[committed] libstdc++: Define std::hash (LWG 3657)

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux, pushed to trunk.

-- >8 --

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.
---
 libstdc++-v3/include/bits/fs_path.h                    | 10 ++++++++++
 .../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 9e06fa679d8..d6202fd275a 100644
--- a/libstdc++-v3/include/bits/fs_path.h
+++ b/libstdc++-v3/include/bits/fs_path.h
@@ -1416,6 +1416,16 @@ 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 6bc6296b5a8..0dcea6efc64 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();
 }
-- 
2.34.1