Microsoft/TypeScript
Ver no GitHubNarrowing by type predicate fails to produce intersection type with weak type
Open
#58.518 aberto em 13 de mai. de 2024
BugDomain: check: Control FlowHelp Wanted
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Description
🔎 Search Terms
- predicate
- intersection
- weak
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
⏯ Playground Link
💻 Code
interface A {
label?: string;
a?: string; // make required to eliminate error
}
interface B {
label?: string; // remove shared property to eliminate error
b: boolean;
}
function isB(thing: object): thing is B {
return 'b' in thing;
}
function test(thing: A) {
thing.a; // ok
if (isB(thing)) {
thing.a; // error?
}
}
🙁 Actual behavior
The type of thing inside the conditional is B.
🙂 Expected behavior
The type of thing inside the conditional should be A & B.
Additional information about the issue
This only happens if:
Ais a "weak" type (all optional properties).AandBhave at least one overlapping property.
If either of these isn't the case, then you get the expected intersection type:
This came up in vscode while testing https://github.com/microsoft/TypeScript/pull/58495#issuecomment-2105358748. The issue is there already but is latent because control flow analysis can't see that a type predicate becomes a type assertion without that PR.