Function operator +

tests/catch.hpp:11262–11268

    }

    auto operator + ( StringRef const& lhs, StringRef const& rhs ) -> std::string {
        std::string str;
        str.reserve( lhs.size() + rhs.size() );
        str += lhs;
        str += rhs;
        return str;
    }
    auto operator + ( StringRef const& lhs, const char* rhs ) -> std::string {
        return std::string( lhs ) + std::string( rhs );
    }