PHPUnit made some progress with regards to error handling which renders our current way of dealing with deprecations incompatible. I'd like to leverage PHPUnit's error handler instead of having our error handler collect deprecations.

As a first step, I'd like to propose a lightweight replacement for ExpectDeprecationTrait. It comed with a method assertDeprecation() that takes an expected deprecation message and a callable that is supposed to trigger that deprecation. I'm registering a temporary error handler that filters out the expected deprecation while letting everything else bubble up.

An interesting side effect is that tests which only trigger expected deprecations don't have to be added to the legacy group anymore. This is especially useful for testing tooling around deprecations in the Config and DependencyInjection components.

With this new trait, I was able to execute the affected tests with PHPUnit 11.1 locally.

public function testDeprecatedMethod()
{
    $someObject = new SomeClass();

    $returnValue = self::assertDeprecation(
        'Since symfony/some-component 7.1: This is deprecated, do something else instead.',
        static fn () => $someObject->someDeprecatedMethod(47, 11),
    );

    self::assertSame('foo', $returnValue);
}