Issue #3517614 by alexpott, longwave, cmlara, catch, quietone, mstrelan:...

    // restored during ::tearDown().

    $handler = Error::currentErrorHandler();

    if (!$handler instanceof TestErrorHandler) {

      throw new \RuntimeException(sprintf('%s registered its own error handler (%s) without restoring the previous one before or during tear down. This can cause unpredictable test results. Ensure the test cleans up after itself.',

        $this->name(),

        self::getCallableName($handler),

      ));

      throw new \RuntimeException(sprintf('%s registered its own error handler without restoring the previous one before or during tear down. This can cause unpredictable test results. Ensure the test cleans up after itself.', $this->name()));

    }

    restore_error_handler();

    DeprecationHandler::expectDeprecation($message);

  }

  /**

   * Returns a callable as a string suitable for inclusion in a message.

   *

   * @param callable $callable

   *   The callable.

   *

   * @return string

   *   The string suitable for inclusion in a message.

   *

   * @see https://stackoverflow.com/questions/34324576/print-name-or-definition-of-callable-in-php

   */

  private static function getCallableName(callable $callable): string {

    switch (TRUE) {

      case is_string($callable) && strpos($callable, '::'):

        return '[static] ' . $callable;

      case is_string($callable):

        return '[function] ' . $callable;

      case is_array($callable) && is_object($callable[0]):

        return '[method] ' . get_class($callable[0]) . '->' . $callable[1];

      case is_array($callable):

        return '[static] ' . $callable[0] . '::' . $callable[1];

      case $callable instanceof \Closure:

        return '[closure]';

      case is_object($callable):

        return '[invokable] ' . get_class($callable);

      default:

        return '[unknown]';

    }

  }

}