My suspicion is that this has a number of problems. Doing instanceof *Type is rarely right so it's not hard to poke holes into the logic - this handles the mixed scenario but what about something else that "may be" an array or int? What if we do int|array + int|array? What piece of code will handle that?

Also this condition will catch TemplateMixedType, maybe there's a situation which will cause generics to be lost.

Also - MixedType is subtractable, meaning there can be a mixed~array|int type which means "this is mixed but it's definitely not int nor array". My point is - doing isSuperTypeOf() is much safer and usually more correct than instanceof :) More about it here: https://phpstan.org/developing-extensions/type-system#querying-a-specific-type