWwBodies

 This bug, exposed by Trac #12562 was very obscure, and has been
 lurking for a long time.  What happened was that, in the
 worker/wrapper split

   a tyvar binder for a worker function
   accidentally shadowed an in-scope term variable
   that was mentioned in the body of the function

 It's jolly hard to provoke, so I have not even attempted to make
 a test case.  There's a Note [Freshen WW arguments] to explain.

 Interestingly, fixing the bug (which meant fresher type variables)
 revealed a second lurking bug: I'd failed to apply the substitution to
 the coercion in the second last case of mkWWArgs, which introduces a
 Cast.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12562#comment:13>
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