Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php

@@ -22,6 +22,20 @@ use Drupal\Tests\UnitTestCase;
  */
 class SafeMarkupTest extends UnitTestCase {
 
+  /**
+   * The error message of the last error in the error handler.
+   *
+   * @var string
+   */
+  protected $lastErrorMessage;
+
+  /**
+   * The error number of the last error in the error handler.
+   *
+   * @var int
+   */
+  protected $lastErrorNumber;
+
   /**
    * {@inheritdoc}
    */
@@ -159,6 +173,40 @@ class SafeMarkupTest extends UnitTestCase {
 
     return $tests;
   }
+    /**
+   * Custom error handler that saves the last error.
+   *
+   * We need this custom error handler because we cannot rely on the error to
+   * exception conversion as __toString is never allowed to leak any kind of
+   * exception.
+   *
+   * @param int $error_number
+   *   The error number.
+   * @param string $error_message
+   *   The error message.
+   */
+  public function errorHandler($error_number, $error_message) {
+    $this->lastErrorNumber = $error_number;
+    $this->lastErrorMessage = $error_message;
+  }
+
+  /**
+   * String formatting with SafeMarkup::format() and an unsupported placeholder.
+   *
+   * When you call SafeMarkup::format() with an unsupported placeholder, an
+   * InvalidArgumentException should be thrown.
+   */
+  public function testUnexpectedFormat() {
+
+    // We set a custom error handler because of https://github.com/sebastianbergmann/phpunit/issues/487
+    set_error_handler([$this, 'errorHandler']);
+    // We want this to trigger an error.
+    $error = SafeMarkup::format('Broken placeholder: ~placeholder', ['~placeholder' => 'broken'])->__toString();
+    restore_error_handler();
+
+    $this->assertEquals(E_USER_ERROR, $this->lastErrorNumber);
+    $this->assertEquals('Invalid placeholder: ~placeholder', $this->lastErrorMessage);
+  }
 
 }