Implement LWG-3734 Inconsistency in inout_ptr and out_ptr for empty case

stl/inc/memory

@@ -4179,7 +4179,15 @@ public:
     explicit out_ptr_t(_SmartPtr& _Smart_ptr_, _ArgsT... _Args_) noexcept(
         is_nothrow_constructible_v<tuple<_ArgsT...>, _ArgsT...>) /* strengthened */
         : _Smart_ptr(_Smart_ptr_),
-          _Mypair(_One_then_variadic_args_t{}, tuple<_ArgsT...>{_STD forward<_ArgsT>(_Args_)...}) {}
+          _Mypair(_One_then_variadic_args_t{}, tuple<_ArgsT...>{_STD forward<_ArgsT>(_Args_)...}) {
+        constexpr bool _Is_resettable = requires { _Smart_ptr.reset(); }; // TRANSITION, DevCom-10291456
+        if constexpr (_Is_resettable) {
+            _Smart_ptr.reset();
+        } else {
+            static_assert(is_constructible_v<_SmartPtr>, "the adapted pointer type must be default constructible.");
+            _Smart_ptr = _SmartPtr();
+        }
+    }
 
     out_ptr_t(const out_ptr_t&) = delete;
 

tests/std/tests/P1132R7_out_ptr/test.cpp

@@ -76,8 +76,22 @@ void test_shared_ptr() {
         assert(*int_ptr == 32);
     }
 
+    // LWG-3734 Inconsistency in inout_ptr and out_ptr for empty case
+    {
+        const auto f = [](void** ptr) { *ptr = new int(42); };
+
+        {
+            auto temp_adaptor = out_ptr(int_ptr, deleter);
+            assert(int_ptr.get() == nullptr);
+            f(temp_adaptor);
+        }
+
+        assert(count == 2);
+        assert(*int_ptr == 42);
+    }
+
     int_ptr.reset();
-    assert(count == 2);
+    assert(count == 3);
 }
 
 template <class Ptr, class Pointer = void, class... Args>
@@ -119,6 +133,19 @@ void test_smart_ptr(Args&&... args) {
         assert(*int_ptr == 19);
     }
 
+    // LWG-3734 Inconsistency in inout_ptr and out_ptr for empty case
+    {
+        const auto f = [](void** ptr) { *ptr = new int(42); };
+
+        {
+            auto temp_adaptor = out_ptr<int*>(int_ptr);
+            assert(int_ptr.get() == nullptr);
+            f(temp_adaptor);
+        }
+
+        assert(*int_ptr == 42);
+    }
+
     // LWG-3594 inout_ptr - inconsistent release() in destructor
     {
         const auto f = [](int** ptr) {
@@ -141,6 +168,10 @@ struct resettable_ptr {
 
     explicit resettable_ptr(int* p) : ptr(p) {}
 
+    void reset() {
+        ptr.reset();
+    }
+
     void reset(int* p, reset_tag) {
         ptr.reset(p);
     }
@@ -163,6 +194,7 @@ struct constructible_ptr {
 
     unique_ptr<int> ptr;
 
+    constructible_ptr() = default;
     explicit constructible_ptr(int* p) : ptr(p) {}
     explicit constructible_ptr(int* p, reset_tag) : ptr(p) {}