Issue #3068076 by alexpott: Properly deprecate Cache::validateTags()

core/lib/Drupal/Core/Cache/Cache.php

@@ -96,12 +96,14 @@ public static function mergeMaxAges($a = Cache::PERMANENT, $b = Cache::PERMANENT
    * @param string[] $tags
    *   An array of cache tags.
    *
-   * @deprecated
-   *   Use assert(Inspector::assertAllStrings($tags));
+   * @deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use
+   *   assert(\Drupal\Component\Assertion\Inspector::assertAllStrings($tags))
+   *   instead.
    *
    * @throws \LogicException
    */
   public static function validateTags(array $tags) {
+    @trigger_error(__NAMESPACE__ . '\Cache::validateTags() is deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use assert(\Drupal\Component\Assertion\Inspector::assertAllStrings($tags)) instead.', E_USER_DEPRECATED);
     if (empty($tags)) {
       return;
     }

core/tests/Drupal/Tests/Core/Cache/CacheTest.php

@@ -44,6 +44,8 @@ public function validateTagsProvider() {
    * @covers ::validateTags
    *
    * @dataProvider validateTagsProvider
+   * @expectedDeprecation Drupal\Core\Cache\Cache::validateTags() is deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use assert(\Drupal\Component\Assertion\Inspector::assertAllStrings($tags)) instead.
+   * @group legacy
    */
   public function testValidateTags(array $tags, $expected_exception_message) {
     if ($expected_exception_message !== FALSE) {