[string.op.append]

constexpr basic_string& operator+=(const basic_string& str);

Effects: Equivalent to: return append(str);

template<class T>
  constexpr basic_string& operator+=(const T& t);

Constraints:

Effects: Equivalent to: basic_string_view<charT, traits> sv = t; return append(sv);

constexpr basic_string& operator+=(const charT* s);

Effects: Equivalent to: return append(s);

constexpr basic_string& operator+=(charT c);

Effects: Equivalent to: return append(size_type{1}, c);

constexpr basic_string& operator+=(initializer_list<charT> il);

Effects: Equivalent to: return append(il);

27 Strings library [strings]