TypeScript使用strictNullChecks进行的控制流分析不会缩小联合类型的范围

Question

我从书中得到了以下代码https://hamednourhani.gitbooks.io/typescript-book/

interface Square {
    kind: "square";
    size: number;
}

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

interface Circle {
    kind: "circle";
    radius: number;
}

type Shape = Square | Rectangle | Circle;

//Type of function: function area1(s: Square | Rectangle | Circle): number | undefined
function area1(s: Shape) {
    if (s.kind === "square") {
        return s.size * s.size;
    }
    else if (s.kind === "rectangle") {
        return s.width * s.height;
    }
    else if (s.kind === "circle") {
        return Math.PI * s.radius * s.radius;
    }
    else {
        //Type 'Square | Rectangle' is not assignable to type 'never'.
        const _exhaustiveCheck: never = s;
    }
}

//Function lacks ending return statement and return type does not include 'undefined'.
function area2(s: Shape): number {
    switch (s.kind) {
        case "square": return s.size * s.size;
        case "rectangle": return s.width * s.height;
        case "circle": return Math.PI * s.radius * s.radius;
        default: const _exhaustiveCheck: never = s; //Type of s is 'never'
    }
}

问题（--strictNullChecks）：

• 为什么表达式中的2个函数有不同的类型？我预计在这两种情况下，s都不会是
• 为什么区域2函数希望返回类型为未定义的编号？在我看来，这里永远不会发生未定义的事情。我错了吗