Allow make_optional for T&, add test

Author: smallp-o-p

libcxx/include/optional

@@ -1437,9 +1437,13 @@ swap(optional<_Tp>& __x, optional<_Tp>& __y) noexcept(noexcept(__x.swap(__y))) {
   __x.swap(__y);
 }
 
+struct __make_optional_barrier_tag {
+  explicit __make_optional_barrier_tag() = default; 
+};
+
 template <
 #    if _LIBCPP_STD_VER >= 26
-    bool = false,
+    __make_optional_barrier_tag = __make_optional_barrier_tag{},
 #    endif
     class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr optional<decay_t<_Tp>> make_optional(_Tp&& __v) {
@@ -1448,17 +1452,11 @@ _LIBCPP_HIDE_FROM_ABI constexpr optional<decay_t<_Tp>> make_optional(_Tp&& __v)
 
 template <class _Tp, class... _Args>
 _LIBCPP_HIDE_FROM_ABI constexpr optional<_Tp> make_optional(_Args&&... __args) {
-#    if _LIBCPP_STD_VER >= 26
-  static_assert(!is_reference_v<_Tp>, "make_optional<T&, Args...> is disallowed");
-#    endif
   return optional<_Tp>(in_place, std::forward<_Args>(__args)...);
 }
 
 template <class _Tp, class _Up, class... _Args>
 _LIBCPP_HIDE_FROM_ABI constexpr optional<_Tp> make_optional(initializer_list<_Up> __il, _Args&&... __args) {
-#    if _LIBCPP_STD_VER >= 26
-  static_assert(!is_reference_v<_Tp>, "make_optional<T&, Args...> is disallowed");
-#    endif
   return optional<_Tp>(in_place, __il, std::forward<_Args>(__args)...);
 }
 

libcxx/test/std/utilities/optional/optional.specalg/make_optional_explicit.pass.cpp

@@ -19,9 +19,26 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <string_view>
 
 #include "test_macros.h"
 
+template <typename T>
+constexpr bool test_ref() {
+  T i{0};
+  auto opt = std::make_optional<T&>(i);
+
+#if TEST_STD_VER < 26
+  assert((std::is_same_v<decltype(opt), std::optional<T>>));
+#else
+  assert((std::is_same_v<decltype(opt), std::optional<T&>>));
+#endif
+
+  assert(*opt == 0);
+
+  return true;
+}
+
 int main(int, char**)
 {
     {
@@ -43,6 +60,12 @@ int main(int, char**)
         auto opt = std::make_optional<std::string>(4u, 'X');
         assert(*opt == "XXXX");
     }
+    using namespace std::string_view_literals;
+
+    static_assert(test_ref<int>());
+    assert((test_ref<int>()));
+    static_assert(test_ref<double>());
+    assert((test_ref<double>()));
 
-  return 0;
+    return 0;
 }