this(this) must be cheap and O(1)

[Trass3r]

> This means that objects with large state would need to use things like 
> COW and/or reference counting.

Isn't an expensive-to-copy type supposed to be a class anyway?

> I'd go as far as requiring this(this) to be nothrow, but perhaps it 
> would be best to see whether that is a necessity.

Don't memory allocations prevent a function from being nothrow?
Then this would make it impossible to properly wrap an array in a struct.