When using this code:

public class App : GLib.Object {
  private double fraction = 1.0;
  public double get_value () { return fraccion; }
  public async void update () {
    try {
      yield do_stuff ();
      var lc = yield return_stuff ();
      for (int i = 0; i < lc.length; i++) {
        GLib.Idle.add (update.callback);
        yield;
        fraccion += 3.5/(lc.length)*i;
        yield update_calc ();
      }
      do_more_stuff.begin (()=>{
        message ("More stuff done");
      });
    } catch (GLib.Error e) {
      warning ("Error al conectar: "+e.message);
    }
    GLib.Idle.add (update.callback);
    yield;
  }
  public async void do_stuff () throws GLib.Error {
    message ("Doing");
  }
  public async void do_more_stuff () throws GLib.Error {
    message ("Doing More");
  }
  public async GLib.Queue<string> return_stuff () throws GLib.Error {
    return  new GLib.Queue<string> ();
  }
  public async void update_calc () throws GLib.Error {
    fraction += 1;
  }
  public int main () {
    var app = new App ();
    app.update.begin (()=>{
      message ("Done Update");
    });
    int p = 1;
    while (app.get_value () < 100 || p < 100000) { p++; }
  }
}

valac 0.36.4 and at least 0.37.91, fails to find  "lc" defined as:

var lc = yield return_stuff ();

in async update () method.