From d194fcd5a183118bcbc992c92158faf9016dd3b7 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Sat, 28 Sep 2024 13:26:14 -0700
Subject: [PATCH] Deduce the return type of `queue::emplace` and
 `stack::emplace` (#4980)

---
 stl/inc/queue                                       |  2 +-
 stl/inc/stack                                       |  2 +-
 stl/inc/xmemory                                     |  6 ++++++
 .../VSO_2252142_wrong_C5046/test.compile.pass.cpp   | 13 +++----------
 4 files changed, 11 insertions(+), 12 deletions(-)

diff --git a/stl/inc/queue b/stl/inc/queue
index 9c0262663c..9fc6d300e0 100644
--- a/stl/inc/queue
+++ b/stl/inc/queue
@@ -135,7 +135,7 @@ public:
 #endif // _HAS_CXX23
 
     template <class... _Valty>
-    _CONTAINER_EMPLACE_RETURN emplace(_Valty&&... _Val) {
+    _ADAPTOR_EMPLACE_RETURN emplace(_Valty&&... _Val) {
 #if _HAS_CXX17
         return c.emplace_back(_STD forward<_Valty>(_Val)...);
 #else // ^^^ _HAS_CXX17 / !_HAS_CXX17 vvv
diff --git a/stl/inc/stack b/stl/inc/stack
index 2640586df5..623640ad34 100644
--- a/stl/inc/stack
+++ b/stl/inc/stack
@@ -120,7 +120,7 @@ public:
 #endif // _HAS_CXX23
 
     template <class... _Valty>
-    _CONTAINER_EMPLACE_RETURN emplace(_Valty&&... _Val) {
+    _ADAPTOR_EMPLACE_RETURN emplace(_Valty&&... _Val) {
 #if _HAS_CXX17
         return c.emplace_back(_STD forward<_Valty>(_Val)...);
 #else // ^^^ _HAS_CXX17 / !_HAS_CXX17 vvv
diff --git a/stl/inc/xmemory b/stl/inc/xmemory
index 92371e2c4a..dfc3ad3b7e 100644
--- a/stl/inc/xmemory
+++ b/stl/inc/xmemory
@@ -2639,6 +2639,12 @@ _NODISCARD _Elem* _UIntegral_to_buff(_Elem* _RNext, _UTy _UVal) { // used by bot
 }
 _STD_END
 
+#if _HAS_CXX17
+#define _ADAPTOR_EMPLACE_RETURN decltype(auto)
+#else // ^^^ _HAS_CXX17 ^^^ / vvv !_HAS_CXX17 vvv
+#define _ADAPTOR_EMPLACE_RETURN void
+#endif // ^^^ !_HAS_CXX17 ^^^
+
 #if _HAS_CXX17
 #define _CONTAINER_EMPLACE_RETURN reference
 #else // ^^^ _HAS_CXX17 ^^^ / vvv !_HAS_CXX17 vvv
diff --git a/tests/std/tests/VSO_2252142_wrong_C5046/test.compile.pass.cpp b/tests/std/tests/VSO_2252142_wrong_C5046/test.compile.pass.cpp
index 8fa63feda4..52462f3aa2 100644
--- a/tests/std/tests/VSO_2252142_wrong_C5046/test.compile.pass.cpp
+++ b/tests/std/tests/VSO_2252142_wrong_C5046/test.compile.pass.cpp
@@ -4,8 +4,6 @@
 #include <deque>
 #include <forward_list>
 #include <list>
-#include <queue>
-#include <stack>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -21,12 +19,6 @@ struct convertible_to_any {
     operator T() &&; // not defined, only used in unevaluated context
 };
 
-template <class Cont, class = void>
-constexpr bool has_emplace = false;
-template <class Cont>
-constexpr bool has_emplace<Cont,
-    void_t<decltype(declval<Cont&>().emplace(declval<convertible_to_any<typename Cont::value_type>>()))>> = true;
-
 template <class Cont, class = void>
 constexpr bool has_emplace_back = false;
 template <class Cont>
@@ -51,6 +43,7 @@ STATIC_ASSERT(has_emplace_front<deque<S2>>);
 STATIC_ASSERT(has_emplace_front<forward_list<S2>>);
 STATIC_ASSERT(has_emplace_back<list<S2>>);
 STATIC_ASSERT(has_emplace_front<list<S2>>);
-STATIC_ASSERT(has_emplace<queue<S2>>);
-STATIC_ASSERT(has_emplace<stack<S2>>);
 STATIC_ASSERT(has_emplace_back<vector<bool>>); // Cannot trigger this bug, but for consistency
+
+// N4988 [queue.defn] and [stack.defn] require the container adaptors to have `decltype(auto) emplace(Args&&... args)`,
+// allowing them to adapt both C++14-era and C++17-era containers.