Refactor use of std::nontype to use std::constant_wrapper instead per P3948

include/beman/utf_view/detail/constant_wrapper_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_CONSTANT_WRAPPER_POLYFILL_HPP
+#define BEMAN_UTF_VIEW_CONSTANT_WRAPPER_POLYFILL_HPP
+
+#include <utility>
+#include <version>
+
+namespace beman::utf_view::detail {
+
+#ifdef __cpp_lib_constant_wrapper
+
+template <auto X, typename T>
+using constant_wrapper = std::constant_wrapper<X, T>;
+
+template<auto X> constexpr auto cw{std::cw<X>};
+
+#else
+
+template<auto X, typename T>
+struct constant_wrapper {
+  static constexpr const auto value = X;
+};
+
+template<auto X> constexpr auto cw{constant_wrapper<X, decltype(X)>{}};
+
+#endif
+
+
+} // namespace beman::utf_view::detail
+
+#endif // BEMAN_UTF_VIEW_CONSTANT_WRAPPER_POLYFILL_HPP

include/beman/utf_view/to_utf_view.hpp

@@ -11,7 +11,7 @@
 #include <beman/utf_view/detail/concepts.hpp>
 #include <beman/utf_view/detail/constexpr_unless_msvc.hpp>
 #include <beman/utf_view/detail/fake_inplace_vector.hpp>
-#include <beman/utf_view/detail/nontype_t_polyfill.hpp>
+#include <beman/utf_view/detail/constant_wrapper_polyfill.hpp>
 #include <bit>
 #include <cassert>
 #include <concepts>
@@ -131,9 +131,13 @@ class to_utf_view : public std::ranges::view_interface<to_utf_view<V, E, ToType>
   constexpr to_utf_view()
     requires std::default_initializable<V>
   = default;
-  /* PAPER:   constexpr explicit to_utf_view(V base, nontype_t<E>, to_utf_tag_t<ToType>); */
+  /* PAPER:   template <auto E2> */
+  /* PAPER:     constexpr explicit to_utf_view(V base, constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>) */
+  /* PAPER:       requires (constant_wrapper<E2, to_utf_view_error_kind>::value == E); */
   /* !PAPER */
-  constexpr explicit to_utf_view(V base, detail::nontype_t<E>, to_utf_tag_t<ToType>)
+  template <auto E2>
+  constexpr explicit to_utf_view(V base, detail::constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>)
+    requires (detail::constant_wrapper<E2, to_utf_view_error_kind>::value == E)
       : base_(std::move(base)) { }
   /* PAPER */
 
@@ -202,8 +206,8 @@ class to_utf_view : public std::ranges::view_interface<to_utf_view<V, E, ToType>
   /* PAPER */
 };
 
-template <class R, to_utf_view_error_kind E, exposition_only_code_unit ToType>
-to_utf_view(R&&, detail::nontype_t<E>, to_utf_tag_t<ToType>) -> to_utf_view<std::views::all_t<R>, E, ToType>;
+template <class R, auto E2, exposition_only_code_unit ToType>
+to_utf_view(R&&, detail::constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>) -> to_utf_view<std::views::all_t<R>, detail::constant_wrapper<E2, to_utf_view_error_kind>::value, ToType>;
 
 template <std::ranges::input_range V, to_utf_view_error_kind E, exposition_only_code_unit ToType>
   requires std::ranges::view<V> && exposition_only_code_unit<std::ranges::range_value_t<V>>
@@ -884,14 +888,14 @@ namespace detail {
           return std::ranges::empty_view<std::expected<ToType, utf_transcoding_error>>{};
         }
       } else if constexpr (detail::is_to_utf_view_v<T>) {
-        return to_utf_view(std::forward<R>(r).base(), detail::nontype<E>, to_utf_tag<ToType>);
+        return to_utf_view(std::forward<R>(r).base(), detail::cw<E>, to_utf_tag<ToType>);
       } else if constexpr (detail::is_to_utf_subrange_v<T>) {
         return to_utf_view(
             std::ranges::subrange(r.begin().base(), r.end().base()),
-            detail::nontype<E>,
+            detail::cw<E>,
             to_utf_tag<ToType>);
-      }  else {
-        return to_utf_view(std::forward<R>(r), detail::nontype<E>, to_utf_tag<ToType>);
+      } else {
+        return to_utf_view(std::forward<R>(r), detail::cw<E>, to_utf_tag<ToType>);
       }
     }
   };

papers/P2728.md

@@ -496,14 +496,14 @@ expression-equivalent to:
   - Otherwise, `empty_view<expected<Char, utf_trancoding_error>>{}`.
 
 - Otherwise, if the type of `E` is a (possibly cv-qualified) specialization of
-  `to_utf_view`, then `to_utf_view(E.base(), nontype<Error>, to_utf_tag<Char>)`.
+  `to_utf_view`, then `to_utf_view(E.base(), cw<Error>, to_utf_tag<Char>)`.
 
 - Otherwise, if the type of `E` is *cv* `subrange<to_utf_view::@*iterator*@,
   to_utf_view::@*iterator*@, subrange_kind::unsized>` for some specialization of
   `to_utf_view`, then `to_utf_view(subrange(E.begin().base(), E.end().base()),
-  nontype<Error>, to_utf_tag<Char>)`.
+  cw<Error>, to_utf_tag<Char>)`.
 
-- Otherwise, `to_utf_view(E, nontype<Error>, to_utf_tag<Char>)`.
+- Otherwise, `to_utf_view(E, cw<Error>, to_utf_tag<Char>)`.
 
 #### 24.7.?.2 Enumeration `utf_transcoding_error` [range.transcoding.error.transcoding] {-}
 
@@ -569,7 +569,9 @@ private:
 
 public:
   constexpr to_utf_view() requires default_initializable<V> = default;
-  constexpr explicit to_utf_view(V base, nontype_t<E>, to_utf_tag_t<ToType>);
+  template <auto E2>
+    constexpr explicit to_utf_view(V base, constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>)
+      requires (constant_wrapper<E2, to_utf_view_error_kind>::value == E);
 
   constexpr V base() const& requires copy_constructible<V> { return @*base_*@; }
   constexpr V base() && { return std::move(@*base_*@); }
@@ -607,12 +609,15 @@ public:
   constexpr auto reserve_hint() const requires approximately_sized_range<const V>;
 };
 
-template<class R, to_utf_view_error_kind E, @*code-unit*@ ToType>
-  to_utf_view(R&&, nontype_t<E>, to_utf_tag_t<ToType>) -> to_utf_view<views::all_t<R>, E, ToType>;
+template<class R, auto E2, @*code-unit*@ ToType>
+  to_utf_view(R&&, constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>) 
+    -> to_utf_view<views::all_t<R>, constant_wrapper<E2, to_utf_view_error_kind>::value, ToType>;
 ```
 
 ```cpp
-constexpr explicit to_utf_view(V base, nontype_t<E>, to_utf_tag_t<ToType>);
+template <auto E2>
+  constexpr explicit to_utf_view(V base, constant_wrapper<E2, to_utf_view_error_kind>, to_utf_tag_t<ToType>)
+    requires (constant_wrapper<E2, to_utf_view_error_kind>::value == E);
 ```
 
 _Effects_: Initializes `@*base_*@` with `std::move(base)`.
@@ -1067,6 +1072,10 @@ gives back the original underlying view if it detects that it's reversing anothe
 
 # Changelog
 
+## Changes since R11
+
+- Refactor use of `std::nontype` to use `std::constant_wrapper` instead per P3948.
+
 ## Changes since R10
 
 - Fix the wording around rejecting arrays for the code unit adaptors

src/beman/utf_view/CMakeLists.txt

@@ -22,9 +22,9 @@ target_sources(
             FILES
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/code_unit_view.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/detail/concepts.hpp
+                ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/detail/constant_wrapper_polyfill.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/detail/constexpr_unless_msvc.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/detail/fake_inplace_vector.hpp
-                ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/detail/nontype_t_polyfill.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/endian_view.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/null_term.hpp
                 ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/beman/utf_view/to_utf_view.hpp

tests/beman/utf_view/to_utf_view.t.cpp

@@ -5,6 +5,7 @@
 //    (See accompanying file LICENSE.txt or copy at
 //          https://www.boost.org/LICENSE_1_0.txt)
 
+#include <beman/utf_view/detail/constant_wrapper_polyfill.hpp>
 #include <beman/utf_view/detail/constexpr_unless_msvc.hpp>
 #include <beman/utf_view/null_term.hpp>
 #include <beman/utf_view/to_utf_view.hpp>
@@ -871,7 +872,7 @@ constexpr bool empty_test() {
     std::is_same_v<
         decltype(std::views::empty<char8_t> | to_utf8_or_error),
         std::ranges::empty_view<std::expected<char8_t, utf_transcoding_error>>>);
-  auto empty_utf{to_utf_view{std::views::empty<char8_t>, detail::nontype<to_utf_view_error_kind::replacement>, to_utf8_tag}};
+  auto empty_utf{to_utf_view{std::views::empty<char8_t>, detail::cw<to_utf_view_error_kind::replacement>, to_utf8_tag}};
   if (!empty_utf.empty()) {
     return false;
   }