expectDeprecation() doesn't work in isolated tests

No remarks for #2, except that we need a failing test-only patch. So, extracted that from the patch in #2, and also reuploaded #2.

Ran locally, confirmed the test-only patch fails, the other one passes, so pre-emptively RTBC'ing.

Note this blocks #2936704: Remove REST route deprecations from DeprecationListenerTrait::getSkippedDeprecations(), use ExpectDeprecationTrait::expectDeprecation() instead.

🎉

+++ b/core/tests/Drupal/Tests/Traits/ExpectDeprecationTrait.php
@@ -32,36 +42,53 @@ protected function expectDeprecation($msg) {
+    // @todo something about isolated tests

is this comment still valid?

#9: Can't believe I did not see that :(

#10: wow, I did not realize you could test multiple deprecations in one test, great!

1. +++ b/core/tests/Drupal/Tests/Listeners/DeprecationListenerTrait.php
   @@ -50,6 +69,26 @@ protected function deprecationEndTest($test, $time) {
   +
   +    $r = new \ReflectionProperty($test, 'runTestInSeparateProcess');
   +    $r->setAccessible(TRUE);
   +
   +    return $r->getValue($test);
   

   wow \PHPUnit_Framework_TestCase::setRunTestInSeparateProcess is pretty sad

2. +++ b/core/tests/Drupal/Tests/Traits/ExpectDeprecationTrait.php
   @@ -32,36 +42,54 @@ protected function expectDeprecation($msg) {
   +      if ($trait = $this->getSymfonyTestListenerTrait()) {
   +        // Add the expected deprecation message to the class property.
   +        $reflection_class = new \ReflectionClass($trait);
   +        $expected_deprecations_property = $reflection_class->getProperty('expectedDeprecations');
   +        $expected_deprecations_property->setAccessible(TRUE);
   +        $expected_deprecations = $expected_deprecations_property->getValue($trait);
   +        $expected_deprecations = array_merge($expected_deprecations, $messages);
   +        $expected_deprecations_property->setValue($trait, $expected_deprecations);
   ...
   +        // Register the error handler if necessary.
   +        $previous_error_handler_property = $reflection_class->getProperty('previousErrorHandler');
   +        $previous_error_handler_property->setAccessible(TRUE);
   +        $previous_error_handler = $previous_error_handler_property->getValue($trait);
   +        if (!$previous_error_handler) {
   +          $previous_error_handler = set_error_handler([$trait, 'handleError']);
   +          $previous_error_handler_property->setValue($trait, $previous_error_handler);
   +        }
   +        return;
   

   Offtopic: Do we have an upstream issue to allow us to do that in a nice way?

Offtopic: Do we have an upstream issue to allow us to do that in a nice way?

https://github.com/symfony/symfony/pull/25757

Committed 6a3ba02 and pushed to 8.6.x

Can cherry-pick after the alpha

Cherry picked as 9414fb5 and pushed to 8.5.x.