Tests failing on failing on 9.x core branches because of deprecation error

Well, this is problematic.

Here's the PR where PHPUnit's expectErrorMessage() was deprecate: https://github.com/sebastianbergmann/phpunit/issues/5062

This PR does not explain the rationale for the change in any helpful way. Sebastian says:

There are no replacements. PHPUnit 10 no longer converts E_* to exceptions, therefore E_* can no longer be expected.

Well, er...okay, then?

@alexpott asked this but has so far received no reply:

what is the recommended way of testing that an error is triggered in PHPUnit 10? If you want to assert that an E_USER_WARNING is thrown by your code what is the best way to do this? Should you replace the error handler as above or is there a better way?

Looking into this more closely...I think this worked only by coincidence.

I'm not sure why $this->expectErrorMessage() was called, but I suspect it was simply an innocent mistake on the part of whoever wrote that line. Because PHPUnit defines that method, it didn't die with a "call to undefined method" or anything like that.

Here's what $this->expectErrorMessage() does: https://github.com/sebastianbergmann/phpunit/commit/3812fd713359a21aed38...

It's just a thin wrapper around $this->expectExceptionMessage()!

So, indeed, the validator is legitimately raising a StageValidationException. We are expecting the correct message. We are simply setting up that expectation in a deprecated way.

Therefore, all we have to do here is call $this->expectExceptionMessage() directly instead, and this problem will go away forever. For clarity and explicitness, we should probably also add a $this->expectException(StageValidationException::class); beforehand, too.

Yes, core has been using that method for years. 👍