Implement overloading

[pcwalton]

Synopsis:

mod foo { overload fn f(int x) { log x; } }
mod bar { overload fn f(str s) { log s; } }
import foo::f;
import bar::f;
fn main() {
    f(3);
    f("hello world");
}

Result:

3
hello world

[nikomatsakis]

interfaces now offer a limited form of overloading.

[graydon]

Closing as this is obsolete given ifaces.