[libc++] Implement LWG3657 std::hash<filesystem::path>

[ldionne]

Summary

This is implemented as a DR on top of C++17.

[jloser]

Seems reasonable to me. We should close https://reviews.llvm.org/D125394 when/if we land your review here.

[ldionne]

Rebase onto main

[ldionne]

Fix namespace qualification.

[philnik]

LGTM % nits.

libcxx/include/__filesystem/path.h

1094

I don't think we have to inherit from unary_function here, and I don't really see a point if we don't have to.

libcxx/test/std/input.output/filesystems/class.path/path.member/path.compare.pass.cpp

138–139

Unfortunatley, we provide std::filesystem as an extension in C++11 and C++14 under std::__fs::filesystem.

[ldionne]

Fix test in C++11/C++14

[ldionne]

libcxx/include/__filesystem/path.h

1094

The std::hash base template is specified to have those members in C++17 even though they are deprecated. So to be strictly conforming, I think it's better to provide them.

[ldionne]

Fix backdeployment issue

libcxx/docs/Status/Cxx2bIssues.csv

 "`3619 <https://wg21.link/LWG3619>`__","Specification of ``vformat_to`` contains ill-formed ``formatted_size`` calls","February 2022","|Nothing to do|","","|format|"
 "`3621 <https://wg21.link/LWG3621>`__","Remove feature-test macro ``__cpp_lib_monadic_optional`` ","February 2022","|Complete|","15.0"
 "`3632 <https://wg21.link/LWG3632>`__","``unique_ptr`` ""Mandates: This constructor is not selected by class template argument deduction""","February 2022","|Nothing to do|",""
 "`3643 <https://wg21.link/LWG3643>`__","Missing ``constexpr`` in ``std::counted_iterator`` ","February 2022","","","|ranges|"
 "`3648 <https://wg21.link/LWG3648>`__","``format`` should not print ``bool`` with ``'c'`` ","February 2022","|Complete|","15.0","|format|"
 "`3649 <https://wg21.link/LWG3649>`__","[fund.ts.v2] Reinstate and bump ``__cpp_lib_experimental_memory_resource`` feature test macro","February 2022","",""
 "`3650 <https://wg21.link/LWG3650>`__","Are ``std::basic_string`` 's ``iterator`` and ``const_iterator`` constexpr iterators?","February 2022","|Nothing to do|",""
 "`3654 <https://wg21.link/LWG3654>`__","``basic_format_context::arg(size_t)`` should be ``noexcept`` ","February 2022","|Complete|","15.0","|format|"
-"`3657 <https://wg21.link/LWG3657>`__","``std::hash<std::filesystem::path>`` is not enabled","February 2022","",""
+"`3657 <https://wg21.link/LWG3657>`__","``std::hash<std::filesystem::path>`` is not enabled","February 2022","|Complete|","17.0"
 "`3660 <https://wg21.link/LWG3660>`__","``iterator_traits<common_iterator>::pointer`` should conform to §[iterator.traits]","February 2022","|Complete|","14.0","|ranges|"
 "`3661 <https://wg21.link/LWG3661>`__","``constinit atomic<shared_ptr<T>> a(nullptr);`` should work","February 2022","",""
 "","","","","",""
 "`3564 <https://wg21.link/LWG3564>`__","``transform_view::iterator<true>::value_type`` and ``iterator_category`` should use ``const F&``","July 2022","","","|ranges|"
 "`3617 <https://wg21.link/LWG3617>`__","``function``/``packaged_task`` deduction guides and deducing ``this``","July 2022","",""
 "`3656 <https://wg21.link/LWG3656>`__","Inconsistent bit operations returning a count","July 2022","|Complete|","15.0",""
 "`3659 <https://wg21.link/LWG3659>`__","Consider ``ATOMIC_FLAG_INIT`` undeprecation","July 2022","|Complete|","15.0"
 "`3670 <https://wg21.link/LWG3670>`__","``Cpp17InputIterators`` don't have integer-class difference types","July 2022","","","|ranges|"

libcxx/include/__filesystem/path.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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 _LIBCPP___FILESYSTEM_PATH_H
 #define _LIBCPP___FILESYSTEM_PATH_H
 
 #include <__algorithm/replace.h>
 #include <__algorithm/replace_copy.h>
 #include <__availability>
 #include <__config>
+#include <__functional/unary_function.h>
+#include <__fwd/hash.h>
 #include <__iterator/back_insert_iterator.h>
 #include <__iterator/iterator_traits.h>
 #include <cstddef>
 #include <string>
 #include <string_view>
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_LOCALIZATION)

 
 _LIBCPP_FUNC_VIS
 size_t hash_value(const path& __p) noexcept;
 
 _LIBCPP_AVAILABILITY_FILESYSTEM_POP
 
 _LIBCPP_END_NAMESPACE_FILESYSTEM
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <>
+struct _LIBCPP_AVAILABILITY_FILESYSTEM hash<_VSTD_FS::path> : __unary_function<_VSTD_FS::path, size_t> {
+  _LIBCPP_HIDE_FROM_ABI size_t operator()(_VSTD_FS::path const& __p) const noexcept {
+    return _VSTD_FS::hash_value(__p);
+  }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
 #endif // _LIBCPP_CXX03_LANG
 
 #endif // _LIBCPP___FILESYSTEM_PATH_H

libcxx/include/filesystem

class path {
         template<class charT, class traits>
           friend basic_istream<charT, traits>&
             operator>>(basic_istream<charT, traits>& is, path& p);
     };
 
     void swap(path& lhs, path& rhs) noexcept;
     size_t hash_value(const path& p) noexcept;
 
+    // [fs.path.hash], hash support
+    template<> struct hash<filesystem::path>;
+
     template <class Source>
       path u8path(const Source& source);
     template <class InputIterator>
       path u8path(InputIterator first, InputIterator last);
 
     class filesystem_error;
 
     class directory_entry {

libcxx/test/std/input.output/filesystems/class.path/path.member/path.compare.pass.cpp

#endif
     { // check hash values
       auto h1 = hash_value(p1);
       auto h2 = hash_value(p2);
       assert((h1 == h2) == (p1 == p2));
       // check signature
       ASSERT_SAME_TYPE(size_t, decltype(hash_value(p1)));
       ASSERT_NOEXCEPT(hash_value(p1));
     }
+    { // check std::hash
+      auto h1 = std::hash<fs::path>()(p1);
+      auto h2 = std::hash<fs::path>()(p2);
+      assert((h1 == h2) == (p1 == p2));
+      // check signature
+      ASSERT_SAME_TYPE(size_t, decltype(std::hash<fs::path>()(p1)));
+      ASSERT_NOEXCEPT(std::hash<fs::path>()(p1));
+    }
   }
 }
 
 int CompareElements(std::vector<std::string> const& LHS, std::vector<std::string> const& RHS) {
   bool IsLess = std::lexicographical_compare(LHS.begin(), LHS.end(), RHS.begin(), RHS.end());
   if (IsLess)
     return -1;
 

libcxx/test/std/input.output/filesystems/class.path/path.nonmember/hash.tested_elswhere.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
-// The "hash_value" function is tested as part of [path.compare]
-// in class.path/path.members/path.compare.pass.cpp
-int main(int, char**) {
-  return 0;
-}
+// The std::hash specialization is tested as part of [path.compare]
+// in libcxx/test/std/input.output/filesystems/class.path/path.member/path.compare.pass.cpp

libcxx/test/std/input.output/filesystems/class.path/path.nonmember/hash_value.tested_elswhere.compile.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
-
-// The "hash_value" function is tested as part of [path.compare]
-// in class.path/path.members/path.compare.pass.cpp
-int main(int, char**) {
-  return 0;
-}
+// The `hash_value` function is tested as part of [path.compare]
+// in libcxx/test/std/input.output/filesystems/class.path/path.member/path.compare.pass.cpp