basic_format_arg() noexcept;
template<class T> explicit basic_format_arg(const T& v) noexcept;
explicit basic_format_arg(float n) noexcept;
explicit basic_format_arg(double n) noexcept;
explicit basic_format_arg(long double n) noexcept;
explicit basic_format_arg(const char_type* s);
template<class traits>
explicit basic_format_arg(basic_string_view<char_type, traits> s) noexcept;
template<class traits, class Allocator>
explicit basic_format_arg(
const basic_string<char_type, traits, Allocator>& s) noexcept;
explicit basic_format_arg(nullptr_t) noexcept;
template<class T> explicit basic_format_arg(const T* p) noexcept;
explicit operator bool() const noexcept;
template<class T> explicit handle(const T& val) noexcept;
void format(basic_format_parse_context<char_type>& parse_ctx, Context& format_ctx) const;
template<class Visitor, class Context>
decltype(auto) visit_format_arg(Visitor&& vis, basic_format_arg<Context> arg);