[GHC] #13317: exprIsConApp_maybe should deal better with strings

[GHC]

#13317: exprIsConApp_maybe should deal better with strings
-------------------------------------+-------------------------------------
           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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 Consider
 {{{
 x :: Addr#
 x = "foo"#

 y = case unpackCString# x of
        [] -> ...
        (x:xs) -> ...
 }}}
 `exprIsConApp_maybe` has a special case for literal strings; see `Note
 [exprIsConApp_maybe on literal strings]` in `CoreSubst`.  But it only
 works if `unpackCString#` is applied to a literal, not to a variable bound
 to a literal (like `x`).

 The fix is easy.  Instead of this code
 {{{
         , [Lit (MachStr str)] <- args
         = dealWithStringLiteral fun str co
 }}}
 we want to use `exprIsLiteral_maybe`, thus
 {{{
         , Just (MachStr str) <- exprIsLiteral_maybe ... arg
         = dealWithStringLiteral fun str co
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13317>
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