Type narrowed to never, incorrectly

[ljharb]

TypeScript Version: latest

Search Terms: n/a

Code

function f(a?: {}) {
    if (!a) {
        // filters out falsy values:
        //  - `null`
        //  - `undefined`
        //  - `false`
        //  - `NaN`
        //  - `0` / `-0`
        //  - `''`
        return {};
    }

    // here, the type of `a` has been narrowed from "optional {}"
    // to just "{} minus `object`" (object-coercible, ie, anything but null/undefined, but not `object`)
    // effectively, this is `string | number | symbol | boolean | Function`

    if (typeof a !== 'object') {
        // `a` here could be the following values:
        //   - a function
        //   - `true``
        //   - a non-zero non-`NaN`` number
        //   - a non-empty string
        //   - a symbol
        return a; // however, the type inferred here is `never`
    }

    // here, `a` has a verifiably certain type of `object`.
    // because of the type guard above.
    return a; // however, the type is reported as `{}` 
}

Expected behavior: The type of a inside the if block will be either {}, or better, string | number | symbol | boolean | Function.

Actual behavior: The type of a inside the if block is never, which is flat out incorrect.

Playground Link: link

Related Issues: not sure how to search for it

[DanielRosenwasser]

It's kind of funny, but we were actually talking about #18196 right as you were opening this issue.

But filing this as a duplicate of #18196

[ljharb]

This may be addressed by #27157; it can be closed or left open til the next release, as you like.

[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.