Why does a switch statement on an enum throw 'not comparable to type' error?

Question

While doing a Lynda tutorial on Typescript ( https://www.lynda.com/Visual-Studio-tutorials/TypeScript-types-part-2/543000/565613-4.html#tab ), I hit a snag. The sample code

Answer 1

That's because the compiler already knows that the case temperature.hot will never happen: the variable temp is given the enum literal type temperature.cold, which can only be assigned that value itself (or null if there are no strict null checks). As temperature.hot is not a compatible value here, the compiler throws an error.

If we discard the information about the literal (by casting or retrieving the value from a function):

function how_cold(celsius: number): temperature {
    return celsius > 40 ? temperature.hot : temperature.cold;
}

The code will then compile:

let temp = how_cold(35); // type is "temperature"

switch (temp) {
    case temperature.cold:
        console.log("Brrr....");
        break;
    case temperature.hot:
        console.log("Yikes...")
        break;
}

Alternatively, prepending + to the value works because it converts the value to a number, which will also widen the type's scope and make it compatible with all enum variants, as well as other numbers.

let temp = temperature.cold;

switch (+temp) {
    case temperature.cold:
        console.log("Brrr....");
        break;
    case temperature.hot:
        console.log("Yikes...")
        break;
    case 5:
        console.log("What??");
        break;
}