Support For Incomplete Types In reference_wrapper (#393)

* Implement P0357R3
* Update LLVM to get skip of libcxx\test\std\utilities\function.objects\refwrap\weak_result.pass.cpp, and exclude "// REQUIRES: c++98 || c++03 || c++11 || c++14 || c++17" as a 'magic comment'.

Co-authored-by: Billy O'Neal <billy.oneal@gmail.com>
Co-authored-by: Casey Carter <cartec69@gmail.com>

llvm-project

@@ -1 +1 @@
-Subproject commit 0b969fa9ccf595abc31942e5d14be784707e960c
+Subproject commit 85ee4ff4e474788abc7abdc939114281c2a68ec1

stl/inc/type_traits

@@ -1845,7 +1845,11 @@ template <class _Ty, class _Uty>
 struct _Refwrap_has_ctor_from<_Ty, _Uty, void_t<decltype(_Refwrap_ctor_fun<_Ty>(_STD declval<_Uty>()))>> : true_type {};
 
 template <class _Ty>
-class reference_wrapper : public _Weak_types<_Ty>::type { // stand-in for an assignable reference
+class reference_wrapper
+#if !_HAS_CXX20
+    : public _Weak_types<_Ty>::type
+#endif // !_HAS_CXX20
+{
 public:
     static_assert(is_object_v<_Ty> || is_function_v<_Ty>,
         "reference_wrapper<T> requires T to be an object type or a function type.");

stl/inc/yvals_core.h

@@ -130,6 +130,7 @@
 // P0318R1 unwrap_reference, unwrap_ref_decay
 // P0325R4 to_array()
 // P0356R5 bind_front()
+// P0357R3 Supporting Incomplete Types In reference_wrapper
 // P0439R0 enum class memory_order
 // P0457R2 starts_with()/ends_with() For basic_string/basic_string_view
 // P0458R2 contains() For Ordered And Unordered Associative Containers

tests/libcxx/magic_comments.txt

@@ -2,4 +2,5 @@
 // REQUIRES: c++11 || c++14
 // REQUIRES: c++98 || c++03
 // REQUIRES: c++98 || c++03 || c++11 || c++14
+// REQUIRES: c++98 || c++03 || c++11 || c++14 || c++17
 // UNSUPPORTED: c++14, c++17, c++2a