Provide AbslHashValue for std::filesystem::path in C++17

This is somewhat tricky to implement because path equality is not
straightforward. See
https://github.com/abseil/abseil-cpp/pull/1560#issuecomment-1799983471
for discussion.

This re-lands 3bd86026c93da5a40006fd53403dff9d5f5e30e3 with a fix
for iOS 13 unavailability of std::filesystem::path.

Roll-forward of 524ebb7ea91d2955dc4c68c7e6fd2bed620621b5.

Closes #655
Closes #1560

PiperOrigin-RevId: 583365100
Change-Id: Id49735c49d123e0cd6a620a2b5b5a12d94129f94

1 files changed, 41 insertions, 0 deletions

diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index 111f375b..59fe8dea 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -48,6 +48,10 @@
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+#include <filesystem>  // NOLINT
+#endif
+
 #ifdef ABSL_HAVE_STD_STRING_VIEW
 #include <string_view>
 #endif
@@ -480,6 +484,43 @@ TEST(HashValueTest, U32StringView) {
 #endif
 }
 
+TEST(HashValueTest, StdFilesystemPath) {
+#ifndef ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE
+  GTEST_SKIP() << "std::filesystem::path is unavailable on this platform";
+#else
+  EXPECT_TRUE((is_hashable<std::filesystem::path>::value));
+
+  // clang-format off
+  const auto kTestCases = std::make_tuple(
+      std::filesystem::path(),
+      std::filesystem::path("/"),
+#ifndef __GLIBCXX__
+      // libstdc++ has a known issue normalizing "//".
+      // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106452
+      std::filesystem::path("//"),
+#endif
+      std::filesystem::path("/a/b"),
+      std::filesystem::path("/a//b"),
+      std::filesystem::path("a/b"),
+      std::filesystem::path("a/b/"),
+      std::filesystem::path("a//b"),
+      std::filesystem::path("a//b/"),
+      std::filesystem::path("c:/"),
+      std::filesystem::path("c:\\"),
+      std::filesystem::path("c:\\/"),
+      std::filesystem::path("c:\\//"),
+      std::filesystem::path("c://"),
+      std::filesystem::path("c://\\"),
+      std::filesystem::path("/e/p"),
+      std::filesystem::path("/s/../e/p"),
+      std::filesystem::path("e/p"),
+      std::filesystem::path("s/../e/p"));
+  // clang-format on
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(kTestCases));
+#endif
+}
+
 TEST(HashValueTest, StdArray) {
   EXPECT_TRUE((is_hashable<std::array<int, 3>>::value));