[libc++] Always initialize __tree::{,const_}iterator

libcxx/include/__tree

@@ -646,12 +646,7 @@ public:
   using reference         = value_type&;
   using pointer           = __rebind_pointer_t<_NodePtr, value_type>;
 
-  _LIBCPP_HIDE_FROM_ABI __tree_iterator() _NOEXCEPT
-#if _LIBCPP_STD_VER >= 14
-      : __ptr_(nullptr)
-#endif
-  {
-  }
+  _LIBCPP_HIDE_FROM_ABI __tree_iterator() _NOEXCEPT : __ptr_(nullptr) {}
 
   _LIBCPP_HIDE_FROM_ABI reference operator*() const { return __get_np()->__value_; }
   _LIBCPP_HIDE_FROM_ABI pointer operator->() const { return pointer_traits<pointer>::pointer_to(__get_np()->__value_); }
@@ -720,12 +715,7 @@ public:
   using reference         = const value_type&;
   using pointer           = __rebind_pointer_t<_NodePtr, const value_type>;
 
-  _LIBCPP_HIDE_FROM_ABI __tree_const_iterator() _NOEXCEPT
-#if _LIBCPP_STD_VER >= 14
-      : __ptr_(nullptr)
-#endif
-  {
-  }
+  _LIBCPP_HIDE_FROM_ABI __tree_const_iterator() _NOEXCEPT : __ptr_(nullptr) {}
 
 private:
   typedef __tree_iterator<_Tp, __node_pointer, difference_type> __non_const_iterator;

libcxx/test/extensions/libcxx/always_initialize_iterators.pass.cpp

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// Make sure that that the iterator types of the associative containers is initialized even when default initialized.
+// This is an extension because the standard only requires initialization when an iterator is value initialized, and
+// that only since C++14. We guarantee that iterators are always initialized, even in C++11.
+
+#include <cassert>
+#include <map>
+#include <set>
+
+template <class Iter>
+void test() {
+  {
+    Iter iter1;
+    Iter iter2;
+    assert(iter1 == iter2);
+  }
+  {
+    Iter iter1;
+    Iter iter2{};
+    assert(iter1 == iter2);
+  }
+  {
+    Iter iter1{};
+    Iter iter2;
+    assert(iter1 == iter2);
+  }
+}
+
+template <class Container>
+void test_container() {
+  test<typename Container::iterator>();
+  test<typename Container::const_iterator>();
+  test<typename Container::reverse_iterator>();
+  test<typename Container::const_reverse_iterator>();
+}
+
+int main(int, char**) {
+  test_container<std::map<int, int>>();
+  test_container<std::multimap<int, int>>();
+  test_container<std::set<int>>();
+  test_container<std::multiset<int>>();
+
+  return 0;
+}