[libc++] Unify __can_reference and __is_referenceable_v (llvm#133278)

Both traits do the same thing. This patch renames __can_reference to
__referenceable and moves it to the is_referenceable header.

Author: ldionne

libcxx/include/__iterator/common_iterator.h

@@ -28,6 +28,7 @@
 #include <__memory/addressof.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/is_pointer.h>
+#include <__type_traits/is_referenceable.h>
 #include <__utility/declval.h>
 #include <variant>
 
@@ -157,7 +158,7 @@ class common_iterator {
       ++*this;
       return __tmp;
     } else if constexpr (requires(_Iter& __i) {
-                           { *__i++ } -> __can_reference;
+                           { *__i++ } -> __referenceable;
                          } || !__can_use_postfix_proxy<_Iter>) {
       return std::__unchecked_get<_Iter>(__hold_)++;
     } else {

libcxx/include/__iterator/concepts.h

@@ -38,6 +38,7 @@
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/is_primary_template.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_referenceable.h>
 #include <__type_traits/remove_cv.h>
 #include <__type_traits/remove_cvref.h>
 #include <__utility/forward.h>
@@ -132,7 +133,7 @@ concept incrementable = regular<_Ip> && weakly_incrementable<_Ip> && requires(_I
 // [iterator.concept.iterator]
 template <class _Ip>
 concept input_or_output_iterator = requires(_Ip __i) {
-  { *__i } -> __can_reference;
+  { *__i } -> __referenceable;
 } && weakly_incrementable<_Ip>;
 
 // [iterator.concept.sentinel]

libcxx/include/__iterator/iter_move.h

@@ -14,6 +14,7 @@
 #include <__config>
 #include <__iterator/iterator_traits.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_referenceable.h>
 #include <__type_traits/remove_cvref.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
@@ -90,7 +91,7 @@ inline constexpr auto iter_move = __iter_move::__fn{};
 
 template <__dereferenceable _Tp>
   requires requires(_Tp& __t) {
-    { ranges::iter_move(__t) } -> __can_reference;
+    { ranges::iter_move(__t) } -> __referenceable;
   }
 using iter_rvalue_reference_t = decltype(ranges::iter_move(std::declval<_Tp&>()));
 

libcxx/include/__iterator/iterator_traits.h

@@ -32,6 +32,7 @@
 #include <__type_traits/is_object.h>
 #include <__type_traits/is_primary_template.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_referenceable.h>
 #include <__type_traits/is_valid_expansion.h>
 #include <__type_traits/nat.h>
 #include <__type_traits/remove_const.h>
@@ -48,15 +49,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
-template <class _Tp>
-using __with_reference _LIBCPP_NODEBUG = _Tp&;
-
-template <class _Tp>
-concept __can_reference = requires { typename __with_reference<_Tp>; };
-
 template <class _Tp>
 concept __dereferenceable = requires(_Tp& __t) {
-  { *__t } -> __can_reference; // not required to be equality-preserving
+  { *__t } -> __referenceable; // not required to be equality-preserving
 };
 
 // [iterator.traits]
@@ -138,9 +133,9 @@ struct __has_iterator_typedefs {
 namespace __iterator_traits_detail {
 template <class _Ip>
 concept __cpp17_iterator = requires(_Ip __i) {
-  { *__i } -> __can_reference;
+  { *__i } -> __referenceable;
   { ++__i } -> same_as<_Ip&>;
-  { *__i++ } -> __can_reference;
+  { *__i++ } -> __referenceable;
 } && copyable<_Ip>;
 
 template <class _Ip>

libcxx/include/__ranges/transform_view.h

@@ -38,6 +38,7 @@
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_object.h>
 #include <__type_traits/is_reference.h>
+#include <__type_traits/is_referenceable.h>
 #include <__type_traits/maybe_const.h>
 #include <__type_traits/remove_cvref.h>
 #include <__utility/forward.h>
@@ -63,7 +64,7 @@ concept __regular_invocable_with_range_ref = regular_invocable<_Fn, range_refere
 template <class _View, class _Fn>
 concept __transform_view_constraints =
     view<_View> && is_object_v<_Fn> && regular_invocable<_Fn&, range_reference_t<_View>> &&
-    __can_reference<invoke_result_t<_Fn&, range_reference_t<_View>>>;
+    __is_referenceable_v<invoke_result_t<_Fn&, range_reference_t<_View>>>;
 
 #  if _LIBCPP_STD_VER >= 23
 template <input_range _View, move_constructible _Fn>

libcxx/include/__type_traits/is_referenceable.h

@@ -24,6 +24,11 @@ inline const bool __is_referenceable_v = false;
 template <class _Tp>
 inline const bool __is_referenceable_v<_Tp, __void_t<_Tp&> > = true;
 
+#if _LIBCPP_STD_VER >= 20
+template <class _Tp>
+concept __referenceable = __is_referenceable_v<_Tp>;
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_IS_REFERENCEABLE_H