[GHC] #14154: Some cocktail of features causes GHC panic

[GHC]

#14154: Some cocktail of features causes GHC panic
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      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 goldfire):

 Could we fix one case this with a well-placed zonk?

 More broadly (re: could this happen elsewhere?) I've grown increasingly
 uncomfortable with the way GHC handles tyvars and zonking. When the type-
 checker fills in a metavariable, it then assumes that the metavar and its
 new value are equal. Except, of course, pure code can (and will, to our
 dismay) spot the difference. So, we could make `tc` versions of various
 type-manipulation functions (we indeed already do) and make them monadic,
 looking through metavars (this bit is new). That's disappointing, somehow.
 But I don't really see a better option. If we did this -- and did it
 reliably -- I think we could remove `zonkTcType` and friends (keeping only
 `zonkTcTypeToType` and friends).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14154#comment:7>
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