[GHC] #13260: panic on unboxed string literal in pattern

[GHC]

#13260: panic on unboxed string literal in pattern
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           Reporter:  rwbarton       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           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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 I don't think this program is meaningful, but it shouldn't cause a panic.
 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 g y = case y of
   "a"# -> True
   _ -> False
 }}}
 {{{
 [1 of 1] Compiling CaseOnString     ( x/CaseOnString.hs, x/CaseOnString.o
 )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         litValue "a"#
 }}}
 The panic is also in at least 7.8, 7.10, and HEAD.

 `"a"#` has type `Addr#`, and I suppose that if we allow `case` on `Addr#`
 then it ought to use pointer equality. But the literal `"a"#` has no
 particular address, so matching against it in a `case` doesn't make sense.
 So I think we should just disallow unboxed string literals as patterns.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13260>
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