struct Window {
mut:
    calls   int
    on_init fn (mut w main.Window)        = unsafe { nil }
    cb      fn (mut w main.Window, x int) = unsafe { nil }
}

fn (mut w Window) run() {
    w.on_init(mut w)
    w.cb(mut w, w.calls)
}

fn (mut w Window) method_with_mut_receiver() {
    w.calls++
}

fn test_mut_lambda_expr() {
    mut window := Window{
        on_init: |mut w| w.method_with_mut_receiver()
        cb:      |mut w, mut y| unsafe {
            if true {
                assert y == 1
                w.calls++
            }
        }
    }
    window.run()
    assert window.calls == 2
}