[gcc r11-10114] libstdc++: Define std::hash<std::filesystem::path> (LWG 3657)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:c8af57eefe1ac4c1f230979ee0714852c6f77c17

commit r11-10114-gc8af57eefe1ac4c1f230979ee0714852c6f77c17
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 25 18:21:57 2022 +0100

    libstdc++: Define std::hash<std::filesystem::path> (LWG 3657)
    
    This DR was approved at the February 2022 plenary.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/fs_path.h (hash<filesystem::path>): Define.
            * testsuite/27_io/filesystem/path/nonmember/hash_value.cc:
            Check std::hash specialization.
    
    (cherry picked from commit e3c5e8360b4e4799e1e2daf74282629248690f23)

Diff:
---
 libstdc++-v3/include/bits/fs_path.h                            | 10 ++++++++++
 .../testsuite/27_io/filesystem/path/nonmember/hash_value.cc    | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/libstdc++-v3/include/bits/fs_path.h b/libstdc++-v3/include/bits/fs_path.h
index a1f780a9c9b..79085c0c1bd 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -1361,6 +1361,16 @@ extern template class __shared_ptr<const filesystem::filesystem_error::_Impl>;
 
 /// @endcond
 
+// _GLIBCXX_RESOLVE_LIB_DEFECTS
+// 3657. std::hash<std::filesystem::path> is not enabled
+template<>
+  struct hash<filesystem::path>
+  {
+    size_t
+    operator()(const filesystem::path& __p) const noexcept
+    { return filesystem::hash_value(__p); }
+  };
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 
diff --git a/libstdc++-v3/testsuite/27_io/filesystem/path/nonmember/hash_value.cc b/libstdc++-v3/testsuite/27_io/filesystem/path/nonmember/hash_value.cc
index 7b7c912a6df..be15b314149 100644
--- a/libstdc++-v3/testsuite/27_io/filesystem/path/nonmember/hash_value.cc
+++ b/libstdc++-v3/testsuite/27_io/filesystem/path/nonmember/hash_value.cc
@@ -42,9 +42,19 @@ test02()
   }
 }
 
+void
+test03()
+{
+  std::hash<path> h;
+  // LWG 3657. std::hash<std::filesystem::path> is not enabled
+  for (const path p : __gnu_test::test_paths)
+    VERIFY( h(p) == hash_value(p) );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }