no type narrowing after if statement if function with return type never called inside

[greg-hornby-roam]

Tried my hardest to find if this was a duplicate error, so sorry if it is.

{
    let foo: string | number;
    if (typeof foo === "string") {
        throw "end";
    }
    foo; //Should Be: number; Is: number; Correct
}

In the code snippet above, it's correct that after the if statement, foo should be narrowed to type number, because if it is type string, then an error is thrown. However if abstracting the throw statement away into a function whose return type is never, the narrowing does not occur.

{
    let end = function(): never {
        throw "end";
    }
    let foo: string | number;
    if (typeof foo === "string") {
        end();
    }
    foo; //Should Be: number; Is: string | number; Incorrect
}

Playground Link: http://www.typescriptlang.org/play/#src=%7B%0D%0A%20%20%20%20let%20foo%3A%20string%20%7C%20number%3B%0D%0A%20%20%20%20if%20(typeof%20foo%20%3D%3D%3D%20%22string%22)%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20%22end%22%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20foo%3B%20%2F%2FShould%20Be%3A%20number%3B%20Is%3A%20number%3B%20Correct%0D%0A%7D%0D%0A%0D%0A%0D%0A%7B%0D%0A%20%20%20%20let%20end%20%3D%20function()%3A%20never%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20%22end%22%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20let%20foo%3A%20string%20%7C%20number%3B%0D%0A%20%20%20%20if%20(typeof%20foo%20%3D%3D%3D%20%22string%22)%20%7B%0D%0A%20%20%20%20%20%20%20%20end()%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20foo%3B%20%2F%2FShould%20Be%3A%20number%3B%20Is%3A%20string%20%7C%20number%3B%20Incorrect%0D%0A%7D

[RyanCavanaugh]

Duplicate #12825

[ajafff]

TypeScript's control flow analysis cannot use type information, because type information is based on control flow. That would be a circular dependency.

I wrote a lint rule to enforce using return or throw when calling functions that return never: https://github.com/fimbullinter/wotan/blob/master/packages/mimir/docs/return-never-call.md
For a guide on how to use it, see this documentation: https://github.com/fimbullinter/wotan/blob/master/packages/wotan/README.md

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.