[GHC] #11444: 8.0 rc1 panics in applyTypeToArgs

[GHC]

#11444: 8.0 rc1 panics in applyTypeToArgs
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        Reporter:  osa1              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Omer, any thoughts about comment:5?

 On comment:2

 * Assume for now that `reallyUnsafePtrEquality# @ a_a1Ts x_X1wq y_X1ws` is
 ''not'' ok-for-speculation. Then the transformation you show above should
 not happen. Look at this code in `Simplify`:
 {{{
   -- 2b.  Turn the case into a let, if
   --      a) it binds only the case-binder
   --      b) unlifted case: the scrutinee is ok-for-speculation
   --           lifted case: the scrutinee is in HNF (or will later be
 demanded)
   | all_dead_bndrs
   , if is_unlifted
     then exprOkForSpeculation scrut  -- See Note [Case elimination:
 unlifted case]
     else exprIsHNF scrut             -- See Note [Case elimination: lifted
 case]
       || scrut_is_demanded_var scrut
   = do { tick (CaseElim case_bndr)
        ; env' <- simplNonRecX env case_bndr scrut
        ; simplExprF env' rhs cont }
 }}}
   We are in the unlifted case, and `exprOkForSpeculation` is false, so
 this case should not fire.  But I think it is firing.  Why?

 Can you check that this really is happening (e.g. there is a `CaseElim` on
 this case binder)?  And figure out why?

 Can you make a small test case so I can see?

 Separately, it's true that `exprOkForSpeculation` is being conservative;
 see this comment in `CoreUtils.app_ok`
 {{{
         | otherwise
         -> primop_ok op                   -- A bit conservative: we don't
 really need
         && all (expr_ok primop_ok) args   -- to care about lazy arguments,
 but this is easy
 }}}
 Let me know if you need more help.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11444#comment:6>
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