Match expressions for std::visit using simple macros.

match.cpp

#include <iostream>
#include <string>
#include <variant>
using namespace std;

#define MATCH(x)                                \
	using __X_TYPE = std::decay_t<decltype(x)>; \
	if constexpr (false)

#define CASE(T) \
	} else if constexpr (std::is_same_v<__X_TYPE, T>) {

#define DEFAULT \
	} else {

int main() {
	variant<int, string, double> i = 5, s = string("hello"), d = 23.4;

	auto func = [](auto&& x) {
		MATCH(x) {
			CASE(int) {
				cout << "It's an int = " << x << "!\n";
			}

			CASE(string) {
				cout << "It's a string = " << x << "!\n";
			}

			DEFAULT {
				cout << "It's a double = " << x << "!\n";
			}
		}
	};

	visit(func, i);
	visit(func, s);
	visit(func, d);

	return 0;
}

I am not so sure that I find these constexpr-if statements so horrible that we should poison the code with macros that don't really add anything of value.