Implemented https://wg21.link/LWG4071

Author: H-G-Hristov

libcxx/include/__functional/reference_wrapper.h

@@ -72,36 +72,50 @@ class _LIBCPP_TEMPLATE_VIS reference_wrapper : public __weak_result_type<_Tp> {
 
   // [refwrap.comparisons], comparisons
 
-  friend constexpr bool operator==(reference_wrapper __x, reference_wrapper __y) {
-    static_assert(is_convertible_v<decltype(__x.get() == __y.get()), bool>);
-
+  friend constexpr bool operator==(reference_wrapper __x, reference_wrapper __y)
+    requires requires {
+      { __x.get() == __y.get() } -> __boolean_testable;
+    }
+  {
     return __x.get() == __y.get();
   }
 
-  friend constexpr bool operator==(reference_wrapper __x, const _Tp& __y) {
-    static_assert(is_convertible_v<decltype(__x.get() == __y), bool>);
-
+  friend constexpr bool operator==(reference_wrapper __x, const _Tp& __y)
+    requires requires {
+      { __x.get() == __y } -> __boolean_testable;
+    }
+  {
     return __x.get() == __y;
   }
 
   friend constexpr bool operator==(reference_wrapper __x, reference_wrapper<const _Tp> __y)
-    requires(!is_const_v<_Tp>)
+    requires(!is_const_v<_Tp>) && requires {
+      { __x.get() == __y.get() } -> __boolean_testable;
+    }
   {
-    static_assert(is_convertible_v<decltype(__x.get() == __y.get()), bool>);
-
     return __x.get() == __y.get();
   }
 
-  friend constexpr __synth_three_way_result<_Tp> operator<=>(reference_wrapper __x, reference_wrapper __y) {
-    return std::__synth_three_way(__x.get(), __y.get());
+  // `operator<=>`: Checks the constraints of `synth-three-way` as per https://wg21.link/LWG4071 directly
+
+  friend constexpr auto operator<=>(reference_wrapper __x, reference_wrapper __y)
+    requires(_Tp t)
+  {
+    { t < t } -> __boolean_testable;
   }
+  { return std::__synth_three_way(__x.get(), __y.get()); }
 
-  friend constexpr __synth_three_way_result<_Tp> operator<=>(reference_wrapper __x, const _Tp& __y) {
-    return std::__synth_three_way(__x.get(), __y);
+  friend constexpr auto operator<=>(reference_wrapper __x, const _Tp& __y)
+    requires(_Tp t)
+  {
+    { t < t } -> __boolean_testable;
   }
+  { return std::__synth_three_way(__x.get(), __y); }
 
-  friend constexpr __synth_three_way_result<_Tp> operator<=>(reference_wrapper __x, reference_wrapper<const _Tp> __y)
-    requires(!is_const_v<_Tp>)
+  friend constexpr auto operator<=>(reference_wrapper __x, reference_wrapper<const _Tp> __y)
+    requires(!is_const_v<_Tp>) && requires(_Tp t) {
+      { t < t } -> __boolean_testable;
+    }
   {
     return std::__synth_three_way(__x.get(), __y.get());
   }