[GHC] #13798: Invalid transformation in simplOptExpr

[GHC]

#13798: Invalid transformation in simplOptExpr
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Prior to
 [https://git.haskell.org/ghc.git/commitdiff/1722fa106e10e63160bb2322e2ccb830fd5b9ab3
 this commit], `CoreOpt.simple_opt_expr` transforms
 {{{
      case e of (b :: t1 ~# t2)
        DEFAULT -> rhs
 ==>
      rhs
 }}}
 That was plain wrong when `e` diverges, or calls `error`.  Hence the
 commit.

 But the commit does this
 {{{
      case Coercible_sc e of (b :: t1 ~# t2)
        DEFAULT -> rhs
 ==>
      rhs
 }}}
 narrowing the scope of the transformation
 to when the scrutinee is application of the Coercible superclass selector
 {{{
   Coercible_sc :: Coercible a b -> (a ~R# b)
 }}}
 Here's the code from `CoreOpt`:
 {{{
       | isDeadBinder b
       , [(DEFAULT, _, rhs)] <- as
       , isCoercionType (varType b)
       , (Var fun, _args) <- collectArgs e
       , fun `hasKey` coercibleSCSelIdKey
          -- without this last check, we get #11230
       = go rhs
 }}}
 But that's bizarrely ad-hoc.  And, worse, it is flat-out wrong... what if
 `e` diverges?

 It's not actually hurting anyone right now, but what we should
 really do is use `exprOkForSpeculation`; and teach `exprOkForSpeculation`
 how to deal with class-op selectors.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13798>
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