exprIsConApp_maybe should deal better with strings

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

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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