Missing visibility check for private abstract trait methods

Description

The following code:

<?php

trait T {
    public abstract function test();
}

class P {
    private function test() {}
}

class C extends P {
    use T;
}

This passes without errors. There's an intentional BC layer that skips visibility checks for abstract trait methods (see Zend/tests/traits/abstract_method_4.phpt). However, this should not include private parent methods, because the trait cannot call those. The whole BC layer is questionable today, so it might be better to remove it entirely.

PHP Version

All supported branches

Operating System

No response

Reproduce the example and read Zend/tests/traits/abstract_method_4.phpt, which documents the existing BC behavior for abstract trait methods. Trace the visibility-checking path for a private parent method, then add or adjust a regression test so the intended behavior is explicit and the relevant supported branches pass.