panic on unboxed string literal in pattern

I don't think this program is meaningful, but it shouldn't cause a panic.

{-# 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.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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