constexpr string operator""s(const char* str, size_t len);
constexpr u8string operator""s(const char8_t* str, size_t len);
constexpr u16string operator""s(const char16_t* str, size_t len);
constexpr u32string operator""s(const char32_t* str, size_t len);
constexpr wstring operator""s(const wchar_t* str, size_t len);