"The last feature": overridable methods in interfaces

[Andrei Alexandrescu]

Walter has now implemented final methods in interfaces and also 
contracts in interfaces, both of which I think are just awesome.

We figured that essentially he artificially disallows interfaces from 
providing bodies for methods. I think that's a gratuitous limitation; 
the only distinguishing quality of an interface is that it has no state. 
  Other than that, interfaces can always offer overridable functions 
that by default offer functionality in terms of existing interface 
functions. For example:

interface Stack(T)
{
     void push(T);
     void pop();
     @property ref T top();
     @property bool empty();
     T belowTop()
     {
         auto t = top;
         pop();
         auto result = top;
         push(t);
     }
}

The default implementation of belowTop does a fair amount of work. A 
particular implementation might just use that or override it with a more 
efficient implementation.

Many more examples can be imagined, but I'm looking for a killer one, or 
perhaps a killer counterexample (e.g. when would an interface-defined 
method be really bad?)

Your thoughts welcome.


Andrei