[clang] [lld] [llvm] [flang] [libcxx] [libc++][streams] P1759R6: Native handles and file streams (PR #76632)

[Hristo Hristov via cfe-commits]

https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/76632

>From 1165b11477ab59122a4db35221fcefe3c9505387 Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sat, 30 Dec 2023 17:34:56 +0200
Subject: [PATCH 1/7] [libc++][streams] P1759R6: Native handles and file
 streams

Implements: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p1759r6.html

- https://eel.is/c++draft/filebuf
- https://eel.is/c++draft/ifstream
- https://eel.is/c++draft/ofstream
- https://eel.is/c++draft/fstream
---
 libcxx/docs/FeatureTestMacroTable.rst         |  2 +-
 libcxx/docs/ReleaseNotes/18.rst               |  1 +
 libcxx/docs/Status/Cxx2cPapers.csv            |  2 +-
 libcxx/include/fstream                        | 59 +++++++++++++++++++
 libcxx/include/version                        |  2 +-
 libcxx/src/CMakeLists.txt                     |  1 +
 libcxx/src/fstream.cpp                        | 33 +++++++++++
 .../fstream.version.compile.pass.cpp          | 16 ++---
 .../version.version.compile.pass.cpp          | 16 ++---
 .../generate_feature_test_macro_components.py |  1 -
 10 files changed, 107 insertions(+), 26 deletions(-)
 create mode 100644 libcxx/src/fstream.cpp

diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index ad12b109023154..0427615a9636fd 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -418,7 +418,7 @@ Status
     --------------------------------------------------- -----------------
     ``__cpp_lib_freestanding_variant``                  *unimplemented*
     --------------------------------------------------- -----------------
-    ``__cpp_lib_fstream_native_handle``                 *unimplemented*
+    ``__cpp_lib_fstream_native_handle``                 ``202306L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_function_ref``                          *unimplemented*
     --------------------------------------------------- -----------------
diff --git a/libcxx/docs/ReleaseNotes/18.rst b/libcxx/docs/ReleaseNotes/18.rst
index fa60a581652d6a..e6f1adf4d5529b 100644
--- a/libcxx/docs/ReleaseNotes/18.rst
+++ b/libcxx/docs/ReleaseNotes/18.rst
@@ -58,6 +58,7 @@ Implemented Papers
 - P2870R3 - Remove basic_string::reserve()
 - P2909R4 - Fix formatting of code units as integers (Dude, where’s my ``char``?)
 - P0521R0 - Proposed Resolution for CA 14 (shared_ptr use_count/unique)
+- P1759R6 - Native handles and file streams
 
 
 Improvements and New Features
diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index ff83648aa76830..1d7807a4291a99 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -19,7 +19,7 @@
 "`P2757R3 <https://wg21.link/P2757R3>`__","LWG","Type-checking format args","Varna June 2023","","","|format|"
 "`P2637R3 <https://wg21.link/P2637R3>`__","LWG","Member ``visit``","Varna June 2023","","","|format|"
 "`P2641R4 <https://wg21.link/P2641R4>`__","CWG, LWG","Checking if a ``union`` alternative is active","Varna June 2023","","",""
-"`P1759R6 <https://wg21.link/P1759R6>`__","LWG","Native handles and file streams","Varna June 2023","","",""
+"`P1759R6 <https://wg21.link/P1759R6>`__","LWG","Native handles and file streams","Varna June 2023","|Complete|","18.0",""
 "`P2697R1 <https://wg21.link/P2697R1>`__","LWG","Interfacing ``bitset`` with ``string_view``","Varna June 2023","|Complete|","18.0",""
 "`P1383R2 <https://wg21.link/P1383R2>`__","LWG","More ``constexpr`` for ``<cmath>`` and ``<complex>``","Varna June 2023","","",""
 "`P2734R0 <https://wg21.link/P2734R0>`__","LWG","Adding the new SI prefixes","Varna June 2023","|Complete|","17.0",""
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 7a4e15b55d56fe..9609ff420220fa 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -73,6 +73,7 @@ public:
     typedef typename traits_type::int_type int_type;
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
+    using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type; // Since C++26
 
     basic_ifstream();
     explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in);
@@ -85,6 +86,7 @@ public:
     void swap(basic_ifstream& rhs);
 
     basic_filebuf<char_type, traits_type>* rdbuf() const;
+    native_handle_type native_handle() const noexcept; // Since C++26
     bool is_open() const;
     void open(const char* s, ios_base::openmode mode = ios_base::in);
     void open(const string& s, ios_base::openmode mode = ios_base::in);
@@ -110,6 +112,7 @@ public:
     typedef typename traits_type::int_type int_type;
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
+    using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type; // Since C++26
 
     basic_ofstream();
     explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out);
@@ -122,6 +125,8 @@ public:
     void swap(basic_ofstream& rhs);
 
     basic_filebuf<char_type, traits_type>* rdbuf() const;
+    native_handle_type native_handle() const noexcept; // Since C++26
+
     bool is_open() const;
     void open(const char* s, ios_base::openmode mode = ios_base::out);
     void open(const string& s, ios_base::openmode mode = ios_base::out);
@@ -148,6 +153,7 @@ public:
     typedef typename traits_type::int_type int_type;
     typedef typename traits_type::pos_type pos_type;
     typedef typename traits_type::off_type off_type;
+    using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type; // Since C++26
 
     basic_fstream();
     explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
@@ -160,6 +166,7 @@ public:
     void swap(basic_fstream& rhs);
 
     basic_filebuf<char_type, traits_type>* rdbuf() const;
+    native_handle_type native_handle() const noexcept; // Since C++26
     bool is_open() const;
     void open(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
     void open(const string& s, ios_base::openmode mode = ios_base::in|ios_base::out);
@@ -219,6 +226,13 @@ public:
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
   typedef typename traits_type::state_type state_type;
+#  if _LIBCPP_STD_VER >= 26
+#    if defined(_LIBCPP_WIN32API)
+  using native_handle_type = void*; // HANDLE
+#    else
+  using native_handle_type = int; // POSIX file descriptor
+#    endif
+#  endif
 
   // 27.9.1.2 Constructors/destructor:
   basic_filebuf();
@@ -245,6 +259,9 @@ public:
 #  endif
   _LIBCPP_HIDE_FROM_ABI basic_filebuf* __open(int __fd, ios_base::openmode __mode);
   basic_filebuf* close();
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle();
+#  endif
 
   _LIBCPP_HIDE_FROM_ABI inline static const char* __make_mdstring(ios_base::openmode __mode) _NOEXCEPT;
 
@@ -1024,6 +1041,9 @@ public:
   typedef typename traits_type::int_type int_type;
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
+#  if _LIBCPP_STD_VER >= 26
+  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+#  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_ifstream();
   _LIBCPP_HIDE_FROM_ABI explicit basic_ifstream(const char* __s, ios_base::openmode __mode = ios_base::in);
@@ -1041,6 +1061,9 @@ public:
   _LIBCPP_HIDE_FROM_ABI void swap(basic_ifstream& __rhs);
 
   _LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+#  endif
   _LIBCPP_HIDE_FROM_ABI bool is_open() const;
   void open(const char* __s, ios_base::openmode __mode = ios_base::in);
 #  ifdef _LIBCPP_HAS_OPEN_WITH_WCHAR
@@ -1116,6 +1139,14 @@ inline basic_filebuf<_CharT, _Traits>* basic_ifstream<_CharT, _Traits>::rdbuf()
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
+// #  if _LIBCPP_STD_VER >= 26
+// template <class _CharT, class _Traits>
+// inline basic_filebuf<_CharT, _Traits>::native_handle_type
+// basic_ifstream<_CharT, _Traits>::native_handle() const noexcept {
+//   return rdbuf()->native_handle();
+// }
+// #  endif
+
 template <class _CharT, class _Traits>
 inline bool basic_ifstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
@@ -1171,6 +1202,9 @@ public:
   typedef typename traits_type::int_type int_type;
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
+#  if _LIBCPP_STD_VER >= 26
+  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+#  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_ofstream();
   _LIBCPP_HIDE_FROM_ABI explicit basic_ofstream(const char* __s, ios_base::openmode __mode = ios_base::out);
@@ -1190,6 +1224,9 @@ public:
   _LIBCPP_HIDE_FROM_ABI void swap(basic_ofstream& __rhs);
 
   _LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+#  endif
   _LIBCPP_HIDE_FROM_ABI bool is_open() const;
   void open(const char* __s, ios_base::openmode __mode = ios_base::out);
 #  ifdef _LIBCPP_HAS_OPEN_WITH_WCHAR
@@ -1266,6 +1303,14 @@ inline basic_filebuf<_CharT, _Traits>* basic_ofstream<_CharT, _Traits>::rdbuf()
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
+// #  if _LIBCPP_STD_VER >= 26
+// template <class _CharT, class _Traits>
+// inline basic_filebuf<_CharT, _Traits>::native_handle_type
+// basic_ofstream<_CharT, _Traits>::native_handle() const noexcept {
+//   return rdbuf()->native_handle();
+// }
+// #  endif
+
 template <class _CharT, class _Traits>
 inline bool basic_ofstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
@@ -1321,6 +1366,9 @@ public:
   typedef typename traits_type::int_type int_type;
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
+#  if _LIBCPP_STD_VER >= 26
+  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+#  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_fstream();
   _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(const char* __s,
@@ -1345,6 +1393,9 @@ public:
   _LIBCPP_HIDE_FROM_ABI void swap(basic_fstream& __rhs);
 
   _LIBCPP_HIDE_FROM_ABI basic_filebuf<char_type, traits_type>* rdbuf() const;
+#  if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept { return rdbuf()->native_handle(); }
+#  endif
   _LIBCPP_HIDE_FROM_ABI bool is_open() const;
   _LIBCPP_HIDE_FROM_ABI void open(const char* __s, ios_base::openmode __mode = ios_base::in | ios_base::out);
 #  ifdef _LIBCPP_HAS_OPEN_WITH_WCHAR
@@ -1420,6 +1471,14 @@ inline basic_filebuf<_CharT, _Traits>* basic_fstream<_CharT, _Traits>::rdbuf() c
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
+// #  if _LIBCPP_STD_VER >= 26
+// template <class _CharT, class _Traits>
+// inline basic_filebuf<_CharT, _Traits>::native_handle_type
+// basic_fstream<_CharT, _Traits>::native_handle() const noexcept {
+//   return rdbuf()->native_handle();
+// }
+// #  endif
+
 template <class _CharT, class _Traits>
 inline bool basic_fstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
diff --git a/libcxx/include/version b/libcxx/include/version
index 768710ef5c84aa..a2bb11b62a44d3 100644
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -495,7 +495,7 @@ __cpp_lib_within_lifetime                               202306L <type_traits>
 // # define __cpp_lib_freestanding_optional                202311L
 // # define __cpp_lib_freestanding_string_view             202311L
 // # define __cpp_lib_freestanding_variant                 202311L
-// # define __cpp_lib_fstream_native_handle                202306L
+# define __cpp_lib_fstream_native_handle                202306L
 // # define __cpp_lib_function_ref                         202306L
 // # define __cpp_lib_hazard_pointer                       202306L
 // # define __cpp_lib_linalg                               202311L
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 329964a001363b..b931460ec76297 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -14,6 +14,7 @@ set(LIBCXX_SOURCES
   filesystem/filesystem_error.cpp
   filesystem/path_parser.h
   filesystem/path.cpp
+  fstream.cpp
   functional.cpp
   hash.cpp
   include/apple_availability.h
diff --git a/libcxx/src/fstream.cpp b/libcxx/src/fstream.cpp
new file mode 100644
index 00000000000000..c680ff638fe7e5
--- /dev/null
+++ b/libcxx/src/fstream.cpp
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <__config>
+#include <cassert>
+#include <cstdio>
+#include <fstream>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+template <class _CharT, class _Traits>
+basic_filebuf<_CharT, _Traits>::native_handle_type basic_filebuf<_CharT, _Traits>::native_handle() {
+  assert(is_open());
+  // __file_ is a FILE*
+#  if defined(_LIBCPP_WIN32API)
+  // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
+  intptr_t __handle = _get_osfhandle(::fileno(__file_));
+  if (__handle == -1)
+    return nullptr;
+  return reinterpret_cast<void*>(__handle);
+#  else
+  return ::fileno(__file_);
+#  endif
+}
+#endif
+
+_LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/fstream.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/fstream.version.compile.pass.cpp
index 981f5420ff7e0f..eab0313b2c1ef7 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/fstream.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/fstream.version.compile.pass.cpp
@@ -56,17 +56,11 @@
 
 #elif TEST_STD_VER > 23
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_fstream_native_handle
-#     error "__cpp_lib_fstream_native_handle should be defined in c++26"
-#   endif
-#   if __cpp_lib_fstream_native_handle != 202306L
-#     error "__cpp_lib_fstream_native_handle should have the value 202306L in c++26"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_fstream_native_handle
-#     error "__cpp_lib_fstream_native_handle should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_fstream_native_handle
+#   error "__cpp_lib_fstream_native_handle should be defined in c++26"
+# endif
+# if __cpp_lib_fstream_native_handle != 202306L
+#   error "__cpp_lib_fstream_native_handle should have the value 202306L in c++26"
 # endif
 
 #endif // TEST_STD_VER > 23
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
index 650a14b019ed87..af7037a1392fbf 100644
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
@@ -6490,17 +6490,11 @@
 #   endif
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_fstream_native_handle
-#     error "__cpp_lib_fstream_native_handle should be defined in c++26"
-#   endif
-#   if __cpp_lib_fstream_native_handle != 202306L
-#     error "__cpp_lib_fstream_native_handle should have the value 202306L in c++26"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_fstream_native_handle
-#     error "__cpp_lib_fstream_native_handle should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_fstream_native_handle
+#   error "__cpp_lib_fstream_native_handle should be defined in c++26"
+# endif
+# if __cpp_lib_fstream_native_handle != 202306L
+#   error "__cpp_lib_fstream_native_handle should have the value 202306L in c++26"
 # endif
 
 # if !defined(_LIBCPP_VERSION)
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
index 2f506f32f565cb..cb7c532c05251f 100755
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -570,7 +570,6 @@ def add_version_header(tc):
             "name": "__cpp_lib_fstream_native_handle",
             "values": {"c++26": 202306},  # P1759R6 Native handles and file streams
             "headers": ["fstream"],
-            "unimplemented": True,
         },
         {
             "name": "__cpp_lib_function_ref",

>From 9d4b10e4739c44e867ae40dc942fc6577212d94e Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sat, 30 Dec 2023 22:28:46 +0200
Subject: [PATCH 2/7] Added tests

---
 libcxx/include/fstream                        | 70 ++++++++++---------
 libcxx/src/fstream.cpp                        | 17 +++--
 .../filebuf.members/native_handle.pass.cpp    | 52 ++++++++++++++
 .../fstream.members/native_handle.pass.cpp    | 57 +++++++++++++++
 .../ifstream.members/native_handle.pass.cpp   | 57 +++++++++++++++
 .../ofstream.members/native_handle.pass.cpp   | 59 ++++++++++++++++
 .../file.streams/fstreams/test_helpers.h      | 33 +++++++++
 7 files changed, 305 insertions(+), 40 deletions(-)
 create mode 100644 libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
 create mode 100644 libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
 create mode 100644 libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
 create mode 100644 libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
 create mode 100644 libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h

diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 9609ff420220fa..f0cb5bb50fc9b6 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -215,8 +215,37 @@ _LIBCPP_PUSH_MACROS
 
 #if !defined(_LIBCPP_HAS_NO_FILESYSTEM)
 
+#  if defined(_LIBCPP_WIN32API)
+#    define WIN32_LEAN_AND_MEAN
+#    define NOMINMAX
+#    include <io.h>
+#    include <windows.h>
+#  endif
+
+#include <fcntl.h>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#  if _LIBCPP_STD_VER >= 26
+#    if defined(_LIBCPP_WIN32API)
+using __filebuf_native_handle_type = void*; // HANDLE
+#    else
+using __filebuf_native_handle_type = int; // POSIX file descriptor
+#    endif
+_LIBCPP_HIDE_FROM_ABI __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept;
+__filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
+#    if defined(_LIBCPP_WIN32API)
+  // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
+  intptr_t __handle = _get_osfhandle(::fileno(__file));
+  if (__handle == -1)
+    return nullptr;
+  return reinterpret_cast<void*>(__handle);
+#    else
+  return ::fileno(__file);
+#    endif
+}
+#  endif
+
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS basic_filebuf : public basic_streambuf<_CharT, _Traits> {
 public:
@@ -227,11 +256,7 @@ public:
   typedef typename traits_type::off_type off_type;
   typedef typename traits_type::state_type state_type;
 #  if _LIBCPP_STD_VER >= 26
-#    if defined(_LIBCPP_WIN32API)
-  using native_handle_type = void*; // HANDLE
-#    else
-  using native_handle_type = int; // POSIX file descriptor
-#    endif
+  using native_handle_type = __filebuf_native_handle_type;
 #  endif
 
   // 27.9.1.2 Constructors/destructor:
@@ -260,7 +285,10 @@ public:
   _LIBCPP_HIDE_FROM_ABI basic_filebuf* __open(int __fd, ios_base::openmode __mode);
   basic_filebuf* close();
 #  if _LIBCPP_STD_VER >= 26
-  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle();
+  _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
+    assert(is_open());
+    return std::__filebuf_native_handle(__file_);
+  }
 #  endif
 
   _LIBCPP_HIDE_FROM_ABI inline static const char* __make_mdstring(ios_base::openmode __mode) _NOEXCEPT;
@@ -1042,7 +1070,7 @@ public:
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
 #  if _LIBCPP_STD_VER >= 26
-  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+  using native_handle_type = typename basic_filebuf<_CharT, _Traits>::native_handle_type;
 #  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_ifstream();
@@ -1139,14 +1167,6 @@ inline basic_filebuf<_CharT, _Traits>* basic_ifstream<_CharT, _Traits>::rdbuf()
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
-// #  if _LIBCPP_STD_VER >= 26
-// template <class _CharT, class _Traits>
-// inline basic_filebuf<_CharT, _Traits>::native_handle_type
-// basic_ifstream<_CharT, _Traits>::native_handle() const noexcept {
-//   return rdbuf()->native_handle();
-// }
-// #  endif
-
 template <class _CharT, class _Traits>
 inline bool basic_ifstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
@@ -1203,7 +1223,7 @@ public:
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
 #  if _LIBCPP_STD_VER >= 26
-  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+  using native_handle_type = typename basic_filebuf<_CharT, _Traits>::native_handle_type;
 #  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_ofstream();
@@ -1303,14 +1323,6 @@ inline basic_filebuf<_CharT, _Traits>* basic_ofstream<_CharT, _Traits>::rdbuf()
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
-// #  if _LIBCPP_STD_VER >= 26
-// template <class _CharT, class _Traits>
-// inline basic_filebuf<_CharT, _Traits>::native_handle_type
-// basic_ofstream<_CharT, _Traits>::native_handle() const noexcept {
-//   return rdbuf()->native_handle();
-// }
-// #  endif
-
 template <class _CharT, class _Traits>
 inline bool basic_ofstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
@@ -1367,7 +1379,7 @@ public:
   typedef typename traits_type::pos_type pos_type;
   typedef typename traits_type::off_type off_type;
 #  if _LIBCPP_STD_VER >= 26
-  using native_handle_type = typename basic_filebuf<charT, traits>::native_handle_type;
+  using native_handle_type = typename basic_filebuf<_CharT, _Traits>::native_handle_type;
 #  endif
 
   _LIBCPP_HIDE_FROM_ABI basic_fstream();
@@ -1471,14 +1483,6 @@ inline basic_filebuf<_CharT, _Traits>* basic_fstream<_CharT, _Traits>::rdbuf() c
   return const_cast<basic_filebuf<char_type, traits_type>*>(&__sb_);
 }
 
-// #  if _LIBCPP_STD_VER >= 26
-// template <class _CharT, class _Traits>
-// inline basic_filebuf<_CharT, _Traits>::native_handle_type
-// basic_fstream<_CharT, _Traits>::native_handle() const noexcept {
-//   return rdbuf()->native_handle();
-// }
-// #  endif
-
 template <class _CharT, class _Traits>
 inline bool basic_fstream<_CharT, _Traits>::is_open() const {
   return __sb_.is_open();
diff --git a/libcxx/src/fstream.cpp b/libcxx/src/fstream.cpp
index c680ff638fe7e5..e5611e2d48b0a6 100644
--- a/libcxx/src/fstream.cpp
+++ b/libcxx/src/fstream.cpp
@@ -7,25 +7,28 @@
 //===----------------------------------------------------------------------===//
 
 #include <__config>
-#include <cassert>
 #include <cstdio>
 #include <fstream>
 
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 26
-template <class _CharT, class _Traits>
-basic_filebuf<_CharT, _Traits>::native_handle_type basic_filebuf<_CharT, _Traits>::native_handle() {
-  assert(is_open());
-  // __file_ is a FILE*
+_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
 #  if defined(_LIBCPP_WIN32API)
   // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
-  intptr_t __handle = _get_osfhandle(::fileno(__file_));
+  intptr_t __handle = _get_osfhandle(::fileno(__file));
   if (__handle == -1)
     return nullptr;
   return reinterpret_cast<void*>(__handle);
 #  else
-  return ::fileno(__file_);
+  return ::fileno(__file);
 #  endif
 }
 #endif
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
new file mode 100644
index 00000000000000..4104aa061e1546
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17, c++20, c++23
+
+// <fstream>
+
+// class basic_filebuf;
+
+// native_handle_type native_handle() const noexcept;
+
+#include <cassert>
+#include <fstream>
+#include <filesystem>
+#include <utility>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
+#include "platform_support.h"
+#include "test_macros.h"
+#include "../test_helpers.h"
+
+template <typename CharT>
+void test() {
+  std::basic_filebuf<CharT> f;
+  assert(!f.is_open());
+  std::filesystem::path p = get_temp_file_name();
+  f.open(p, std::ios_base::in);
+  assert(f.is_open());
+  assert(is_handle_valid(f.native_handle()));
+  assert(is_handle_valid(std::as_const(f).native_handle()));
+  static_assert(noexcept(f.native_handle()));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
new file mode 100644
index 00000000000000..44da8bd3e18bad
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17, c++20, c++23
+
+// <fstream>
+
+// class basic_fstream;
+
+// native_handle_type native_handle() const noexcept;
+
+#include <cassert>
+#include <fstream>
+#include <filesystem>
+#include <utility>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
+#include "platform_support.h"
+#include "test_macros.h"
+#include "../test_helpers.h"
+
+template <typename CharT>
+void test() {
+  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
+                               typename std::basic_fstream<CharT>::native_handle_type>);
+
+  std::basic_fstream<CharT> f;
+  static_assert(noexcept(f.native_handle()));
+  assert(!f.is_open());
+  std::filesystem::path p = get_temp_file_name();
+  f.open(p);
+  assert(f.is_open());
+  assert(f.native_handle() == f.rdbuf()->native_handle());
+  assert(is_handle_valid(f.native_handle()));
+  assert(is_handle_valid(std::as_const(f).native_handle()));
+  static_assert(noexcept(f.native_handle()));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
new file mode 100644
index 00000000000000..1345b72b158e28
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17, c++20, c++23
+
+// <fstream>
+
+// class basic_ifstream;
+
+// native_handle_type native_handle() const noexcept;
+
+#include <cassert>
+#include <fstream>
+#include <filesystem>
+#include <utility>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
+#include "platform_support.h"
+#include "test_macros.h"
+#include "../test_helpers.h"
+
+template <typename CharT>
+void test() {
+  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
+                               typename std::basic_ifstream<CharT>::native_handle_type>);
+
+  std::basic_ifstream<CharT> f;
+  static_assert(noexcept(f.native_handle()));
+  assert(!f.is_open());
+  std::filesystem::path p = get_temp_file_name();
+  f.open(p);
+  assert(f.is_open());
+  assert(f.native_handle() == f.rdbuf()->native_handle());
+  assert(is_handle_valid(f.native_handle()));
+  assert(is_handle_valid(std::as_const(f).native_handle()));
+  static_assert(noexcept(f.native_handle()));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
new file mode 100644
index 00000000000000..cd69c8087608df
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++11, c++14, c++17, c++20, c++23
+
+// <fstream>
+
+// class basic_ofstream;
+
+// native_handle_type native_handle() const noexcept;
+
+#include <cassert>
+#include <fstream>
+#include <filesystem>
+#include <utility>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#endif
+
+#include "platform_support.h"
+#include "test_macros.h"
+#include "../test_helpers.h"
+
+template <typename CharT>
+void test() {
+  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
+                               typename std::basic_ofstream<CharT>::native_handle_type>);
+
+  std::basic_ofstream<CharT> f;
+  static_assert(noexcept(f.native_handle()));
+  assert(!f.is_open());
+  std::filesystem::path p = get_temp_file_name();
+  f.open(p);
+  assert(f.is_open());
+  assert(f.native_handle() == f.rdbuf()->native_handle());
+  assert(f.native_handle());
+  assert(std::as_const(f).native_handle());
+  assert(is_handle_valid(f.native_handle()));
+  assert(is_handle_valid(std::as_const(f).native_handle()));
+  static_assert(noexcept(f.native_handle()));
+}
+
+int main(int, char**) {
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
new file mode 100644
index 00000000000000..06ddbf8287f039
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
+#define TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
+
+#include <cstdio>
+
+#if defined(_LIBCPP_WIN32API)
+#  define WIN32_LEAN_AND_MEAN
+#  define NOMINMAX
+#  include <io.h>
+#  include <windows.h>
+#else
+#  include <fcntl.h>
+#endif
+
+#if defined(_LIBCPP_WIN32API)
+auto is_handle_valid([[maybe_unused]] HANDLE handle) {
+  // TODO: GetFileInformationByHandle
+  return true;
+};
+#else
+// POSIX
+auto is_handle_valid(int fd) { return fcntl(fd, F_GETFL) != -1 || errno != EBADF; };
+#endif
+
+#endif // TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
\ No newline at end of file

>From 37a871ace443c45dae2c048a91de712a1c369fe4 Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sun, 31 Dec 2023 11:25:48 +0200
Subject: [PATCH 3/7] Added missing include

---
 libcxx/include/fstream | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index f0cb5bb50fc9b6..5598943268276d 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -195,6 +195,7 @@ typedef basic_fstream<wchar_t> wfstream;
 #include <__utility/move.h>
 #include <__utility/swap.h>
 #include <__utility/unreachable.h>
+#include <cassert>
 #include <cstdio>
 #include <filesystem>
 #include <istream>

>From 5aeb33918bd80840469cec803ba054f1f4aeec8e Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sun, 31 Dec 2023 14:38:06 +0200
Subject: [PATCH 4/7] WIP: Addressed some comments and simplified/generalized
 the tests

---
 libcxx/include/fstream                        | 22 +++-----
 libcxx/src/fstream.cpp                        |  9 +--
 .../fstream.members/native_handle.pass.cpp    | 34 +-----------
 .../ifstream.members/native_handle.pass.cpp   | 35 +-----------
 .../ofstream.members/native_handle.pass.cpp   | 36 +-----------
 .../file.streams/fstreams/test_helpers.h      | 55 ++++++++++++++-----
 6 files changed, 60 insertions(+), 131 deletions(-)

diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 5598943268276d..ab854cf534e7fb 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -195,7 +195,6 @@ typedef basic_fstream<wchar_t> wfstream;
 #include <__utility/move.h>
 #include <__utility/swap.h>
 #include <__utility/unreachable.h>
-#include <cassert>
 #include <cstdio>
 #include <filesystem>
 #include <istream>
@@ -223,24 +222,21 @@ _LIBCPP_PUSH_MACROS
 #    include <windows.h>
 #  endif
 
-#include <fcntl.h>
+// #  include <fcntl.h>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #  if _LIBCPP_STD_VER >= 26
 #    if defined(_LIBCPP_WIN32API)
-using __filebuf_native_handle_type = void*; // HANDLE
-#    else
-using __filebuf_native_handle_type = int; // POSIX file descriptor
+using __filebuf_native_handle_type = HANDLE;
+_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_windows_native_handle(FILE* __file);
+#    else // POSIX
+using __filebuf_native_handle_type = int; // File descriptor
 #    endif
-_LIBCPP_HIDE_FROM_ABI __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept;
-__filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
+
+_LIBCPP_HIDE_FROM_ABI inline __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
 #    if defined(_LIBCPP_WIN32API)
-  // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
-  intptr_t __handle = _get_osfhandle(::fileno(__file));
-  if (__handle == -1)
-    return nullptr;
-  return reinterpret_cast<void*>(__handle);
+  return __filebuf_windows_native_handle(__file);
 #    else
   return ::fileno(__file);
 #    endif
@@ -287,7 +283,7 @@ public:
   basic_filebuf* close();
 #  if _LIBCPP_STD_VER >= 26
   _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
-    assert(is_open());
+    _LIBCPP_ASSERT_UNCATEGORIZED(this->is_open(), "File must be opened");
     return std::__filebuf_native_handle(__file_);
   }
 #  endif
diff --git a/libcxx/src/fstream.cpp b/libcxx/src/fstream.cpp
index e5611e2d48b0a6..814d912e7d22b3 100644
--- a/libcxx/src/fstream.cpp
+++ b/libcxx/src/fstream.cpp
@@ -20,17 +20,14 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 26
-_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
 #  if defined(_LIBCPP_WIN32API)
+_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_windows_native_handle(FILE* __file) noexcept {
   // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
   intptr_t __handle = _get_osfhandle(::fileno(__file));
   if (__handle == -1)
     return nullptr;
-  return reinterpret_cast<void*>(__handle);
-#  else
-  return ::fileno(__file);
+  return reinterpret_cast<__filebuf_native_handle_type>(__handle);
 #  endif
-}
 #endif
 
-_LIBCPP_END_NAMESPACE_STD
+  _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
index 44da8bd3e18bad..900475069a1c9b 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/fstream.members/native_handle.pass.cpp
@@ -14,43 +14,13 @@
 
 // native_handle_type native_handle() const noexcept;
 
-#include <cassert>
-#include <fstream>
-#include <filesystem>
-#include <utility>
-
-#if defined(_LIBCPP_WIN32API)
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
-#  include <io.h>
-#  include <windows.h>
-#endif
-
-#include "platform_support.h"
 #include "test_macros.h"
 #include "../test_helpers.h"
 
-template <typename CharT>
-void test() {
-  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
-                               typename std::basic_fstream<CharT>::native_handle_type>);
-
-  std::basic_fstream<CharT> f;
-  static_assert(noexcept(f.native_handle()));
-  assert(!f.is_open());
-  std::filesystem::path p = get_temp_file_name();
-  f.open(p);
-  assert(f.is_open());
-  assert(f.native_handle() == f.rdbuf()->native_handle());
-  assert(is_handle_valid(f.native_handle()));
-  assert(is_handle_valid(std::as_const(f).native_handle()));
-  static_assert(noexcept(f.native_handle()));
-}
-
 int main(int, char**) {
-  test<char>();
+  test_native_handle<char, std::basic_fstream<char>>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  test<wchar_t>();
+  test_native_handle<wchar_t, std::basic_fstream<wchar_t>>();
 #endif
 
   return 0;
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
index 1345b72b158e28..ffe5e922225da8 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
@@ -14,44 +14,15 @@
 
 // native_handle_type native_handle() const noexcept;
 
-#include <cassert>
-#include <fstream>
-#include <filesystem>
-#include <utility>
-
-#if defined(_LIBCPP_WIN32API)
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
-#  include <io.h>
-#  include <windows.h>
-#endif
-
-#include "platform_support.h"
 #include "test_macros.h"
 #include "../test_helpers.h"
 
-template <typename CharT>
-void test() {
-  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
-                               typename std::basic_ifstream<CharT>::native_handle_type>);
-
-  std::basic_ifstream<CharT> f;
-  static_assert(noexcept(f.native_handle()));
-  assert(!f.is_open());
-  std::filesystem::path p = get_temp_file_name();
-  f.open(p);
-  assert(f.is_open());
-  assert(f.native_handle() == f.rdbuf()->native_handle());
-  assert(is_handle_valid(f.native_handle()));
-  assert(is_handle_valid(std::as_const(f).native_handle()));
-  static_assert(noexcept(f.native_handle()));
-}
-
 int main(int, char**) {
-  test<char>();
+  test_native_handle<char, std::basic_ifstream<char>>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  test<wchar_t>();
+  test_native_handle<wchar_t, std::basic_ifstream<wchar_t>>();
 #endif
 
   return 0;
 }
+
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
index cd69c8087608df..45f0841dad18cb 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/native_handle.pass.cpp
@@ -14,45 +14,13 @@
 
 // native_handle_type native_handle() const noexcept;
 
-#include <cassert>
-#include <fstream>
-#include <filesystem>
-#include <utility>
-
-#if defined(_LIBCPP_WIN32API)
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
-#  include <io.h>
-#  include <windows.h>
-#endif
-
-#include "platform_support.h"
 #include "test_macros.h"
 #include "../test_helpers.h"
 
-template <typename CharT>
-void test() {
-  static_assert(std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type,
-                               typename std::basic_ofstream<CharT>::native_handle_type>);
-
-  std::basic_ofstream<CharT> f;
-  static_assert(noexcept(f.native_handle()));
-  assert(!f.is_open());
-  std::filesystem::path p = get_temp_file_name();
-  f.open(p);
-  assert(f.is_open());
-  assert(f.native_handle() == f.rdbuf()->native_handle());
-  assert(f.native_handle());
-  assert(std::as_const(f).native_handle());
-  assert(is_handle_valid(f.native_handle()));
-  assert(is_handle_valid(std::as_const(f).native_handle()));
-  static_assert(noexcept(f.native_handle()));
-}
-
 int main(int, char**) {
-  test<char>();
+  test_native_handle<char, std::basic_ofstream<char>>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  test<wchar_t>();
+  test_native_handle<wchar_t, std::basic_ofstream<wchar_t>>();
 #endif
 
   return 0;
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
index 06ddbf8287f039..7691c04c750448 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
+++ b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
@@ -9,25 +9,52 @@
 #ifndef TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
 #define TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
 
-#include <cstdio>
-
-#if defined(_LIBCPP_WIN32API)
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
-#  include <io.h>
-#  include <windows.h>
-#else
-#  include <fcntl.h>
-#endif
-
-#if defined(_LIBCPP_WIN32API)
+#if _LIBCPP_STD_VER >= 26
+
+#  include <cassert>
+#  include <cstdio>
+#  include <fstream>
+#  include <filesystem>
+#  include <utility>
+
+#  if defined(_LIBCPP_WIN32API)
+#    define WIN32_LEAN_AND_MEAN
+#    define NOMINMAX
+#    include <io.h>
+#    include <windows.h>
+#  else
+#    include <fcntl.h>
+#  endif
+
+#  include "platform_support.h"
+
+#  if defined(_LIBCPP_WIN32API)
 auto is_handle_valid([[maybe_unused]] HANDLE handle) {
   // TODO: GetFileInformationByHandle
   return true;
 };
-#else
+#  else
 // POSIX
 auto is_handle_valid(int fd) { return fcntl(fd, F_GETFL) != -1 || errno != EBADF; };
-#endif
+#  endif
+
+template <typename CharT, typename StreamT>
+void test_native_handle() {
+  static_assert(
+      std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type, typename StreamT::native_handle_type>);
+
+  StreamT f;
+  static_assert(noexcept(f.native_handle()));
+  assert(!f.is_open());
+  std::filesystem::path p = get_temp_file_name();
+  f.open(p);
+  assert(f.is_open());
+  assert(f.native_handle() == f.rdbuf()->native_handle());
+  assert(is_handle_valid(f.native_handle()));
+  assert(is_handle_valid(std::as_const(f).native_handle()));
+  static_assert(noexcept(f.native_handle()));
+}
+
+#endif // _LIBCPP_STD_VER >= 26
 
 #endif // TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
\ No newline at end of file

>From 822633dd56bf65b25a265aafb3c16b7e1f071bb4 Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sun, 31 Dec 2023 14:48:56 +0200
Subject: [PATCH 5/7] WIP: Fixed formatting error

---
 .../fstreams/filebuf.members/native_handle.pass.cpp        | 7 -------
 .../fstreams/ifstream.members/native_handle.pass.cpp       | 1 -
 .../std/input.output/file.streams/fstreams/test_helpers.h  | 4 ++--
 3 files changed, 2 insertions(+), 10 deletions(-)

diff --git a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
index 4104aa061e1546..790cc91effc822 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
@@ -19,13 +19,6 @@
 #include <filesystem>
 #include <utility>
 
-#if defined(_LIBCPP_WIN32API)
-#  define WIN32_LEAN_AND_MEAN
-#  define NOMINMAX
-#  include <io.h>
-#  include <windows.h>
-#endif
-
 #include "platform_support.h"
 #include "test_macros.h"
 #include "../test_helpers.h"
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
index ffe5e922225da8..95a95cb69560b7 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/native_handle.pass.cpp
@@ -25,4 +25,3 @@ int main(int, char**) {
 
   return 0;
 }
-
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
index 7691c04c750448..a3069549315611 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
+++ b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
@@ -30,7 +30,7 @@
 
 #  if defined(_LIBCPP_WIN32API)
 auto is_handle_valid([[maybe_unused]] HANDLE handle) {
-  // TODO: GetFileInformationByHandle
+  // TODO: Maybe test HANDLE with GetFileInformationByHandle??
   return true;
 };
 #  else
@@ -57,4 +57,4 @@ void test_native_handle() {
 
 #endif // _LIBCPP_STD_VER >= 26
 
-#endif // TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H
\ No newline at end of file
+#endif // TEST_STD_INPUT_OUTPUT_FILE_STREAMS_FSTREAMS_TEST_HELPERS_H

>From ecaf77e4da4fa56db9a07b738bcc0b75fa2b7b5a Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Sun, 31 Dec 2023 15:18:13 +0200
Subject: [PATCH 6/7] WIP: Refactored `fstream->native_handle()` implementation

---
 libcxx/include/fstream                        | 38 ++++++++-----------
 libcxx/src/fstream.cpp                        |  4 +-
 .../file.streams/fstreams/test_helpers.h      |  7 ++--
 3 files changed, 20 insertions(+), 29 deletions(-)

diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index ab854cf534e7fb..6f6e36f259f381 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -215,32 +215,12 @@ _LIBCPP_PUSH_MACROS
 
 #if !defined(_LIBCPP_HAS_NO_FILESYSTEM)
 
-#  if defined(_LIBCPP_WIN32API)
-#    define WIN32_LEAN_AND_MEAN
-#    define NOMINMAX
-#    include <io.h>
-#    include <windows.h>
-#  endif
-
-// #  include <fcntl.h>
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #  if _LIBCPP_STD_VER >= 26
 #    if defined(_LIBCPP_WIN32API)
-using __filebuf_native_handle_type = HANDLE;
-_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_windows_native_handle(FILE* __file);
-#    else // POSIX
-using __filebuf_native_handle_type = int; // File descriptor
-#    endif
-
-_LIBCPP_HIDE_FROM_ABI inline __filebuf_native_handle_type __filebuf_native_handle(FILE* __file) noexcept {
-#    if defined(_LIBCPP_WIN32API)
-  return __filebuf_windows_native_handle(__file);
-#    else
-  return ::fileno(__file);
+_LIBCPP_EXPORTED_FROM_ABI void* __filebuf_windows_native_handle(FILE* __file);
 #    endif
-}
 #  endif
 
 template <class _CharT, class _Traits>
@@ -253,7 +233,13 @@ public:
   typedef typename traits_type::off_type off_type;
   typedef typename traits_type::state_type state_type;
 #  if _LIBCPP_STD_VER >= 26
-  using native_handle_type = __filebuf_native_handle_type;
+#    if defined(_LIBCPP_WIN32API)
+  using native_handle_type = void*; // HANDLE
+#    elif __has_include(<unistd.h>)
+  using native_handle_type = int; // POSIX file descriptor
+#    else
+#      error "Provide a file native handle!"
+#    endif
 #  endif
 
   // 27.9.1.2 Constructors/destructor:
@@ -284,7 +270,13 @@ public:
 #  if _LIBCPP_STD_VER >= 26
   _LIBCPP_HIDE_FROM_ABI native_handle_type native_handle() const noexcept {
     _LIBCPP_ASSERT_UNCATEGORIZED(this->is_open(), "File must be opened");
-    return std::__filebuf_native_handle(__file_);
+#    if defined(_LIBCPP_WIN32API)
+    return __filebuf_windows_native_handle(__file_);
+#    elif __has_include(<unistd.h>)
+    return ::fileno(__file_);
+#    else
+#      error "Provide a way to determine the file native handle!
+#    endif
   }
 #  endif
 
diff --git a/libcxx/src/fstream.cpp b/libcxx/src/fstream.cpp
index 814d912e7d22b3..27150389627594 100644
--- a/libcxx/src/fstream.cpp
+++ b/libcxx/src/fstream.cpp
@@ -21,12 +21,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 26
 #  if defined(_LIBCPP_WIN32API)
-_LIBCPP_EXPORTED_FROM_ABI __filebuf_native_handle_type __filebuf_windows_native_handle(FILE* __file) noexcept {
+_LIBCPP_EXPORTED_FROM_ABI void* __filebuf_windows_native_handle(FILE* __file) noexcept {
   // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
   intptr_t __handle = _get_osfhandle(::fileno(__file));
   if (__handle == -1)
     return nullptr;
-  return reinterpret_cast<__filebuf_native_handle_type>(__handle);
+  return reinterpret_cast<void*>(__handle);
 #  endif
 #endif
 
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
index a3069549315611..fd6d732638ad32 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
+++ b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
@@ -29,13 +29,12 @@
 #  include "platform_support.h"
 
 #  if defined(_LIBCPP_WIN32API)
-auto is_handle_valid([[maybe_unused]] HANDLE handle) {
+bool is_handle_valid([[maybe_unused]] HANDLE handle) {
   // TODO: Maybe test HANDLE with GetFileInformationByHandle??
   return true;
 };
-#  else
-// POSIX
-auto is_handle_valid(int fd) { return fcntl(fd, F_GETFL) != -1 || errno != EBADF; };
+#  else // POSIX
+bool is_handle_valid(int fd) { return fcntl(fd, F_GETFL) != -1 || errno != EBADF; };
 #  endif
 
 template <typename CharT, typename StreamT>

>From a066b7957ee85e6c208e531c12b6adf96a8ef3f8 Mon Sep 17 00:00:00 2001
From: Zingam <zingam at outlook.com>
Date: Mon, 1 Jan 2024 09:17:09 +0200
Subject: [PATCH 7/7] Addressed comments

---
 libcxx/include/fstream                        |  2 +-
 libcxx/src/fstream.cpp                        |  9 ++++---
 .../filebuf.members/native_handle.pass.cpp    | 14 ++++++++--
 .../file.streams/fstreams/test_helpers.h      | 27 ++++++++++++++-----
 4 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 6f6e36f259f381..287c4280f18e99 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -238,7 +238,7 @@ public:
 #    elif __has_include(<unistd.h>)
   using native_handle_type = int; // POSIX file descriptor
 #    else
-#      error "Provide a file native handle!"
+#      error "Provide a native file handle!"
 #    endif
 #  endif
 
diff --git a/libcxx/src/fstream.cpp b/libcxx/src/fstream.cpp
index 27150389627594..f063b7020cb373 100644
--- a/libcxx/src/fstream.cpp
+++ b/libcxx/src/fstream.cpp
@@ -19,15 +19,18 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 26
-#  if defined(_LIBCPP_WIN32API)
+#if defined(_LIBCPP_WIN32API)
+
+// Confirm that `HANDLE` is `void*`
+static_assert(std::same_as<HANDLE, void*>);
+
 _LIBCPP_EXPORTED_FROM_ABI void* __filebuf_windows_native_handle(FILE* __file) noexcept {
   // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/get-osfhandle?view=msvc-170
   intptr_t __handle = _get_osfhandle(::fileno(__file));
   if (__handle == -1)
     return nullptr;
   return reinterpret_cast<void*>(__handle);
-#  endif
+
 #endif
 
   _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
index 790cc91effc822..c87bda5382c85e 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/native_handle.pass.cpp
@@ -30,9 +30,19 @@ void test() {
   std::filesystem::path p = get_temp_file_name();
   f.open(p, std::ios_base::in);
   assert(f.is_open());
-  assert(is_handle_valid(f.native_handle()));
-  assert(is_handle_valid(std::as_const(f).native_handle()));
+#if defined(_LIBCPP_WIN32API)
+  using HandleT = void*;
+#elif __has_include(<unistd.h>) // POSIX
+  using HandleT = int;
+#else
+#  error "Provide a native file handle!"
+#endif
+  std::same_as<HandleT> decltype(auto) handle = f.native_handle();
+  assert(is_handle_valid(handle));
+  std::same_as<HandleT> decltype(auto) const_handle = std::as_const(f).native_handle();
+  assert(is_handle_valid(const_handle));
   static_assert(noexcept(f.native_handle()));
+  static_assert(noexcept(std::as_const(f).native_handle()));
 }
 
 int main(int, char**) {
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
index fd6d732638ad32..e32c8fb5fc9de3 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
+++ b/libcxx/test/std/input.output/file.streams/fstreams/test_helpers.h
@@ -12,9 +12,11 @@
 #if _LIBCPP_STD_VER >= 26
 
 #  include <cassert>
+#  include <concepts>
 #  include <cstdio>
 #  include <fstream>
 #  include <filesystem>
+#  include <type_traits>
 #  include <utility>
 
 #  if defined(_LIBCPP_WIN32API)
@@ -29,12 +31,15 @@
 #  include "platform_support.h"
 
 #  if defined(_LIBCPP_WIN32API)
-bool is_handle_valid([[maybe_unused]] HANDLE handle) {
-  // TODO: Maybe test HANDLE with GetFileInformationByHandle??
+bool is_handle_valid([[void* handle) {
+  if (LPBY_HANDLE_FILE_INFORMATION & pFileInformation; !GetFileInformationByHandle(handle, &lpFileInformation))
+    return false;
   return true;
 };
-#  else // POSIX
+#  elif __has_include(<unistd.h>) // POSIX
 bool is_handle_valid(int fd) { return fcntl(fd, F_GETFL) != -1 || errno != EBADF; };
+#  else
+#    error "Provide a native file handle!"
 #  endif
 
 template <typename CharT, typename StreamT>
@@ -43,15 +48,25 @@ void test_native_handle() {
       std::is_same_v<typename std::basic_filebuf<CharT>::native_handle_type, typename StreamT::native_handle_type>);
 
   StreamT f;
-  static_assert(noexcept(f.native_handle()));
+
   assert(!f.is_open());
   std::filesystem::path p = get_temp_file_name();
   f.open(p);
   assert(f.is_open());
   assert(f.native_handle() == f.rdbuf()->native_handle());
-  assert(is_handle_valid(f.native_handle()));
-  assert(is_handle_valid(std::as_const(f).native_handle()));
+#  if defined(_LIBCPP_WIN32API)
+  using HandleT = void*;
+#  elif __has_include(<unistd.h>) // POSIX
+  using HandleT = int;
+#  else
+#    error "Provide a native file handle!"
+#  endif
+  std::same_as<HandleT> decltype(auto) handle = f.native_handle();
+  assert(is_handle_valid(handle));
+  std::same_as<HandleT> decltype(auto) const_handle = std::as_const(f).native_handle();
+  assert(is_handle_valid(const_handle));
   static_assert(noexcept(f.native_handle()));
+  static_assert(noexcept(std::as_const(f).native_handle()));
 }
 
 #endif // _LIBCPP_STD_VER >= 26