boost::text::v1::is_stream_safe
// In header: <boost/text/stream_safe.hpp> template<code_point_iter I, std::sentinel_for< I > S> constexpr bool is_stream_safe(I first, S last);
Returns true iff [first, last) is in stream-safe format.
See Also:
https://unicode.org/reports/tr15/#Stream_Safe_Text_Format