Coerce does not work with function arguments

[ankrgyl]

fn down(a int) {
    a + 1
}

fn up(a bigint) {
    a + 1
}

let i int = 3::int;
let b bigint = 3::bigint;

-- down(i);
down(b);

up(i);
-- up(b);

Both the call to down and up fail here.