Add printer for absl::monostate / std::monostate

[alexweej]

We support variant, but it looks like monostate is still printing as 0-byte object.

Propose we print it as {} or (). Hopefully the bikeshedding is not too strong on this one. I think spelling it "monostate" is going to be too verbose where this appears in nested values several times.

[sbenzaquen]

Printing the type when it is statically known is not that useful. But printing {} is better than "0 byte object..."
I think this is a good change. Feel free to send the PR if you want to do the work.

[gennadiycivil]

Thank you very much for this report. The best way to approach this would be to create a proper PR and submit it for consideration.

[gennadiycivil]

@alex-weej any updates?

[kuzkry]

@gennadiycivil, this could be added to #2438 which was rejected (I posted my thoughts on why it should be reconsidered there, but I got no answer). Can I ask you or anyone else to have a second look into that?

[kuzkry]

After recent addition of std::variant printer, we'd print a type (if RTTI is available) and a value like this:

// for regular types
std::variant<int> full = 100;
::testing::internal::UniversalPrint(full, &std::cout); // hack for brevity

gives back:
('int(index = 0)' with value 100)

whereas:

// for std::monostate
std::variant<std::monostate> empty;
::testing::internal::UniversalPrint(empty, &std::cout);

returns:
('std::monostate(index = 0)' with value 1-byte object <00>)

The question is whether it's good enough.