[GHC] #12104: Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell into a hole"

[GHC]

#12104: Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell
into a hole"
-------------------------------------+-------------------------------------
        Reporter:  antalsz           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * cc: conal (added)
 * component:  Compiler => Compiler (Type checker)


Comment:

 Conal Elliott reported the same panic in
 https://mail.haskell.org/pipermail/ghc-devs/2016-April/011687.html, when
 writing a custom GHC plugin.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12104#comment:2>
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