Resolved: How to discriminate an union against “never” type?

type TShape = {
    id: string;
}

type TCircle = TShape & {
    radius: number;
    size: never;
}

type TSquare = TShape & {
    radius: never;
    size: number;
}

function getArea(shape: TCircle | TSquare): number {
    if (typeof shape.radius === "number") {
        return shape.radius * shape.radius * Math.PI;
    }
    else {
        return shape.radius * shape.size;  //no error?
    }
}

type TSquare = TShape & {
    size: number;
}

function getArea(shape: TCircle | TSquare): number {
    if ("radius" in shape) {
        return shape.radius * shape.radius * Math.PI;
    }
    else {
        return shape.radius * shape.size;  // this should be an error this time
    }
}