Hi internals, The #[\Attribute] attribute is used to mark classes as being usable as attributes. It currently does not trigger any error if added to a trait, interface, enum, or abstract class. However, if any of those *were* used as an attribute, calling ReflectionAttribute::newInstance() would fail; if you are not going to call ReflectionAttribute::newInstance(), then you don't really need to mark the trait/interface/enum/abstract class as an attribute.
I have proposed https://github.com/php/php-src/pull/19154 to add validation to the #[Attribute] attribute, and it was suggested that I post here on the mailing list in case there were any objections. So, are there? -Daniel
