[GHC] #12785: GHC panic, `tcTyVarDetails` is missing a case

[GHC]

#12785: GHC panic, `tcTyVarDetails` is missing a case
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:  yes
      Blocked By:                    |             Blocking:
 Related Tickets:  #12590            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"f4a14d6c535bdf52b19f441fe185ea13d62fdc24/ghc"
 f4a14d6c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f4a14d6c535bdf52b19f441fe185ea13d62fdc24"
 Use substTyUnchecked in TcMType.new_meta_tv_x

 Sadly, one of the indirect callers of this function doesn't yet
 enforce the in-scope set.  It's in the TcHsType.tc_infer_args swamp,
 so I'm not going to wade in there today.

 The assertion tripped when investigating Trac #12785; but this patch
 does NOT fix the actual bug reported there.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:6>
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