I used the GTK+ Sample given on the vala web page.
I just replaced
source.label = "Thank you";
with
button.label = "Thank you";

Here is the code:
using Gtk;

static int main (string[] args) {
    Gtk.init (ref args);

    var window = new Window (WindowType.TOPLEVEL);
    window.title = "First GTK+ Program";
    window.set_default_size (300, 50);
    window.position = WindowPosition.CENTER;
    window.destroy += Gtk.main_quit;

    var button = new Button.with_label ("Click me!");
    button.clicked += (source) => {
        button.label = "Thank you";
    };

    window.add (button);
    window.show_all ();

    Gtk.main ();
    return 0;
}

When compiling this, I get a GCC compilation error telling me that the "button" variable is not accessible.

I don't know if the vala language allows to access to "button" from the inline button.clicked hanlder. If i isn't, I should get a valac error. If it is allowed, the generated code must be wrong.

Regards