Call to function returning never can not be used to narrow type

[bwrrp]

TypeScript Version: 2.3.2

A function that never returns (return type never) can not be used in a type guard to narrow the type after the branch.

Code

class Thing { };

function diverges(): never {
    throw new Error('Never returns');
}

function problem(thing: Thing | null): Thing {
    if (thing === null) {
        diverges();
        // throw new Error('Works!');
    }

    return thing; // fails type check
}

Expected behavior:

Replacing the call to diverges() with a return or throw statement causes thing to be marked as type Thing after the branch, as code will never flow from the if (thing === null) branch to the code below. I expected a call to a function with return type never to achieve the same, as this should indicate that the call never returns.

Actual behavior:

The code does not compile due to an error, as the type of thing is not narrowed to Thing:
"Type 'Thing | null' is not assignable to type 'Thing'. Type 'null' is not assignable to type 'Thing'."

[krryan]

I suspect this won't happen, because it is possible to return a never-type value, which means it's possible for that function to complete and return and control to move on down to return thing;.

[RyanCavanaugh]

Duplicate #12825. The recommended workaround is to write return diverges();

[RyanCavanaugh]

I suspect this won't happen, because it is possible to return a never-type value

The intent of never is that it shouldn't be possible.

[bwrrp]

Sorry, must have missed that one in my search. Thanks for the explanation!