Andrei's interface requests

[Christopher Wright]

Andrei has put up a few enhancement requests in the bugzilla:
http://d.puremagic.com/issues/show_bug.cgi?id=2784
http://d.puremagic.com/issues/show_bug.cgi?id=2785
http://d.puremagic.com/issues/show_bug.cgi?id=2786

These are intended to change interfaces from being solely a tool of 
polymorphism into a way of specifying type constraints. The additions 
recommended can only be used at compile time (constructors, static 
functions, and nested types).

I grant that it would be quicker and clearer to write:
interface Foo
{
    static void stuff();
    this (int);
}
template Template(Arg : Foo) {}

than to write:
template Template(Arg) if (is (typeof (new Arg(0)) && 
isStaticFunction!(Arg, "stuff")) {}

However, I'm not sure whether this would be used much at all, and it 
deserves some discussion.

One detail of #2785 seems problematic:
interface Foo
{
     extern void bar(int, Foo, double);
}

meaning that a non-member function bar should exist that accepts an int, 
*the implementor of Foo*, and a double.

This is a huge and silent divergence from the standard meaning of using 
the interface name; it would allow:
class C : Foo
{
	static void bar(int, C, double);
}

and disallow:
class C : Foo
{
	static void bar(int, Foo, double);
}

Thoughts? Any concerns that I have not raised? I don't do sufficient 
metaprogramming to find any of this useful, I admit.