Do not deduce the return type of `unique,remove[_if]` of `[forward_]list` (#4975)

stl/inc/forward_list

@@ -1208,12 +1208,12 @@ public:
         _Nodeptr* _Tail;
     };
 
-    auto remove(const _Ty& _Val) { // erase each element matching _Val
+    _LIST_REMOVE_RETURN remove(const _Ty& _Val) { // erase each element matching _Val
         return remove_if([&](const _Ty& _Other) -> bool { return _Other == _Val; });
     }
 
     template <class _Pr1>
-    auto remove_if(_Pr1 _Pred) { // erase each element satisfying _Pr1
+    _LIST_REMOVE_RETURN remove_if(_Pr1 _Pred) { // erase each element satisfying _Pr1
         _Flist_node_remove_op _Op(*this);
         auto _Firstb       = _Unchecked_before_begin();
         size_type _Removed = 0;
@@ -1235,12 +1235,12 @@ public:
 #endif
     }
 
-    auto unique() { // erase each element matching previous
+    _LIST_REMOVE_RETURN unique() { // erase each element matching previous
         return unique(equal_to<>{});
     }
 
     template <class _Pr2>
-    auto unique(_Pr2 _Pred) { // erase each element satisfying _Pred with previous
+    _LIST_REMOVE_RETURN unique(_Pr2 _Pred) { // erase each element satisfying _Pred with previous
         _Flist_node_remove_op _Op(*this);
         auto _First        = _Unchecked_begin();
         size_type _Removed = 0;

stl/inc/list

@@ -1662,12 +1662,12 @@ public:
         _Nodeptr* _Tail;
     };
 
-    auto remove(const _Ty& _Val) { // erase each element matching _Val
+    _LIST_REMOVE_RETURN remove(const _Ty& _Val) { // erase each element matching _Val
         return remove_if([&](const _Ty& _Other) -> bool { return _Other == _Val; });
     }
 
     template <class _Pr1>
-    auto remove_if(_Pr1 _Pred) { // erase each element satisfying _Pred
+    _LIST_REMOVE_RETURN remove_if(_Pr1 _Pred) { // erase each element satisfying _Pred
         auto& _My_data = _Mypair._Myval2;
         _List_node_remove_op _Op(*this);
         const auto _Last         = _My_data._Myhead;
@@ -1688,12 +1688,12 @@ public:
 #endif
     }
 
-    auto unique() { // erase each element matching previous
+    _LIST_REMOVE_RETURN unique() { // erase each element matching previous
         return unique(equal_to<>{});
     }
 
     template <class _Pr2>
-    auto unique(_Pr2 _Pred) { // erase each element satisfying _Pred with previous
+    _LIST_REMOVE_RETURN unique(_Pr2 _Pred) { // erase each element satisfying _Pred with previous
         _List_node_remove_op _Op(*this);
         const _Nodeptr _Phead    = _Mypair._Myval2._Myhead;
         _Nodeptr _Pprev          = _Phead->_Next;

stl/inc/xmemory

@@ -2649,6 +2649,12 @@ _STD_END
 #define _CONTAINER_EMPLACE_RETURN void
 #endif // ^^^ !_HAS_CXX17 ^^^
 
+#if _HAS_CXX20
+#define _LIST_REMOVE_RETURN size_type
+#else // ^^^ _HAS_CXX20 ^^^ / vvv !_HAS_CXX20 vvv
+#define _LIST_REMOVE_RETURN void
+#endif // ^^^ !_HAS_CXX20 ^^^
+
 #pragma pop_macro("new")
 _STL_RESTORE_CLANG_WARNINGS
 #pragma warning(pop)