Mocking callables by mocking __invoke() on stdClass

[morozov]

Q	A
PHPUnit version	8.3.0
PHP version	7.3.8
Installation Method	Composer

As of 20b0b21, PHPUnit triggers a warning when a partial mock is created with a non-existing method. While this behavior looks reasonable per se, it breaks the technique of mocking callables which is not natively supported by PHPUnit:

public function testClosureIsCalled() : void
{
    $closure = $this->createPartialMock(stdClass::class, ['__invoke']);

    $closure->expects(
        $this->exactly(2)
    )->method('__invoke');

    $closure();
    $closure();
}

While I'm not proposing any specific solution to the problem, I'd like the developers to be aware of it. Escalating this warning to a failure in a future PHPUnit version will break existing tests without a known way to rework them.

[sebastianbergmann]

If at all, this should be taken care of in #3536.