typeof switch filters union types but does not narrow them

[jack-williams]

TypeScript Version: 83fe1ea

Search Terms: switch typeof narrowing narrows

Code

// @strictNullChecks: true
function narrowingNarrows(x: {} | undefined) {
    switch (typeof x) {
        case 'string': x; return; // x is {}, should be string
        case 'number': x; return; // x is {}, should be number
    } 
}

Expected behavior:

Switching on typeof should narrow union members in addition to filtering.

• In the first clause, undefined should be filtered and {} should be narrowed to string.
• In the second clause, undefined should be filtered and {} should be narrowed to number.

Actual behavior:

Switching on typeof only filters unions.

• In the first clause, undefined is filtered but {} is not narrowed.
• In the second clause, undefined is filtered but {} is not narrowed.

Playground Link: Link: N/A (Playground not on this version).

[jack-williams]

Should this have a fixed label or should it wait on merging a PR?

[weswigham]

The PR's in a milestone (even if not yet merged), so the fixed label is fine. It's good for making sure nobody else picks up the issue accidentally while it's still open.

[jack-williams]

Cheers @weswigham. Might be worth adding a fixed label to #27180 as it is handled by the same PR.

[RyanCavanaugh]

@weswigham assigning to you for tracking purposes

[jack-williams]

@RyanCavanaugh @weswigham

Is there any merit in trying to unify the if and switch narrowing code under typeof, at least to some degree?