The number type is assignable to any enum with implicit values

[gvergnaud]

Bug Report

🔎 Search Terms

enum, extends, number

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about number being assignable to Enum elements.

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.4.4#code/PTAEHUAsFMDtQCbQGYEtboOagIbzgK4C2ANKNDgMaTkA20RcALqJtEwOQDOuBTA9kRxNUlHLVoBPXFy6pM8Abnjom0NgCcAUISKgAKtC4sA3ltAXQAeVjQS5y-oDu-ewF8tWkKABC0MQRc0KD8yKBMkKg8TsHQAB4ADv4shsYAdDbBSgBGwTiy8rA42fTh-OEwoLDEuRrhkklaTA1ZAIygALwGRizxarAIPNVEtaAA-OEaBMEAXKDI4kEA3F5gHQB8k9Oe3j58VfwsEcGHMHVOONI4GvwEA00t4QBMnVU10HV9cIPdxuNbs3mi2gK28ESi5AAbuICMIjGUAaAABRjACEAEpPFpKPxYH9kHdXki4nNUkx0Z1NmZLKAuE5UExqMi4hTqTTLGIgr8mBlbHM2ez2Rp2AQNPAAESZcUOQUeGXsznBMlpZz8fnywUWYVMUUSqUayxyzWIFA4Ai0Jjq4007xxUCQfL1JJVaCQj5kXIBLmuj7SRWgCFcAgJBL8IIIBG5e14BD0BAGwU4vEsSRzWxuupdOKea2WCI3JwuwsAUQ0Nw0SI44J4XEgt1oEfTH2jIbgHExxo8NI8bhWWgJsCRysy6JWA6HPRVLlH-buSKeAAZR6BvPpIjW6+aELAq07-DBKABrD37BmIfhGMgxgMsekSCoFoA

Playground link with relevant code

💻 Code

// When defining an enum, each element get's automatically assign to an integer
enum Test {
    One,
    Two,
}

// Because of this we expect Test.One to be assignable to the number type
type t1 = Test extends number ? true : false;
// => true

// But not the other way around
type t2 = number extends Test ? true : false;
// this evaluates to true (?!)


const fun = (x: Test) => {
    switch (x) {
        case Test.One: {
            return "One"
        }

        case Test.Two: {
            return "One"
        }

        default: {
            // x has type never, because every case is supposed to be handled
            const y: never = x

            throw new Error('this should never happen')
        }
    }
};

fun(Test.One);
fun(Test.Two);
fun(20); // This shouldn't typecheck, but it does, and it will throw

🙁 Actual behavior

the type of an enum defined without providing values for its elements is wrongfully considered a superset of number, which means we can provide any number to a function taking an enum as parameter.

Note that this isn't the case for enum with string values:

enum Test {
    One = "One",
    Two = "Two",
}


type t1 = Test extends string ? true : false;
// => true

type t2 = string extends Test ? true : false;
// => false


const fun = (x: Test) => {
    switch (x) {
        case Test.One: {
            return "One"
        }

        case Test.Two: {
            return "One"
        }

        default: {
            const y: never = x
            throw new Error('this should never happen')
        }
    }
};

fun(Test.One);
fun(Test.Two);
fun("20"); // doesn't typecheck

🙂 Expected behavior

number shouldn't be assignable to Test.

[MartinJohns]

Duplicate of #26362, which provides plenty of reasoning for this behaviour. Used search terms: enum number in:title

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.