Do not allow PHPUnit tests to test an interface

I changed Drupal\Tests\Component\Utility\HtmlTest to @coversDefaultClass \Drupal\Component\Utility\ArgumentsResolverInterface and it did start giving errors.

1) Drupal\Tests\Component\Utility\HtmlTest::testCleanCssIdentifier with data set #0 ('abcdefghijklmnopqrstuvwxyz_AB...456789', 'abcdefghijklmnopqrstuvwxyz_AB...456789', array())
@coversDefaultClass does not exist '\Drupal\Component\Utility\ArgumentsResolverInterface': Drupal\Tests\Component\Utility\HtmlTest::testCleanCssIdentifier with data set #0

/Users/jfranks/Sites/devdesktop/drupal/core/tests/Drupal/Tests/Standards/DrupalStandardsListener.php:30
/Users/jfranks/Sites/devdesktop/drupal/core/tests/Drupal/Tests/Standards/DrupalStandardsListener.php:75
/Users/jfranks/Sites/devdesktop/drupal/core/tests/Drupal/Tests/Standards/DrupalStandardsListener.php:154

However, the interface does actually exist, it's just that we aren't allowing it. Should we change the failure to reflect why the test is really failing?

However, the interface does actually exist, it's just that we aren't allowing it. Should we change the failure to reflect why the test is really failing?

I agree that message is not that helpful yet.

Reroll + fixes #4.

There was one instance in core where this was happening, fixed that.

@borisson_ Nice find!

+++ b/core/tests/Drupal/Tests/Listeners/DrupalStandardsListenerTrait.php
@@ -71,6 +71,9 @@ private function checkValidCoversForTest(TestCase $test) {
+      if (!$valid_default_class && interface_exists($default_class)) {
+        $this->fail($test, "@coversDefaultClass refers to an interface '$default_class' and those can not be tested.");
+      }
       if (!$valid_default_class) {

Would it make sense to merge these 2 ifs?

@dawehner:

We could do this instead, but I don't think that's a big improvement.

-      if (!$valid_default_class) {
+      else if (!$valid_default_class) {

@dawehner is #13 what you meant to improve in #12?

+1 for #13

Ok, done.

Is there anything we can do other than a review to get this issue in? Does this need a change notice?

Do you know whether we can ship with an invalid use and have that to be a test?

I think #9 is sufficient to prove that this works?

Well, an automated way which doesn't break in the future would be nice I guess. But yeah I see your point. Well, let's see whether a core committer complains.

Let's open a followup for testing DrupalStandardsListenerTrait as there is no test so far.

Crediting @dawehner and @franksj for reviews and testing.

Committed 7c6f4ae and pushed to 8.6.x. Thanks!