[libc++] Implement LWG3430 disallow implicit conversion of the source arguments to std::filesystem::path when constructing std::basic_*fstream (#85079)

Implement [LWG3430](https://wg21.link/LWG3430).

---------

Signed-off-by: yronglin <[email protected]>

Author: yronglin

libcxx/docs/ReleaseNotes/19.rst

@@ -92,6 +92,11 @@ Deprecations and Removals
   libatomic is not available. If you are one such user, please reach out to the libc++ developers so we can collaborate
   on a path for supporting atomics properly on freestanding platforms.
 
+- LWG3430 disallow implicit conversion of the source arguments to ``std::filesystem::path`` when
+  constructing ``std::basic_*fstream``. This effectively removes the possibility to directly construct
+  a ``std::basic_*fstream`` from a ``std::basic_string_view``, a input-iterator or a C-string, instead
+  you can construct a temporary ``std::basic_string``. This change has been applied to C++17 and later.
+
 
 Upcoming Deprecations and Removals
 ----------------------------------

libcxx/docs/Status/Cxx23Issues.csv

@@ -64,7 +64,7 @@
 `2818 <https://wg21.link/LWG2818>`__,"``::std::`` everywhere rule needs tweaking","June 2021","|Nothing To Do|",""
 `2997 <https://wg21.link/LWG2997>`__,"LWG 491 and the specification of ``{forward_,}list::unique``","June 2021","",""
 `3410 <https://wg21.link/LWG3410>`__,"``lexicographical_compare_three_way`` is overspecified","June 2021","|Complete|","17.0","|spaceship|"
-`3430 <https://wg21.link/LWG3430>`__,"``std::fstream`` & co. should be constructible from string_view","June 2021","",""
+`3430 <https://wg21.link/LWG3430>`__,"``std::fstream`` & co. should be constructible from string_view","June 2021","|Complete|","19.0",""
 `3462 <https://wg21.link/LWG3462>`__,"§[formatter.requirements]: Formatter requirements forbid use of ``fc.arg()``","June 2021","|Nothing To Do|","","|format|"
 `3481 <https://wg21.link/LWG3481>`__,"``viewable_range`` mishandles lvalue move-only views","June 2021","Superseded by `P2415R2 <https://wg21.link/P2415R2>`__","","|ranges|"
 `3506 <https://wg21.link/LWG3506>`__,"Missing allocator-extended constructors for ``priority_queue``","June 2021","|Complete|","14.0"

libcxx/include/fstream

@@ -78,8 +78,8 @@ public:
     basic_ifstream();
     explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in);
     explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in);
-    explicit basic_ifstream(const filesystem::path& p,
-                            ios_base::openmode mode = ios_base::in); // C++17
+    template<class T>
+    explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); // Since C++17
     basic_ifstream(basic_ifstream&& rhs);
 
     basic_ifstream& operator=(basic_ifstream&& rhs);
@@ -117,8 +117,8 @@ public:
     basic_ofstream();
     explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out);
     explicit basic_ofstream(const string& s, ios_base::openmode mode = ios_base::out);
-    explicit basic_ofstream(const filesystem::path& p,
-                            ios_base::openmode mode = ios_base::out); // C++17
+    template<class T>
+    explicit basic_ofstream(const T& s, ios_base::openmode mode = ios_base::out); // Since C++17
     basic_ofstream(basic_ofstream&& rhs);
 
     basic_ofstream& operator=(basic_ofstream&& rhs);
@@ -158,8 +158,8 @@ public:
     basic_fstream();
     explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
     explicit basic_fstream(const string& s, ios_base::openmode mode = ios_base::in|ios_base::out);
-    explicit basic_fstream(const filesystem::path& p,
-                           ios_base::openmode mode = ios_base::in|ios_base::out); C++17
+    template<class T>
+    explicit basic_fstream(const T& s, ios_base::openmode mode = ios_base::in | ios_base::out); // Since C++17
     basic_fstream(basic_fstream&& rhs);
 
     basic_fstream& operator=(basic_fstream&& rhs);
@@ -192,6 +192,8 @@ typedef basic_fstream<wchar_t> wfstream;
 #include <__config>
 #include <__fwd/fstream.h>
 #include <__locale>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_same.h>
 #include <__utility/move.h>
 #include <__utility/swap.h>
 #include <__utility/unreachable.h>
@@ -1101,8 +1103,9 @@ public:
 #  endif
   _LIBCPP_HIDE_FROM_ABI explicit basic_ifstream(const string& __s, ios_base::openmode __mode = ios_base::in);
 #  if _LIBCPP_STD_VER >= 17
+  template <class _Tp, class = enable_if_t<is_same_v<_Tp, filesystem::path>>>
   _LIBCPP_AVAILABILITY_FILESYSTEM_LIBRARY _LIBCPP_HIDE_FROM_ABI explicit basic_ifstream(
-      const filesystem::path& __p, ios_base::openmode __mode = ios_base::in)
+      const _Tp& __p, ios_base::openmode __mode = ios_base::in)
       : basic_ifstream(__p.c_str(), __mode) {}
 #  endif // _LIBCPP_STD_VER >= 17
   _LIBCPP_HIDE_FROM_ABI basic_ifstream(basic_ifstream&& __rhs);
@@ -1255,8 +1258,9 @@ public:
   _LIBCPP_HIDE_FROM_ABI explicit basic_ofstream(const string& __s, ios_base::openmode __mode = ios_base::out);
 
 #  if _LIBCPP_STD_VER >= 17
+  template <class _Tp, class = enable_if_t<is_same_v<_Tp, filesystem::path>>>
   _LIBCPP_AVAILABILITY_FILESYSTEM_LIBRARY _LIBCPP_HIDE_FROM_ABI explicit basic_ofstream(
-      const filesystem::path& __p, ios_base::openmode __mode = ios_base::out)
+      const _Tp& __p, ios_base::openmode __mode = ios_base::out)
       : basic_ofstream(__p.c_str(), __mode) {}
 #  endif // _LIBCPP_STD_VER >= 17
 
@@ -1414,8 +1418,9 @@ public:
                                                ios_base::openmode __mode = ios_base::in | ios_base::out);
 
 #  if _LIBCPP_STD_VER >= 17
+  template <class _Tp, class = enable_if_t<is_same_v<_Tp, filesystem::path>>>
   _LIBCPP_AVAILABILITY_FILESYSTEM_LIBRARY _LIBCPP_HIDE_FROM_ABI explicit basic_fstream(
-      const filesystem::path& __p, ios_base::openmode __mode = ios_base::in | ios_base::out)
+      const _Tp& __p, ios_base::openmode __mode = ios_base::in | ios_base::out)
       : basic_fstream(__p.c_str(), __mode) {}
 #  endif // _LIBCPP_STD_VER >= 17
 

libcxx/test/std/input.output/file.streams/fstreams/fstream.cons/path.pass.cpp

@@ -15,17 +15,50 @@
 // plate <class charT, class traits = char_traits<charT> >
 // class basic_fstream
 
-// explicit basic_fstream(const filesystem::path& s,
-//     ios_base::openmode mode = ios_base::in|ios_base::out);
+// template<class T>
+// explicit basic_fstream(const T& s, ios_base::openmode mode = ios_base::in); // Since C++17
+// Constraints: is_same_v<T, filesystem::path> is true
 
 #include <fstream>
 #include <filesystem>
 #include <cassert>
+#include <type_traits>
+
 #include "test_macros.h"
+#include "test_iterators.h"
 #include "platform_support.h"
 
 namespace fs = std::filesystem;
 
+template <class CharT>
+constexpr bool test_non_convert_to_path() {
+  // String types
+  static_assert(!std::is_constructible_v<std::fstream, std::basic_string_view<CharT>>);
+  static_assert(!std::is_constructible_v<std::fstream, const std::basic_string_view<CharT>>);
+
+  // Char* pointers
+  if constexpr (!std::is_same_v<CharT, char>)
+    static_assert(!std::is_constructible_v<std::fstream, const CharT*>);
+
+  // Iterators
+  static_assert(!std::is_convertible_v<std::fstream, cpp17_input_iterator<const CharT*>>);
+
+  return true;
+}
+
+static_assert(test_non_convert_to_path<char>());
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && !defined(TEST_HAS_OPEN_WITH_WCHAR)
+static_assert(test_non_convert_to_path<wchar_t>());
+#endif // !TEST_HAS_NO_WIDE_CHARACTERS && !TEST_HAS_OPEN_WITH_WCHAR
+
+#ifndef TEST_HAS_NO_CHAR8_T
+static_assert(test_non_convert_to_path<char8_t>());
+#endif // TEST_HAS_NO_CHAR8_T
+
+static_assert(test_non_convert_to_path<char16_t>());
+static_assert(test_non_convert_to_path<char32_t>());
+
 int main(int, char**) {
   fs::path p = get_temp_file_name();
   {

libcxx/test/std/input.output/file.streams/fstreams/ifstream.cons/path.pass.cpp

@@ -17,18 +17,49 @@
 // template <class charT, class traits = char_traits<charT> >
 // class basic_ifstream
 
-// explicit basic_ifstream(const filesystem::path& s,
-//     ios_base::openmode mode = ios_base::in);
+// template<class T>
+// explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); // Since C++17
+// Constraints: is_same_v<T, filesystem::path> is true
 
 #include <cassert>
 #include <filesystem>
 #include <fstream>
 #include <type_traits>
 
 #include "test_macros.h"
+#include "test_iterators.h"
 
 namespace fs = std::filesystem;
 
+template <class CharT>
+constexpr bool test_non_convert_to_path() {
+  // String types
+  static_assert(!std::is_constructible_v<std::ifstream, std::basic_string_view<CharT>>);
+  static_assert(!std::is_constructible_v<std::ifstream, const std::basic_string_view<CharT>>);
+
+  // Char* pointers
+  if constexpr (!std::is_same_v<CharT, char>)
+    static_assert(!std::is_constructible_v<std::ifstream, const CharT*>);
+
+  // Iterators
+  static_assert(!std::is_convertible_v<std::ifstream, cpp17_input_iterator<const CharT*>>);
+
+  return true;
+}
+
+static_assert(test_non_convert_to_path<char>());
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && !defined(TEST_HAS_OPEN_WITH_WCHAR)
+static_assert(test_non_convert_to_path<wchar_t>());
+#endif // !TEST_HAS_NO_WIDE_CHARACTERS && !TEST_HAS_OPEN_WITH_WCHAR
+
+#ifndef TEST_HAS_NO_CHAR8_T
+static_assert(test_non_convert_to_path<char8_t>());
+#endif // TEST_HAS_NO_CHAR8_T
+
+static_assert(test_non_convert_to_path<char16_t>());
+static_assert(test_non_convert_to_path<char32_t>());
+
 int main(int, char**) {
   {
     fs::path p;

libcxx/test/std/input.output/file.streams/fstreams/ofstream.cons/path.pass.cpp

@@ -15,7 +15,9 @@
 // plate <class charT, class traits = char_traits<charT> >
 // class basic_ofstream
 
-// explicit basic_ofstream(const filesystem::path& s, ios_base::openmode mode = ios_base::out);
+// template<class T>
+// explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); // Since C++17
+// Constraints: is_same_v<T, filesystem::path> is true
 
 #include <cassert>
 #include <filesystem>
@@ -24,9 +26,39 @@
 
 #include "platform_support.h"
 #include "test_macros.h"
+#include "test_iterators.h"
 
 namespace fs = std::filesystem;
 
+template <class CharT>
+constexpr bool test_non_convert_to_path() {
+  // String types
+  static_assert(!std::is_constructible_v<std::ofstream, std::basic_string_view<CharT>>);
+  static_assert(!std::is_constructible_v<std::ofstream, const std::basic_string_view<CharT>>);
+
+  // Char* pointers
+  if constexpr (!std::is_same_v<CharT, char>)
+    static_assert(!std::is_constructible_v<std::ofstream, const CharT*>);
+
+  // Iterators
+  static_assert(!std::is_convertible_v<std::ofstream, cpp17_input_iterator<const CharT*>>);
+
+  return true;
+}
+
+static_assert(test_non_convert_to_path<char>());
+
+#if !defined(TEST_HAS_NO_WIDE_CHARACTERS) && !defined(TEST_HAS_OPEN_WITH_WCHAR)
+static_assert(test_non_convert_to_path<wchar_t>());
+#endif // !TEST_HAS_NO_WIDE_CHARACTERS && !TEST_HAS_OPEN_WITH_WCHAR
+
+#ifndef TEST_HAS_NO_CHAR8_T
+static_assert(test_non_convert_to_path<char8_t>());
+#endif // TEST_HAS_NO_CHAR8_T
+
+static_assert(test_non_convert_to_path<char16_t>());
+static_assert(test_non_convert_to_path<char32_t>());
+
 int main(int, char**) {
   fs::path p = get_temp_file_name();
   {

libcxx/test/support/test_macros.h

@@ -385,6 +385,10 @@ inline Tp const& DoNotOptimize(Tp const& value) {
 #   define TEST_HAS_NO_UNICODE
 #endif
 
+#if defined(_LIBCPP_HAS_OPEN_WITH_WCHAR)
+#  define TEST_HAS_OPEN_WITH_WCHAR
+#endif
+
 #if defined(_LIBCPP_HAS_NO_INT128) || defined(_MSVC_STL_VERSION)
 #   define TEST_HAS_NO_INT128
 #endif