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);

Effects: Equivalent to: basic_string<charT, traits, Allocator> r = lhs; r.append(rhs); return r;

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);

Effects: Equivalent to: lhs.append(rhs); return std::move(lhs);

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);

Effects: Equivalent to: lhs.append(rhs); return std::move(lhs); except that both lhs and rhs are left in valid but unspecified states.

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);

Effects: Equivalent to: rhs.insert(0, lhs); return std::move(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);

Effects: Equivalent to: basic_string<charT, traits, Allocator> r = rhs; r.insert(0, lhs); return r;

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

Effects: Equivalent to: basic_string<charT, traits, Allocator> r = rhs; r.insert(r.begin(), lhs); return r;

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

Effects: Equivalent to: rhs.insert(rhs.begin(), lhs); return std::move(rhs);

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

Effects: Equivalent to: basic_string<charT, traits, Allocator> r = lhs; r.push_back(rhs); return r;

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

Effects: Equivalent to: lhs.push_back(rhs); return std::move(lhs);

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

Equivalent to: basic_string<charT, traits, Allocator> r = lhs; r.append(rhs); return r;

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

Equivalent to: lhs.append(rhs); return std::move(lhs);

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

Equivalent to: basic_string<charT, traits, Allocator> r = rhs; r.insert(0, lhs); return r;

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

Equivalent to: rhs.insert(0, lhs); return std::move(rhs);

[Note 2:

Using a specialization of type_identity_t as a parameter type ensures that an object of type basic_string<charT, traits, Allocator> can be concatenated with an object of a type T having an implicit conversion to basic_string_view<charT, traits> ([over.match.oper]).

— end note]