Delete objects of incomplete type

Question

This one made me think:

class X;

void foo(X* p)
{
    delete p;
}

How can we possibly delete p if we do not even know whether

Answer 1

From the standard [expr.delete]:

If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.

So, it's UB if there's nontrivial stuff to do, and it's ok if there isn't. Warnings aren't neccessary for UB.