Narrowing by type predicate fails to produce intersection type with weak type
danvk opened this issue Β· comments
Dan Vanderkam commented
π Search Terms
- predicate
- intersection
- weak
π Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
β― Playground Link
π» 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?
}
}
π Actual behavior
The type of thing
inside the conditional is B
.
π Expected behavior
The type of thing
inside the conditional should be A & B
.
Additional information about the issue
This only happens if:
A
is a "weak" type (all optional properties).A
andB
have 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 #58495 (comment). 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.