[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:  (none)
            Type:  bug               |               Status:  patch
        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:  #13487            |  Differential Rev(s):  Phab:D3495
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"69b9b853e3e68191cdfa8aec0e4da966298a2659/ghc"
 69b9b853/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="69b9b853e3e68191cdfa8aec0e4da966298a2659"
 Add regression test for #12104

 Commit 2f9f1f86849ebc18af409c9b3fd809c9cd464021
 (#13487) fixes #12104 as well. This adds a regression test for the
 program reported in #12104 to keep it fixed.

 Test Plan: make test TEST=T12104

 Reviewers: bgamari, austin

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #12104

 Differential Revision: https://phabricator.haskell.org/D3495
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12104#comment:10>
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