We don't do any type inference across function boundaries, so we largely dodge the issue that I think you are getting at. (Please elaborate if I misunderstand!) We rely on interface and class definitions in order to know what methods are available, and even though the runtime resolves everything at runtime so you can call any method that happens to exist at that time ("duck typing"), we enforce statically that methods do exist where we can. So for example, the following code will work at runtime since the method `g` does exist, our static typechecker will reject it since it does not exist on type `I`.

  <?hh // strict
  interface I { public function f(): void; }
  class C implements I {
    public function f(): void {}
    public function g(): void {}
  }
  function f(I $i): void {
    $i->g();
  }
  f(new C());