Description:
------------
Both abstract trait method validation and promotion of warnings to fatal errors for LSP violation have been implemented in PHP 8. Yay!

Unfortunately due to the timing of these two changes, it can result in a new fatal error where previously there was no warning/notice issued at all. This can catch people upgrading by surprise, and is rather difficult to track down in any sort of automated way.

https://www.reddit.com/r/PHP/comments/hj61po/php_8_class_inheritance_method_signature/fwmc93e/ and https://php.watch/versions/8.0/lsp-errors#trait provide more context and examples.

Test script:
---------------
trait MyTrait {
  abstract protected function foo(): string;
}

class MyClass {
  use MyTrait;
  protected function foo()
  {
  }
}

Expected result:
----------------
Some sort of warning/notice/etc for abstract trait validation is back-ported from 8.0.0, so that the (expected) fatal error the same code would produce under 8 isn't a complete surprise for projects running on modern 7.x.

I recognize this would go against normal conventions and RFC processes for new warnings/errors/deprecations, but given the impact it can have when trying to upgrade, I think it's well worth considering making a process exception.

Actual result:
--------------
7.4.7: no warning, error, notice, etc.

8.0.0-alpha1: fatal error