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
Test case
Differential revisions
BlockedBy
Related
Blocking
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Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

added newcomer label

assigned to @trac-ruperthorlick

Trac metadata

Trac field	Value
Test case	- → parser/should_fail/T13260
Differential revisions	- → D3286

mentioned in commit 6177c0d8

closed

changed milestone to %8.2.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added strings label and removed newcomer label

added Pnormal label