Never type is not inferred when used from helper function

[Hurtak]

TypeScript Version: 3.5.1

Code

type UnionType = "A" | "B"

function getSomething(foo: UnionType): number {
    switch (foo) {
        case "A":
            return 1;
        case "B":
            return 2
        default:
            const _: never = foo;
            return _;
    }
}

function getSomethingInlineNeverFail(foo: UnionType): number {
    switch (foo) {
        case "A":
            return 1;
        default:
            const _: never = foo; // Type '"B"' is not assignable to type 'never'.
            return _;
    }
}

function getSomethingHelperNeverNoFail(foo: UnionType): number {
    switch (foo) {
        case "A":
            return 1;
        default:
            return never(); // No type error
    }
}

function never(): never {
    throw new Error("Should never happen");
}

Expected behavior:
There is type error in getSomethingHelperNeverNoFail function

Actual behavior:
No type error in getSomethingHelperNeverNoFail function

Playground Link: http://www.typescriptlang.org/play/#code/FAFwngDgpgBAqgOwJYHsEBVKwLwwEQCCeMAPvgEJ7DABmArggMYioIwDmUIAyigLZcAFkgTsAFDRQoAXPGRpM0AJSyEdPgCMoAJxgBvYDCMwAzgHckIRoJgSpS-YePPGAQxOxCeaU+d+Y2lx02mwAjADcvn5uHhTeUf5GgSDBbABMCcYAJlA0rnQANiA+idFoJiAwAPqqUABuOjC4kiiRpc7JqdVtxgC+wP20DMysHFy8AiDCogCSCAUiUABy9ToAYq5IBXYycqyKUCowapqNBs7mlta2LQ7niTGeRCXtSUEhMBGZRjl5hcXfMoICrVWoNXTNKThGAAehhMAOMAA5HhKEiYEgTMcUJV3CYkOwEK4NAVYCAUDBwNBkQhVtokQA6QEdd5sKo9Iz9Qb0JgsNBjHj8IQidgACSgBWg2hW4KWKA2Wx2skQ+ywRxOWl09yMlysNh2d0Bj3wz2Zxk6Hy+pV++SKL1eFrYtPBYiU0LhMDllKwMB02hQ2iiXOoPJG-OdOldYLOUSm-rMxygCYAotp-doxHhuIIUIUsonwTBBK4INAEHg3QMgA

[IllusionMH]

Function should accept param of type never, otherwise there should be no checks and TS works correctly.

function getSomethingHelperNeverNoFail(foo: UnionType): number {
    switch (foo) {
        case "A":
            return 1;
        default:
            return never(foo); // you should pass param to function to enable type check
    }
}

function never(val: never): never {
    throw new Error("Should never happen");
}

http://www.typescriptlang.org/play/#code/FAFwngDgpgBAqgOwJYHsEBVKwLwwEQCCeMAPvgEJ7DABmArggMYioIwDmUIAyigLZcAFkgTsAFDRQoAXPGRpM0AJSyEdPgCMoAJxgBvYDCMwAzgHckIRoJgSpS-YePPGAQxOxCeaU+d+Y2lx02mwAjADcvn5uHhTeUf5GgSDBbABMCcYAJlA0rnQANiA+idFoJiAwAPqqUABuOjC4kiiRpc7JqdVtxgC+wP20DMysHFy8AiDCogCSCAUiUABy9ToAYq5IBXYycqyKUCowapqNBs7mlta2LQ7niTGeRCXtSUEhMBGZRjl5hcXfMoICrVWoNXTNKThGAAehhMAOMAA5HhKEiYEgTMcUJV3CYkOwEK4NAVYCAUDBwNBkQhVtokQA6QEdd5sKo9Iz9Qb0JgsNBjHj8IQidgACSgBWg2hW4KWKA2Wx2skQ+ywRxOWl09yMlysNh2d0Bj3wz2Zxk6Hy+pV++SKL1eFrYtPBBuhcJgcspWBgOm0KG0US51B5I35zp0YjqrgKYJ06rpjmcUz9ZmOUFTAFFtH7tGI8NxBChClk0+CYIJXBBoAg8EpIr0gA

UPD. You can even provide helpful exception:

function never(val: never): never {
    throw new Error("Nothing expected, but got: " + val); // or JSON.stringify(val)
}

[Hurtak]

"Function should accept param of type never, otherwise there should be no checks", but why? Shouldn't the never type be inferred from the return value of the function?

[fatcerberus]

never is the bottom type and therefore assignable to all other types. There is no type error in your code.

[IllusionMH]

There are no problem with returning never as described above.

As for handling functions with never -
probably you are looking for #12825

And you can see related discussion in comment: #14490 (comment)

[Hurtak]

Thanks for the links and explanation. Should I close this issue, since there is overlap with linked issues, or is there some value to keeping it open?

[IllusionMH]

Not sure if there are any actionable items left, so most likely this issue should be closed.

However if you are sure that there are some you can let if open for TS team to decide.