Add constraint to emplace, add deleted optional<T&> constructors, add tests

Author: smallp-o-p

libcxx/include/optional

@@ -240,6 +240,7 @@ namespace std {
 #  include <__type_traits/is_trivially_relocatable.h>
 #  include <__type_traits/is_unbounded_array.h>
 #  include <__type_traits/negation.h>
+#  include <__type_traits/reference_constructs_from_temporary.h>
 #  include <__type_traits/remove_const.h>
 #  include <__type_traits/remove_cv.h>
 #  include <__type_traits/remove_cvref.h>
@@ -265,6 +266,11 @@ namespace std {
 _LIBCPP_PUSH_MACROS
 #  include <__undef_macros>
 
+#  if _LIBCPP_STD_VER >= 26
+#    define _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_TP, _UP)                                                            \
+      requires(is_lvalue_reference_v<_TP> && reference_constructs_from_temporary_v<_TP, _UP>)
+#  endif
+
 namespace std // purposefully not using versioning namespace
 {
 
@@ -813,6 +819,41 @@ public:
     this->__construct_from(std::move(__v));
   }
 
+  // deleted optional<T&> constructors
+#    if _LIBCPP_STD_VER >= 26
+  template <class _Up,
+            class... _Args,
+            enable_if_t<is_constructible_v<value_type, initializer_list<_Up>&, _Args...>, int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit optional(in_place_t, initializer_list<_Up> __il, _Args&&... __args) = delete;
+
+  template <class _Up                                                                        = value_type,
+            enable_if_t<_CheckOptionalArgsCtor<_Up>::template __enable_implicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI constexpr optional(_Up&& __v) = delete;
+
+  template <class _Up                                                                        = remove_cv_t<_Tp>,
+            enable_if_t<_CheckOptionalArgsCtor<_Up>::template __enable_explicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit optional(_Up&& __v) = delete;
+
+  template <class _Up, enable_if_t<_CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_implicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 optional(const optional<_Up>& __v) = delete;
+
+  template <class _Up, enable_if_t<_CheckOptionalLikeCtor<_Up, _Up const&>::template __enable_explicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit optional(const optional<_Up>& __v) = delete;
+
+  template <class _Up, enable_if_t<_CheckOptionalLikeCtor<_Up, _Up&&>::template __enable_implicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 optional(optional<_Up>&& __v) = delete;
+
+  template <class _Up, enable_if_t<_CheckOptionalLikeCtor<_Up, _Up&&>::template __enable_explicit<_Up>(), int> = 0>
+  _LIBCPP_OPT_REF_DELETED_CONS_REQUIRES(_Tp, _Up)
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 explicit optional(optional<_Up>&& __v) = delete;
+#    endif
+
 #    if _LIBCPP_STD_VER >= 23
   template <class _Tag,
             class _Fp,
@@ -868,7 +909,12 @@ public:
 
   template <class _Up,
             class... _Args,
-            enable_if_t<is_constructible_v<value_type, initializer_list<_Up>&, _Args...>, int> = 0>
+            enable_if_t<is_constructible_v<value_type, initializer_list<_Up>&, _Args...>
+#    if _LIBCPP_STD_VER >= 26
+                            && !reference_constructs_from_temporary_v<_Tp&, _Up>
+#    endif
+                        ,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) {
     reset();
     this->__construct(__il, std::forward<_Args>(__args)...);

libcxx/test/std/utilities/optional/optional.object/optional.object.ctor/ref_constructs_from_temporary.verify.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// optional
+
+#include <optional>
+#include <utility>
+
+struct X {
+  int i;
+
+  X(int j) : i(j) {}
+};
+
+int main(int, char**) {
+  const std::optional<int> _co(1);
+  std::optional<int> _o(1);
+
+  // expected-error-re@*:* 8 {{call to deleted constructor of 'std::optional<{{.*}}>'}}
+  std::optional<const int&> o1{1};                     // optional(U&&)
+  std::optional<const int&> o2{std::optional<int>(1)}; // optional(optional<U>&&)
+  std::optional<const int&> o3{_co};                   // optional(const optional<U>&)
+  std::optional<const int&> o4{_o};                    // optional(optional<U>&)
+  std::optional<const X&> o5{1};                       // optional(U&&)
+  std::optional<const X&> o6{std::optional<int>(1)};   // optional(optional<U>&&)
+  std::optional<const X&> o7{_co};                     // optional(const optional<U>&)
+  std::optional<const X&> o8{_o};                      // optional(optional<U>&)
+}