[GHC] #13994: STG lint failure on master

[GHC]

#13994: STG lint failure on master
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  stg-lint
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4396
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by osa1):

 After Phab:D4396 it's failing in `GHC.Integer.Type`. This time the problem
 is we have in scrutinee position an unboxed tuple (pair) with `State#` as
 second argument, but binder becomes `Unit#` after unarise becuase `State#`
 has no representation in runtime. Error message:

 {{{
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.5.20180207 for x86_64-unknown-linux):
         ASSERT failed!
   dataConInstArgTys Unit#
   [k0_10, a_11]
   ['TupleRep '[], 'LiftedRep, State# s_a2Hq, a_a2Hr]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1206:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/basicTypes/DataCon.hs:1255:76
 in ghc:DataCon

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 I look into this in more detail later.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13994#comment:5>
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