Reproduce generics union bound issue

[ondrejmirtes]

See phpstan/phpstan#5591

/cc @arnaud-lb Looks like you're digging through some older issues, I've got one that you're more equipped to fix rather than me :) The problem here is that the type is rejected although it's exactly the same one: https://phpstan.org/r/cc421f16-0749-407d-80ac-e05e7624d3f7

Parameter #1 $record of method App\TestClass<TEntity of App\EntityA|App\EntityB>::innerMethod() expects (TEntity of App\EntityA|App\EntityB)|null, (TEntity of App\EntityA|App\EntityB)|null given.

The problem is that we're representing TEntity|null as a UnionType(new TemplateUnionType(EntityA, EntityB), null). Union inside union might not be ideal but I don't know how differently it could work.

The problem is that UnionType::accepts() does not accept the same type it's itself. Can you please look into it and figure out what's the root cause and how it could be fixed?

Thank you :)