constexpr return-type F(const charT* s, size_type pos);is equivalent to return F(basic_string_view(s), pos);
constexpr return-type F(const charT* s, size_type pos, size_type n);is equivalent to return F(basic_string_view(s, n), pos);
constexpr size_type find(basic_string_view str, size_type pos = 0) const noexcept;
constexpr size_type rfind(basic_string_view str, size_type pos = npos) const noexcept;
constexpr size_type find_first_of(basic_string_view str, size_type pos = 0) const noexcept;
constexpr size_type find_last_of(basic_string_view str, size_type pos = npos) const noexcept;
constexpr size_type find_first_not_of(basic_string_view str, size_type pos = 0) const noexcept;
constexpr size_type find_last_not_of(basic_string_view str, size_type pos = npos) const noexcept;