From 5c2d2ad5454ad9b34c86cbb3d3b6aabe8b5000a3 Mon Sep 17 00:00:00 2001
From: Igor Zhukov <fsb4000@yandex.ru>
Date: Sun, 26 Jun 2022 04:50:00 +0700
Subject: [PATCH] add default ctor for node types (#2782)

Co-authored-by: A. Jiang <de34@live.cn>
Co-authored-by: Casey Carter <Casey@Carter.net>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>
---
 stl/inc/forward_list | 5 +++--
 stl/inc/list         | 4 +++-
 stl/inc/type_traits  | 9 +++------
 stl/inc/xtree        | 4 +++-
 4 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/stl/inc/forward_list b/stl/inc/forward_list
index e334451202..ce71eea9b3 100644
--- a/stl/inc/forward_list
+++ b/stl/inc/forward_list
@@ -241,9 +241,10 @@ template <class _Value_type, class _Voidptr>
 struct _Flist_node { // forward_list node
     using _Nodeptr = _Rebind_pointer_t<_Voidptr, _Flist_node>;
 
-    _Nodeptr _Next; // successor node
-    _Value_type _Myval; // the stored value
+    _Nodeptr _Next;
+    _Value_type _Myval = _Returns_exactly<_Value_type>(); // fake a viable constructor to workaround GH-2749
 
+    _Flist_node()                   = default;
     _Flist_node(const _Flist_node&) = delete;
     _Flist_node& operator=(const _Flist_node&) = delete;
 
diff --git a/stl/inc/list b/stl/inc/list
index e5d0956d72..0884228fb2 100644
--- a/stl/inc/list
+++ b/stl/inc/list
@@ -289,8 +289,10 @@ struct _List_node { // list node
     using _Nodeptr   = _Rebind_pointer_t<_Voidptr, _List_node>;
     _Nodeptr _Next; // successor node, or first element if head
     _Nodeptr _Prev; // predecessor node, or last element if head
-    _Value_type _Myval; // the stored value, unused if head
+    _Value_type _Myval = // the stored value, unused if head
+        _Returns_exactly<_Value_type>(); // fake a viable constructor to workaround GH-2749
 
+    _List_node()                  = default;
     _List_node(const _List_node&) = delete;
     _List_node& operator=(const _List_node&) = delete;
 
diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 8ac768676b..22780c2c8f 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -1218,13 +1218,13 @@ struct _Common_type3<void_t<common_type_t<_Ty1, _Ty2>>, _Ty1, _Ty2, _Rest...>
 template <class _Ty1, class _Ty2, class... _Rest>
 struct common_type<_Ty1, _Ty2, _Rest...> : _Common_type3<void, _Ty1, _Ty2, _Rest...> {};
 
+template <class _Ty>
+_Ty _Returns_exactly() noexcept; // not defined
+
 #if _HAS_CXX20
 template <class, class, template <class> class, template <class> class>
 struct basic_common_reference {};
 
-template <class _Ty>
-_Ty _Returns_exactly() noexcept; // not defined
-
 template <class _From>
 struct _Copy_cv_impl {
     template <class _To>
@@ -1614,9 +1614,6 @@ template <class _From, class _To>
 using is_nothrow_convertible = _Is_nothrow_convertible<_From, _To>;
 #endif // _HAS_CXX20
 
-template <class _Ty>
-_Ty _Returns_exactly() noexcept; // not defined
-
 template <class _From, class _To, class = void>
 struct _Invoke_convertible : false_type {};
 
diff --git a/stl/inc/xtree b/stl/inc/xtree
index 106ab11eea..ab7649521f 100644
--- a/stl/inc/xtree
+++ b/stl/inc/xtree
@@ -328,13 +328,15 @@ struct _Tree_node {
     _Nodeptr _Right; // right subtree, or largest element if head
     char _Color; // _Red or _Black, _Black if head
     char _Isnil; // true only if head (also nil) node; TRANSITION, should be bool
-    value_type _Myval; // the stored value, unused if head
+    value_type _Myval = // the stored value, unused if head
+        _Returns_exactly<value_type>(); // fake a viable constructor to workaround GH-2749
 
     enum _Redbl { // colors for link to parent
         _Red,
         _Black
     };
 
+    _Tree_node()                  = default;
     _Tree_node(const _Tree_node&) = delete;
     _Tree_node& operator=(const _Tree_node&) = delete;