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

The instantiation used to deduce the return types of template functions with placeholders in their declared return type happens outside the immediate context of any other template instantiation. Notably, that means the body of such an instantiation isn't an unevaluated context. Fixes DevCom-10745303 / VSO-2252142
microsoft · Sep 18, 2024 · 6209e0b · 6209e0b
1 parent a26f4ad
commit 6209e0b
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Showing 10 changed files with 76 additions and 9 deletions.
diff --git a/stl/inc/deque b/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)...);
 

diff --git a/stl/inc/forward_list b/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

diff --git a/stl/inc/list b/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

diff --git a/stl/inc/queue b/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

diff --git a/stl/inc/stack b/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

diff --git a/stl/inc/vector b/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);
 

diff --git a/stl/inc/xmemory b/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)

diff --git a/tests/std/test.lst b/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
diff --git a/tests/std/tests/VSO_2252142_wrong_C5046/env.lst b/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
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
@@ -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 "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