Description:
------------
Both Pure Enums and Backed Enums implement an internal interface named UnitEnum. UnitEnum includes a static method cases(). So manually defining cases() method on an Enum results in a fatal error as expected(as clarified in enum RFC). However, it seems it's allowed to define cases() in a trait, resulting in cases() being a requirment in the exhibiting enum with somewhat a confused error which makes no sense!. I think we should clarify with a proper error message that abstract cases() is not allowed too. Non-abstract cases() is totally fine since the enum implementation will take precedance over trait so no error message is needed here.

Test script:
---------------
enum Size: int 
{
    use Test;
}

trait Test 
{
    abstract static private function cases(): int;
}

Expected result:
----------------
PHP Fatal error:  Abstract Size::cases() is already implemented

Actual result:
--------------
PHP Fatal error:  Declaration of Size::cases(): array must be compatible with Test::cases(): int in /var/www/html/index.php on line 0