Fix inverted null check in time_zone_name_win.cc

This commit follows up the previous commit [1], which aimed to improve
the code but ended up introducing an inverted null check in
time_zone_name_win.cc. As a result, LoadIcuGetTimeZoneIDForWindowsID()
currently returns nullptr when it should be returning a valid time zone
ID.

A unit test is also added, as the fallback chain in local_time_zone()
makes it difficult to verify the behavior of
LoadIcuGetTimeZoneIDForWindowsID() in isolation. The test ensures that
the function correctly returns a valid time zone ID on Windows.

This is also a preparation for implementing TimeZoneIf with Windows time
APIs (#328).

[1] 27ca173

Author: yukawa

BUILD

@@ -125,6 +125,18 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "time_zone_name_win_test",
+    size = "small",
+    srcs = ["src/time_zone_name_win_test.cc"],
+    target_compatible_with = ["@platforms//os:windows"],
+    deps = [
+        ":time_zone",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "time_zone_fuzz_test",
     srcs = [

CMakeLists.txt

@@ -148,12 +148,26 @@ if (BUILD_TESTING)
     )
   add_test(time_zone_format_test time_zone_format_test)
 
+  if(WIN32)
+    add_executable(time_zone_name_win_test src/time_zone_name_win_test.cc)
+    cctz_target_set_cxx_standard(time_zone_name_win_test)
+    target_include_directories(time_zone_name_win_test PRIVATE ${GTEST_INCLUDE_DIRS})
+    target_link_libraries(time_zone_name_win_test
+      cctz::cctz
+      ${GTEST_BOTH_LIBRARIES}
+      ${CMAKE_THREAD_LIBS_INIT}
+      )
+    add_test(time_zone_name_win_test time_zone_name_win_test)
+  endif()
+
   # tests runs on testdata
+  set(CCTZ_TESTS civil_time_test time_zone_format_test time_zone_lookup_test)
+  if(WIN32)
+    list(APPEND CCTZ_TESTS time_zone_name_win_test)
+  endif()
   set_property(
     TEST
-      civil_time_test
-      time_zone_format_test
-      time_zone_lookup_test
+      ${CCTZ_TESTS}
     PROPERTY
       ENVIRONMENT "TZDIR=${CMAKE_CURRENT_SOURCE_DIR}/testdata/zoneinfo"
     )

src/time_zone_name_win.cc

@@ -112,7 +112,7 @@ ucal_getTimeZoneIDForWindowsID_func LoadIcuGetTimeZoneIDForWindowsID() {
   const auto ucal_getTimeZoneIDForWindowsIDRef =
       AsProcAddress<ucal_getTimeZoneIDForWindowsID_func>(
           icu_dll, "ucal_getTimeZoneIDForWindowsID");
-  if (ucal_getTimeZoneIDForWindowsIDRef != nullptr) {
+  if (ucal_getTimeZoneIDForWindowsIDRef == nullptr) {
     g_unavailable.store(true, std::memory_order_relaxed);
     return nullptr;
   }

src/time_zone_name_win_test.cc

@@ -0,0 +1,31 @@
+// Copyright 2026 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+//   Unless required by applicable law or agreed to in writing, software
+//   distributed under the License is distributed on an "AS IS" BASIS,
+//   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//   See the License for the specific language governing permissions and
+//   limitations under the License.
+
+#include "time_zone_name_win.h"
+
+#include <string>
+
+#include "cctz/time_zone.h"
+#include "gtest/gtest.h"
+
+namespace cctz {
+
+TEST(TimeZoneNameWin, GetWindowsLocalTimeZone) {
+  // On Windows 10 1809+ (where icu.dll is available in System32),
+  // GetWindowsLocalTimeZone() should return a valid IANA time zone name.
+  const std::string tz = GetWindowsLocalTimeZone();
+  ASSERT_FALSE(tz.empty());
+}
+
+}  // namespace cctz