fn cast_interface[T, U](u U) T {
    $if U is $interface {
        if u is T {
            return u
        } else {
            panic('expected t to be ${typeof[T]().name}, got ${typeof[U]().name}')
        }
    } $else {
        $compile_error('not an interface')
    }
}

interface Foo {
    f()
}

struct Bar {}

fn (bar Bar) f() {}

struct Baz {}

fn (baz Baz) f() {}

fn (_ Bar) g() int {
    return 0
}

fn (_ Baz) g() int {
    return 1
}

fn f(foo Foo) int {
    if foo is Bar {
        return cast_interface[Bar, Foo](foo).g()
    }
    if foo is Baz {
        return cast_interface[Baz, Foo](foo).g()
    }
    return -1
}

fn test_main() {
    assert f(Bar{}) == 0
    assert f(Baz{}) == 1
}