Switch not performing exhaustive check when a default with unreachable assert is specified

[fcole90]

Bug Report

🔎 Search Terms

is:issue is:open exhaustive switch

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about switch exhaustiveness and I didn't find anything about this.

⏯ Playground Link

Playground link with relevant code

💻 Code

const assertUnreachable = (x: never): never => {
    throw new Error("Unreachable code")
}

// This is OK for the compiler
const exhaustiveFunctionReturnOK = (value: "a" | "b"): number => {
    switch(value) {
        case "a":
            return 0
        case "b":
            return 1
    }
}

// This is an ERROR for the compiler:
// > Function lacks ending return statement and return type does not include 'undefined'.(2366)
const exhaustiveFunctionMissesReturn = (value: "a" | "b"): number => {
    switch(value) {
        case "a":
            return 0
        case "b":
            return 1
        default:
            assertUnreachable(value)
    }
}

// I think both should be OK instead

🙁 Actual behavior

I have a function with a parameter which is a string union (e.g. value: "a" | "b") and a return of some type (e.g. number). I want to perform an exhaustive switchto decide whatnumberto return based on the value, if it's"a"or"b"`. The compiler correctly identifies that the branches match all the possible values (exhaustive), so it's happy and compiles.

However, if I add a default branch, with an unreachable assert of type never, then the compiler complains that I'm missing a return of type number. This seems weird to me, given that the reason for the assert itself is exactly to ensure that such a branch is actually unreachable, and hence every option of the switch is exhausted.

🙂 Expected behavior

Both behaviours to work and the compiler to understand that a default branch with an unreachable-code assertion is not going to happen and behave like in the case without such a branch, inferring that the only possible return type is number (or whatever type the function is returning).

[fcole90]

Possibly related issue: #36884

[fcole90]

Other possibly related issue: #12825

As for this comment a workaround is to return the assertUnreachable(x: never), like this:

// Now the compiler doesn't complain anymore
const exhaustiveFunctionMissesReturn = (value: "a" | "b"): number => {
    switch(value) {
        case "a":
            return 0
        case "b":
            return 1
        default:
            return assertUnreachable(value)
            //  ^ Note this "return" was added
    }
}

[MartinJohns]

Duplicate of #12825.

[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.