[GHC] #15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into a hole' panic

[GHC]

#15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into
a hole' panic
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        Reporter:  dramforever       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Seems ok in HEAD, I'm happy to say.   I'm not sure about the 8.6 branch.

 Regardless, could someone add a regression test?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15368#comment:1>
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