[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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Comment (by simonpj):

 Ryan and I talked by Skype.  It seems that `dataToTag#` is truly
 polymorphic (works for values of any type), and GHC happily accepts
 {{{
 get :: forall a. a -> Int#
 get x = getTag x
 }}}
 `dataToTag#` will never crash; it'll just fetch a tag from its argument's
 info table.

 Given that, the Right Thing is just to make sure that the optimisatio rule
 in `PrelRules` has enough side conditions.  Currently it says
 {{{
 caseRules _ (App (App (Var f) (Type ty)) v)       -- dataToTag x
   | Just DataToTagOp <- isPrimOpId_maybe f
   = Just (v, tx_con_dtt ty
            , \v -> App (App (Var f) (Type ty)) (Var v))
 }}}
 But it should also check that `ty` is a proper data type; if not, return
 `Nothing` (i.e. don't optimise).  Easy.  Ryan is on the job.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14680#comment:5>
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