Matt, if this were to become a language thing, I don't see how the above could be done so easily. I also wonder how it would handle a function declaration that occurs /after/ the "using new" statement. If A and B are both using new X, and a function f(X, X) is declared after the definitions of A and B, is f(A, A) valid? f(A, B)?

Thank you for the feedback.

My simplistic view of "using A = new X" or "using new A = X" or "using_otherwise_adorned A = X" is simply to be similar to "struct A {}" where A is "clone" of X but not an alias in the type system. I imagine extension methods to add methods would be useful in this picture but somewhat orthogonal.

Perhaps to handle your example of preventing ops in an inappropriate domain, such as the time add, = delete or extension removal would also make sense. That is, using an extension method for adding the delta and an extension deletion for removing the addition of time to time. 

Such a slightly different way of looking at the type system may also make a contribution to the ABI/performance debate. 

I'm not sure if adding data to a clone would also make sense due to the layout changes. I can see it being useful but different from both cloning a type and inheritance as an alternative composition technique.

Kind regards,