Add test program in which string comparison operators are folded.

[fruffy]

Caused by #4837.

Should this be permitted?

[vlstill]

Frankly I don't see much motivation to have this allowed, but if there is some I'd guess it would not harm anything to add this to spec.

[vlstill]

One use case could be on the interface of P4 and CPP -- i.e. this gives the programmer ability to compare (at compile time) a string coming from preprocessor with some pre-defined values and have the result be a valid P4 value. While this can always be done using ifdefs or by changing the macro defines, using this on strings may be a little cleaner.

const bool isFoo = DEVICE == "foo";

instead of

#if DEVICE == "foo"
const bool isFoo = true;
#else
const bool isFoo = false;
#endif

or

#if DEVICE_FOO
const bool isFoo = true;
#else
const bool isFoo = false;
#endif

So this might be useful, although I'm not sure still. On the other hand I would tend towards allowing it (with spec change) as it would make string more like a regular primitive type (there is already match_kind and int that can be compared at compile time, can have variables, and int even has a lot of other operations, so I don't really see why string can't be allowed similar levels of compile-time existence). I'll think about it a bit more and then I guess we will discuss it in P4 LDWG in September.

[fruffy]

p4lang/p4-spec#1306 is the related specification issue.