为什么 Typescript 编译器会抱怨以下代码？
type Foo = {
a: string
}
type Bar = {
b: number
}
type Baz = Foo | Bar;
function f(x: Baz): number {
if (x.a) { // property 'a' does not exist on type Bar!
return 0;
}
if (x.b) { // property 'b' does not exist on type Foo!
return 1;
}
return -1;
}