Eliminate to_utfN_views; make to_utf_view not exposition-only

examples/readme_examples.cpp

@@ -110,7 +110,7 @@ transcode_result<I, O> transcode_to_utf32(I first, S last, O out) {
 
 bool transcode_to_utf32_test() {
   std::u8string_view char8_string{u8"\xf0\x9f\x95\xb4\xef\xbf\xbd"};
-  to_utf16_view<std::u8string_view> utf16_transcoding_view{char8_string};
+  auto utf16_transcoding_view{char8_string | to_utf16};
   std::u32string char32_string{};
   auto transcode_result{transcode_to_utf32(utf16_transcoding_view.begin(),
                                            utf16_transcoding_view.end(),

include/beman/utf_view/detail/nontype_t_polyfill.hpp

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: BSL-1.0
+
+//   Copyright Eddie Nolan and Jonathan Wakely 2023 - 2025.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE.txt or copy at
+//          https://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BEMAN_UTF_VIEW_NONTYPE_T_POLYFILL_HPP
+#define BEMAN_UTF_VIEW_NONTYPE_T_POLYFILL_HPP
+
+#include <utility>
+#include <version>
+
+namespace beman::utf_view::detail {
+
+#ifdef __cpp_lib_function_ref
+
+template <auto V>
+using nontype_t = std::nontype_t<V>;
+
+template<auto V> constexpr auto nontype{std::nontype<V>};
+
+#else
+
+template<auto V>
+struct nontype_t {
+  explicit nontype_t() = default;
+};
+
+template<auto V> constexpr nontype_t<V> nontype{};
+
+#endif
+
+
+} // namespace beman::utf_view::detail
+
+#endif // BEMAN_UTF_VIEW_NONTYPE_T_POLYFILL_HPP