[GHC] #14680: GHC 8.4.1-alpha panics when optimizing function using getTag and tagToEnum#

[GHC]

#14680: GHC 8.4.1-alpha panics when optimizing function using getTag and tagToEnum#
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.4.1-alpha1
      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):  Phab:D4371
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"d8a0e6d322deaa3743c95a11a6b7272577d1f86e/ghc"
 d8a0e6d3/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d8a0e6d322deaa3743c95a11a6b7272577d1f86e"
 Don't apply dataToTag's caseRules for data families

 Commit 193664d42dbceadaa1e4689dfa17ff1cf5a405a0 added a
 special caseRule for `dataToTag`, but this transformation completely
 broke when `dataToTag` was applied to somewith with a type headed by
 a data family, leading to #14680. For now at least, the simplest
 solution is to simply not apply this transformation when the type is
 headed by a data family.

 Test Plan: make test TEST=T14680

 Reviewers: simonpj, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #14680

 Differential Revision: https://phabricator.haskell.org/D4371
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14680#comment:7>
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