[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):  Phab:D2931
       Wiki Page:                    |
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Comment (by goldfire):

 For the record, I was never convinced that Simon's a0899b2... was a good
 idea. I think we went with it thinking "let's try and see what happens".
 And now we see what happens!

 I advocate for reverting that commit. Simon, the message says that it
 slows GHC down, but why? If the check is compiled by a sufficiently smart
 compiler, it shouldn't slow a thing down in the common case. Right after
 the `isTyVar` check (which should be inlined), we do a `tcTyVarDetails`.
 This requires case-splitting on the constructor for the `Var`. All we want
 to do is to replace the default error continuation in the non-`TcTyVar`
 case with something that returns `False`.

 Or is GHC not a sufficiently smart compiler? :)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:10>
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