[GHC] #12549: Panic on ":t datatypeName"

[GHC]

#12549: Panic on ":t datatypeName"
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        Reporter:  johnleo       |                Owner:  johnleo
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.1
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by johnleo):

 Thanks, that also works, and as you say is semantically cleaner.  I did in
 fact use `newMetaTyVarsX`; in fact I had to write it since it didn't
 exist, but it is straightforward.

 {{{
 newMetaTyVarsX :: TCvSubst -> [TyVar] -> TcM (TCvSubst, [TcTyVar])
 -- Just like newMetaTyVars, but start with an existing substitution.
 newMetaTyVarsX subst = mapAccumLM newMetaTyVarX subst
 }}}

 I'll post the code for review in the near future.  Note that I haven't
 handled `tc_infer_args`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12549#comment:24>
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