Fixed util test string_transparent_hash_ut (#311)

Author: Gazizonoki

util/CMakeLists.txt

@@ -96,8 +96,7 @@ if (YDB_SDK_TESTS)
       generic/stack_ut.cpp
       generic/store_policy_ut.cpp
       generic/strbuf_ut.cpp
-      # TODO: uncomment this test after we get the fix
-      # generic/string_transparent_hash_ut.cpp
+      generic/string_transparent_hash_ut.cpp
       generic/string_ut.cpp
       generic/typelist_ut.cpp
       generic/typetraits_ut.cpp

util/generic/string_transparent_hash_ut.cpp

@@ -3,15 +3,27 @@
 #include "strbuf.h"
 
 #include <library/cpp/testing/unittest/registar.h>
-#include <library/cpp/containers/absl_flat_hash/flat_hash_set.h>
 
 #include <util/str_stl.h>
 
+#ifdef __cpp_lib_generic_unordered_lookup
+    #include <unordered_set>
+
+template <class T, class THasher, class TPred>
+using THashSetType = std::unordered_set<T, THasher, TPred>;
+#else
+    // Using Abseil hash set because `std::unordered_set` is transparent only from libstdc++11.
+    // Meanwhile clang-linux-x86_64-release-stl-system autocheck sets OS_SDK=ubuntu-20,
+    // that support libstdc++10 by default.
+    #include <library/cpp/containers/absl_flat_hash/flat_hash_set.h>
+
+template <class T, class THasher, class TPred>
+using THashSetType = absl::flat_hash_set<T, THasher, TPred>;
+#endif
+
 Y_UNIT_TEST_SUITE(StringHashFunctorTests) {
     Y_UNIT_TEST(TestTransparencyWithUnorderedSet) {
-        // Using Abseil hash set because `std::unordered_set` is transparent only from C++20 (while
-        // we stuck with C++17 right now).
-        absl::flat_hash_set<TString, THash<TString>, TEqualTo<TString>> s = {"foo"};
+        THashSetType<TString, THash<TString>, TEqualTo<TString>> s = {"foo"};
         // If either `THash` or `TEqualTo` is not transparent compilation will fail.
         UNIT_ASSERT_UNEQUAL(s.find(TStringBuf("foo")), s.end());
         UNIT_ASSERT_EQUAL(s.find(TStringBuf("bar")), s.end());