Why “t instanceof T” is not allowed where T is a type parameter and t is a variable?

Question

Eclipse says that the instanceof operation is not allowed with Type Parameter due to generic type eraser.

I agree that at runtime, no type information stays. But con

Answer 1

But if I instantiate this class of type Integer, then the corresponding object will have a field t of type Integer

No, it won't. It will have a field of type Object. Just everytime you access it, it will be cast to an Integer.

Consider the following code:

SomeClass c = new SomeClass();
SomeClass untyped = (SomeClass)c; // Which type was it?
SomeClass stringTyped = (SomeClass)untyped; // Now it's STRING??

Works. Gives you a bunch of compiler warnings, but works. Because the field T is actually of type Object and can be cast to anything.