diff --git a/libcxx/include/string b/libcxx/include/string
index 0abdfebcb863f..6a8523ee1e04a 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1312,13 +1312,17 @@ public:
 #  if _LIBCPP_STD_VER >= 23
   template <class _Op>
   _LIBCPP_HIDE_FROM_ABI constexpr void resize_and_overwrite(size_type __n, _Op __op) {
-    __resize_default_init(__n);
-    __erase_to_end(std::move(__op)(data(), _LIBCPP_AUTO_CAST(__n)));
+    size_type __sz  = size();
+    size_type __cap = capacity();
+    if (__n > __cap)
+      __grow_by_without_replace(__cap, __n - __cap, __sz, __sz, 0);
+    __annotate_delete();
+    __set_size(__n);
+    __annotate_new(__n);
+    __erase_to_end(std::move(__op)(data(), auto(__n)));
   }
 #  endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __resize_default_init(size_type __n);
-
 #  if _LIBCPP_STD_VER < 26 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE)
   _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_HIDE_FROM_ABI void reserve() _NOEXCEPT { shrink_to_fit(); }
 #  endif
@@ -1410,8 +1414,6 @@ public:
   _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s);
   _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(size_type __n, value_type __c);
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __append_default_init(size_type __n);
-
   template <class _InputIterator, __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string&
   append(_InputIterator __first, _InputIterator __last) {
@@ -3079,22 +3081,6 @@ basic_string<_CharT, _Traits, _Allocator>::append(size_type __n, value_type __c)
   return *this;
 }
 
-template <class _CharT, class _Traits, class _Allocator>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 inline void
-basic_string<_CharT, _Traits, _Allocator>::__append_default_init(size_type __n) {
-  if (__n == 0)
-    return;
-  size_type __cap = capacity();
-  size_type __sz  = size();
-  if (__cap - __sz < __n)
-    __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
-  __annotate_increase(__n);
-  pointer __p = __get_pointer();
-  __sz += __n;
-  __set_size(__sz);
-  traits_type::assign(__p[__sz], value_type());
-}
-
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c) {
   bool __is_short = !__is_long();
@@ -3433,16 +3419,6 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::re
     __erase_to_end(__n);
 }
 
-template <class _CharT, class _Traits, class _Allocator>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 inline void
-basic_string<_CharT, _Traits, _Allocator>::__resize_default_init(size_type __n) {
-  size_type __sz = size();
-  if (__n > __sz) {
-    __append_default_init(__n - __sz);
-  } else
-    __erase_to_end(__n);
-}
-
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __requested_capacity) {
   if (__requested_capacity > max_size())
diff --git a/libcxx/src/filesystem/format_string.h b/libcxx/src/filesystem/format_string.h
index ad6c57579a0a6..e91475e440480 100644
--- a/libcxx/src/filesystem/format_string.h
+++ b/libcxx/src/filesystem/format_string.h
@@ -34,20 +34,19 @@ inline _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 0) string vformat_string(const ch
 
   va_list apcopy;
   va_copy(apcopy, ap);
-  int ret = ::vsnprintf(buf.data(), buf.size(), msg, apcopy);
+  int size = ::vsnprintf(buf.data(), buf.size(), msg, apcopy);
   va_end(apcopy);
 
   string result;
-  if (static_cast<size_t>(ret) < buf.size()) {
-    result.assign(buf.data(), static_cast<size_t>(ret));
+  if (static_cast<size_t>(size) < buf.size()) {
+    result.assign(buf.data(), static_cast<size_t>(size));
   } else {
     // we did not provide a long enough buffer on our first attempt. The
     // return value is the number of bytes (excluding the null byte) that are
     // needed for formatting.
-    size_t size_with_null = static_cast<size_t>(ret) + 1;
-    result.__resize_default_init(size_with_null - 1);
-    ret = ::vsnprintf(&result[0], size_with_null, msg, ap);
-    _LIBCPP_ASSERT_INTERNAL(static_cast<size_t>(ret) == (size_with_null - 1), "TODO");
+    result.resize_and_overwrite(size, [&](char* res, size_t n) { return ::vsnprintf(res, n, msg, ap); });
+    _LIBCPP_ASSERT_INTERNAL(static_cast<size_t>(size) == result.size(),
+                            "vsnprintf did not result in the same number of characters as the first attempt?");
   }
   return result;
 }
diff --git a/libcxx/test/libcxx-03/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp b/libcxx/test/libcxx-03/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp
deleted file mode 100644
index 8e6e07d659c1a..0000000000000
--- a/libcxx/test/libcxx-03/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp
+++ /dev/null
@@ -1,75 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// <string>
-
-// __resize_default_init(size_type)
-
-#include <string>
-#include <cassert>
-
-#include "test_macros.h"
-
-TEST_CONSTEXPR_CXX20 void write_c_str(char* buf, int size) {
-  for (int i = 0; i < size; ++i) {
-    buf[i] = 'a';
-  }
-  buf[size] = '\0';
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 void test_buffer_usage() {
-  {
-    unsigned buff_size = 125;
-    unsigned used_size = buff_size - 16;
-    S s;
-    s.__resize_default_init(buff_size);
-    write_c_str(&s[0], used_size);
-    assert(s.size() == buff_size);
-    assert(std::char_traits<char>().length(s.data()) == used_size);
-    s.__resize_default_init(used_size);
-    assert(s.size() == used_size);
-    assert(s.data()[used_size] == '\0');
-    for (unsigned i = 0; i < used_size; ++i) {
-      assert(s[i] == 'a');
-    }
-  }
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 void test_basic() {
-  {
-    S s;
-    s.__resize_default_init(3);
-    assert(s.size() == 3);
-    assert(s.data()[3] == '\0');
-    for (int i = 0; i < 3; ++i)
-      s[i] = 'a' + i;
-    s.__resize_default_init(1);
-    assert(s[0] == 'a');
-    assert(s.data()[1] == '\0');
-    assert(s.size() == 1);
-  }
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 bool test() {
-  test_basic<S>();
-  test_buffer_usage<S>();
-
-  return true;
-}
-
-int main(int, char**) {
-  test<std::string>();
-#if TEST_STD_VER > 17
-  static_assert(test<std::string>());
-#endif
-
-  return 0;
-}
diff --git a/libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp b/libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp
index 1acbed55d2b51..a25601698e4ef 100644
--- a/libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp
+++ b/libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp
@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 // UNSUPPORTED: availability-filesystem-missing
 // UNSUPPORTED: no-filesystem
 // ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src
diff --git a/libcxx/test/libcxx/input.output/filesystems/convert_file_time.pass.cpp b/libcxx/test/libcxx/input.output/filesystems/convert_file_time.pass.cpp
index c501969c31167..6e5c5aa52674c 100644
--- a/libcxx/test/libcxx/input.output/filesystems/convert_file_time.pass.cpp
+++ b/libcxx/test/libcxx/input.output/filesystems/convert_file_time.pass.cpp
@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 // UNSUPPORTED: availability-filesystem-missing
 
 // <filesystem>
diff --git a/libcxx/test/libcxx/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp
deleted file mode 100644
index 8e6e07d659c1a..0000000000000
--- a/libcxx/test/libcxx/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp
+++ /dev/null
@@ -1,75 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// <string>
-
-// __resize_default_init(size_type)
-
-#include <string>
-#include <cassert>
-
-#include "test_macros.h"
-
-TEST_CONSTEXPR_CXX20 void write_c_str(char* buf, int size) {
-  for (int i = 0; i < size; ++i) {
-    buf[i] = 'a';
-  }
-  buf[size] = '\0';
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 void test_buffer_usage() {
-  {
-    unsigned buff_size = 125;
-    unsigned used_size = buff_size - 16;
-    S s;
-    s.__resize_default_init(buff_size);
-    write_c_str(&s[0], used_size);
-    assert(s.size() == buff_size);
-    assert(std::char_traits<char>().length(s.data()) == used_size);
-    s.__resize_default_init(used_size);
-    assert(s.size() == used_size);
-    assert(s.data()[used_size] == '\0');
-    for (unsigned i = 0; i < used_size; ++i) {
-      assert(s[i] == 'a');
-    }
-  }
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 void test_basic() {
-  {
-    S s;
-    s.__resize_default_init(3);
-    assert(s.size() == 3);
-    assert(s.data()[3] == '\0');
-    for (int i = 0; i < 3; ++i)
-      s[i] = 'a' + i;
-    s.__resize_default_init(1);
-    assert(s[0] == 'a');
-    assert(s.data()[1] == '\0');
-    assert(s.size() == 1);
-  }
-}
-
-template <class S>
-TEST_CONSTEXPR_CXX20 bool test() {
-  test_basic<S>();
-  test_buffer_usage<S>();
-
-  return true;
-}
-
-int main(int, char**) {
-  test<std::string>();
-#if TEST_STD_VER > 17
-  static_assert(test<std::string>());
-#endif
-
-  return 0;
-}