Don't deduce the return type of emplace{,_back,_front}

stl/inc/deque

@@ -1160,7 +1160,7 @@ private:
 
 public:
     template <class... _Valty>
-    decltype(auto) emplace_front(_Valty&&... _Val) {
+    _CONTAINER_EMPLACE_RETURN emplace_front(_Valty&&... _Val) {
         _Orphan_all();
         _Emplace_front_internal(_STD forward<_Valty>(_Val)...);
 #if _HAS_CXX17
@@ -1169,7 +1169,7 @@ public:
     }
 
     template <class... _Valty>
-    decltype(auto) emplace_back(_Valty&&... _Val) {
+    _CONTAINER_EMPLACE_RETURN emplace_back(_Valty&&... _Val) {
         _Orphan_all();
         _Emplace_back_internal(_STD forward<_Valty>(_Val)...);
 

stl/inc/forward_list

@@ -700,7 +700,7 @@ public:
     }
 
     template <class... _Valty>
-    decltype(auto) emplace_front(_Valty&&... _Val) { // insert element at beginning
+    _CONTAINER_EMPLACE_RETURN emplace_front(_Valty&&... _Val) { // insert element at beginning
         _Insert_after(_Mypair._Myval2._Before_head(), _STD forward<_Valty>(_Val)...);
 
 #if _HAS_CXX17

stl/inc/list

@@ -981,7 +981,7 @@ public:
     }
 
     template <class... _Valty>
-    decltype(auto) emplace_front(_Valty&&... _Val) { // insert element at beginning
+    _CONTAINER_EMPLACE_RETURN emplace_front(_Valty&&... _Val) { // insert element at beginning
         reference _Result = _Emplace(_Mypair._Myval2._Myhead->_Next, _STD forward<_Valty>(_Val)...)->_Myval;
 
 #if _HAS_CXX17
@@ -992,7 +992,7 @@ public:
     }
 
     template <class... _Valty>
-    decltype(auto) emplace_back(_Valty&&... _Val) { // insert element at end
+    _CONTAINER_EMPLACE_RETURN emplace_back(_Valty&&... _Val) { // insert element at end
         reference _Result = _Emplace(_Mypair._Myval2._Myhead, _STD forward<_Valty>(_Val)...)->_Myval;
 
 #if _HAS_CXX17

stl/inc/queue

@@ -135,7 +135,7 @@ public:
 #endif // _HAS_CXX23
 
     template <class... _Valty>
-    decltype(auto) emplace(_Valty&&... _Val) {
+    _CONTAINER_EMPLACE_RETURN emplace(_Valty&&... _Val) {
 #if _HAS_CXX17
         return c.emplace_back(_STD forward<_Valty>(_Val)...);
 #else // ^^^ _HAS_CXX17 / !_HAS_CXX17 vvv

stl/inc/stack

@@ -120,7 +120,7 @@ public:
 #endif // _HAS_CXX23
 
     template <class... _Valty>
-    decltype(auto) emplace(_Valty&&... _Val) {
+    _CONTAINER_EMPLACE_RETURN emplace(_Valty&&... _Val) {
 #if _HAS_CXX17
         return c.emplace_back(_STD forward<_Valty>(_Val)...);
 #else // ^^^ _HAS_CXX17 / !_HAS_CXX17 vvv

stl/inc/vector

@@ -853,7 +853,7 @@ private:
 
 public:
     template <class... _Valty>
-    _CONSTEXPR20 decltype(auto) emplace_back(_Valty&&... _Val) {
+    _CONSTEXPR20 _CONTAINER_EMPLACE_RETURN emplace_back(_Valty&&... _Val) {
         // insert by perfectly forwarding into element at end, provide strong guarantee
         _Ty& _Result = _Emplace_one_at_back(_STD forward<_Valty>(_Val)...);
 #if _HAS_CXX17
@@ -2979,7 +2979,7 @@ public:
     }
 
     template <class... _Valty>
-    _CONSTEXPR20 decltype(auto) emplace_back(_Valty&&... _Val) {
+    _CONSTEXPR20 _CONTAINER_EMPLACE_RETURN emplace_back(_Valty&&... _Val) {
         bool _Tmp(_STD forward<_Valty>(_Val)...);
         push_back(_Tmp);
 

stl/inc/xmemory

@@ -2621,6 +2621,12 @@ _NODISCARD _Elem* _UIntegral_to_buff(_Elem* _RNext, _UTy _UVal) { // used by bot
 }
 _STD_END
 
+#if _HAS_CXX17
+#define _CONTAINER_EMPLACE_RETURN reference
+#else // ^^^ _HAS_CXX17 ^^^ / vvv !_HAS_CXX17 vvv
+#define _CONTAINER_EMPLACE_RETURN void
+#endif // ^^^ !_HAS_CXX17 ^^^
+
 #pragma pop_macro("new")
 _STL_RESTORE_CLANG_WARNINGS
 #pragma warning(pop)

tests/std/test.lst

@@ -766,3 +766,4 @@ tests\VSO_0971246_legacy_await_headers
 tests\VSO_1775715_user_defined_modules
 tests\VSO_1804139_static_analysis_warning_with_single_element_array
 tests\VSO_1925201_iter_traits
+tests\VSO_2252142_wrong_C5046

tests/std/tests/VSO_2252142_wrong_C5046/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_matrix.lst

tests/std/tests/VSO_2252142_wrong_C5046/test.compile.pass.cpp

@@ -0,0 +1,56 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <deque>
+#include <forward_list>
+#include <list>
+#include <queue>
+#include <stack>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+// Test for DevCom-10745303 / VSO-2252142 "C5046 is wrongly triggered in unevaluated context"
+
+#define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
+
+using namespace std;
+
+template <class T>
+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>
+constexpr bool has_emplace_back<Cont,
+    void_t<decltype(declval<Cont&>().emplace_back(declval<convertible_to_any<typename Cont::value_type>>()))>> = true;
+
+template <class Cont, class = void>
+constexpr bool has_emplace_front = false;
+template <class Cont>
+constexpr bool has_emplace_front<Cont,
+    void_t<decltype(declval<Cont&>().emplace_front(declval<convertible_to_any<typename Cont::value_type>>()))>> = true;
+
+namespace {
+    struct S2 {};
+} // namespace
+
+// Was emitting "warning C5046: Symbol involving type with internal linkage not defined"
+// as a consequence of our use of return type deduction for the pertinent container functions.
+STATIC_ASSERT(has_emplace_back<vector<S2>>);
+STATIC_ASSERT(has_emplace_back<deque<S2>>);
+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