Revert "Issue #2662844 by penyaskito, dawehner, catch: cache.backend.chainedfast misbehaves in some situations"

This reverts commit 4eb87a26df.

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

@@ -87,16 +87,8 @@ class ChainedFastBackend implements CacheBackendInterface, CacheTagsInvalidatorI
    *   The fast cache backend.
    * @param string $bin
    *   The cache bin for which the object is created.
-   *
-   * @throws \Exception
-   *   When the consistent cache backend and the fast cache backend are the same
-   *   service.
    */
   public function __construct(CacheBackendInterface $consistent_backend, CacheBackendInterface $fast_backend, $bin) {
-    if ($consistent_backend == $fast_backend) {
-      // @todo: should throw a proper exception. See https://www.drupal.org/node/2751847.
-      trigger_error('Consistent cache backend and fast cache backend cannot use the same service.', E_USER_ERROR);
-    }
     $this->consistentBackend = $consistent_backend;
     $this->fastBackend = $fast_backend;
     $this->bin = 'cache_' . $bin;

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

@@ -33,21 +33,6 @@ class ChainedFastBackendTest extends UnitTestCase {
    */
   protected $bin;
 
-  /**
-   * Tests that chained fast backend cannot be constructed with two instances of
-   * the same service.
-   */
-  public function testConsistentAndFastBackendCannotBeTheSameService() {
-    // ToDo: It should throw a proper exception. See https://www.drupal.org/node/2751847.
-    $this->setExpectedException(\PHPUnit_Framework_Error::class, 'Consistent cache backend and fast cache backend cannot use the same service.');
-    $cache = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');
-    $chained_fast_backend = new ChainedFastBackend(
-      $cache,
-      $cache,
-      'foo'
-    );
-  }
-
   /**
    * Tests a get() on the fast backend, with no hit on the consistent backend.
    */