[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#
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        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):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: simonpj (added)


Comment:

 This regression was introduced in commi
 193664d42dbceadaa1e4689dfa17ff1cf5a405a0 (`Re-engineer caseRules to add
 tagToEnum/dataToTag`). cc'ing simonpj, who authored this.

 From glancing at that diff, I can at least see why this happening. It's
 due to
 [http://git.haskell.org/ghc.git/blob/c65104e1a6875f7879db87877848cc35363c1bf3:/compiler/prelude/PrelRules.hs#l1515
 this line]:

 {{{#!hs
 get_con ty tag = tyConDataCons (tyConAppTyCon ty) !! tag
 }}}

 But we're dealing with a data family here, so oughtn't we be using the
 //representation// tycon here, not `tyConAppTyCon`? It's likely that
 `tyConAppTyCon ty` is returning an empty list when `ty` is headed by a
 data family, which would explain the panic.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14680#comment:2>
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