Add C++26 reference_wrapper

include/preview/__compare/synth_three_way.h

@@ -0,0 +1,35 @@
+//
+// Created by yonggyulee on 2024. 10. 17.
+//
+
+#ifndef PREVIEW_COMPARE_SYNTH_THREE_WAY_H_
+#define PREVIEW_COMPARE_SYNTH_THREE_WAY_H_
+
+#include <type_traits>
+
+#include "preview/__concepts/boolean_testable.h"
+#include "preview/__type_traits/conjunction.h"
+#include "preview/__utility/cxx20_rel_ops.h"
+
+namespace preview {
+namespace detail {
+
+template<typename T, typename U,
+    bool = /* false */ conjunction_v<
+        rel_ops::is_less_than_comparable<const T&, const U&>,
+        rel_ops::is_less_equal_than_comparable<const U&, const T&>
+    >
+>
+struct synth_three_way_possible : std::false_type {};
+
+template<typename T, typename U>
+struct synth_three_way_possible<T, U, true>
+    : conjunction<
+        boolean_testable<decltype(std::declval<const T&>() < std::declval<const U&>())>,
+        boolean_testable<decltype(std::declval<const U&>() < std::declval<const T&>())>
+    > {};
+
+} // namespace detail
+} // namespace preview
+
+#endif // PREVIEW_COMPARE_SYNTH_THREE_WAY_H_

include/preview/__concepts/boolean_testable.h

@@ -14,44 +14,29 @@
 namespace preview {
 namespace detail {
 
-template<typename B>
-struct boolean_testable_impl : convertible_to<B, bool> {};
+template<typename T>
+struct boolean_testable_impl : convertible_to<T, bool> {};
 
 template<typename T, typename = void>
 struct is_explicitly_negatable : std::false_type {};
 template<typename T>
 struct is_explicitly_negatable<T, void_t<decltype(!std::declval<T>())> > : std::true_type {};
 
-template<
-    typename B,
-    bool =
-        conjunction<
-            is_explicitly_negatable<std::add_lvalue_reference_t<std::remove_reference_t<B>>>,
-            is_explicitly_negatable<std::add_rvalue_reference_t<std::remove_reference_t<B>>>
-        >::value
->
+template<typename T, bool = /* false */ is_explicitly_negatable<T>::value>
 struct boolean_testable_stage_2 : std::false_type {};
+template<typename T>
+struct boolean_testable_stage_2<T, true> : boolean_testable_impl<decltype(!std::declval<T>())> {};
 
-template<typename B>
-struct boolean_testable_stage_2<B, true>
-    : conjunction<
-        boolean_testable_impl<decltype(!std::declval< std::add_lvalue_reference_t<std::remove_reference_t<B>> >())>,
-        boolean_testable_impl<decltype(!std::declval< std::add_rvalue_reference_t<std::remove_reference_t<B>> >())>
-      > {};
-
-template<
-    typename B,
-    bool = conjunction<boolean_testable_impl<B>, is_referencable<B>>::value
->
+template<typename T, bool = /* false */ boolean_testable_impl<T>::value>
 struct boolean_testable_stage_1 : std::false_type {};
-
-template<typename B>
-struct boolean_testable_stage_1<B, true> : boolean_testable_stage_2<B> {};
+template<typename T>
+struct boolean_testable_stage_1<T, true> : boolean_testable_stage_2<T> {};
 
 } // namespace detail
 
-template<typename B>
-struct boolean_testable : detail::boolean_testable_stage_1<B> {};
+// TODO: concept
+template<typename T>
+struct boolean_testable : detail::boolean_testable_stage_1<T> {};
 
 } // namespace preview
 

include/preview/__functional/is_reference_wrapper.h

@@ -0,0 +1,21 @@
+//
+// Created by yonggyulee on 2024. 10. 17.
+//
+
+#ifndef PREVIEW_FUNCTIONAL_IS_REFERENCE_WRAPPER_H_
+#define PREVIEW_FUNCTIONAL_IS_REFERENCE_WRAPPER_H_
+
+#include <functional>
+#include <type_traits>
+
+namespace preview {
+
+template<typename T>
+struct is_reference_wrapper : std::false_type {};
+
+template<typename T>
+struct is_reference_wrapper<std::reference_wrapper<T>> : std::true_type {};
+
+} // namespace preview
+
+#endif // PREVIEW_FUNCTIONAL_IS_REFERENCE_WRAPPER_H_