[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:  8.2.1
       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 for merging!  There is a bug #12785 in which Simon changed a
 `substTy` to `substTyUnchecked` and mentions it is due to `tc_infer_args`
 not setting up the in-scope set correctly.  However I'm not sure if fixing
 the `tc_infer_args` problem fixes that bug or if this is a separate issue.
 Richard and Simon, is it enough to track the `tc_infer_args` problem in
 #12785?  If not I can create a new bug for it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12549#comment:29>
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