[omnicognate]

std::monostate is explicitly provided for use with std::variant. It's in the <variant> header. Sometimes people use it for other things, but that's really an abuse, especially given defining your own type suitable for such cases is typically as simple as `struct mytype{};`.

Using one type to represent empty literals for optional, tuple and pointer types, implicitly convertible to all of them, would make the compiler accept many obviously accidental constructs. In a world where the maintainers of C++ are trying their hardest to make the language safer what conceivable benefit would there be?