interface Y { 'i am a very certain type': Y }
var y : Y = <Y> undefined;
function destructure<a, r>(
    something: a | Y,
    haveValue: (value: a) => r,
    haveY: (value: Y) => r
) : r {
    return something === y ? haveY(y) : haveValue(<a> something);
}

var value = Math.random() > 0.5 ? 'hey!' : <Y> undefined;

console.log(destructure(value, text => 'string', y => 'other one')); // <-- complete mess and failure