dynamic classes and duck typing

[Walter Bright]

One thing Java and Python, Ruby, etc., still hold over D is dynamic 
classes, i.e. classes that are only known at runtime, not compile time. 
In D, this:

    s.foo(3);

could be emulated with:

    s.dynamicMethod("foo", 3);

Unfortunately, that makes it impossible to use s with generic code 
(besides looking unappealing). But with a small feature, we can make 
this work:

    struct S
    {
         ...
	T opDynamic(s : string)(args...);
    }

and then s.foo(3), if foo is not a compile time member of s, is 
rewritten as:

    s.opDynamic!("foo")(3);

and opDynamic defers all the nuts-and-bolts of making this work out of 
the language and into the library.

In particular, opDynamic's parameter and return types should all be 
instances of std.variant.

(This has come up in various forms in this n.g. before, but I don't have 
any references handy.)