template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const basic_string<charT, traits, Allocator>& lhs,
const basic_string<charT, traits, Allocator>& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const basic_string<charT, traits, Allocator>& lhs, const charT* rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(basic_string<charT, traits, Allocator>&& lhs,
const basic_string<charT, traits, Allocator>& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(basic_string<charT, traits, Allocator>&& lhs, const charT* rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(basic_string<charT, traits, Allocator>&& lhs,
basic_string<charT, traits, Allocator>&& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const basic_string<charT, traits, Allocator>& lhs,
basic_string<charT, traits, Allocator>&& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const charT* lhs, basic_string<charT, traits, Allocator>&& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const charT* lhs, const basic_string<charT, traits, Allocator>& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(charT lhs, const basic_string<charT, traits, Allocator>& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(charT lhs, basic_string<charT, traits, Allocator>&& rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(const basic_string<charT, traits, Allocator>& lhs, charT rhs);
template<class charT, class traits, class Allocator>
constexpr basic_string<charT, traits, Allocator>
operator+(basic_string<charT, traits, Allocator>&& lhs, charT rhs);