Always discriminate contextual types by existing discriminant property #61097
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@typescript-bot test it |
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@jakebailey Here are the results of running the user tests with tsc comparing Everything looks good! |
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Hey @jakebailey, the results of running the DT tests are ready. There were interesting changes: Branch only errors:Package: knockout Package: react-dates |
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@jakebailey Here they are:
tscComparison Report - baseline..pr
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@jakebailey Here are the results of running the top 400 repos with tsc comparing Something interesting changed - please have a look. Details
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jakebailey
reviewed
Feb 22, 2025
| return false; | ||
| } | ||
| if (p.kind === SyntaxKind.PropertyAssignment) { | ||
| return isPossiblyDiscriminantValue(p.initializer) && isDiscriminantProperty(contextualType, p.symbol.escapedName); |
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I feel like it's at least moderately concerning that this leaves the code with only one use of isPossiblyDiscriminantValue; shouldn't it be the case that if isPossiblyDiscriminantValue returns false, isDiscriminantProperty should have also returned false?
The documentation for isPossiblyDiscriminantValue implies that it's there to avoid circularities, so I'm surprised that it is a problem, and that we aren't somehow introducing circularities by not calling it anymore...
But even more surprisingly, if you just straight up delete isPossiblyDiscriminantValue and its one use (so, make it so that it always returns true), no test fails.
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The documentation for isPossiblyDiscriminantValue implies that it's there to avoid circularities, so I'm surprised that it is a problem, and that we aren't somehow introducing circularities by not calling it anymore...
It may not be about circularities in the form of implicit any. One of the failures here is related to a code like this:
interface KnockoutObservable<T> {
(value: T): void;
}
interface KnockoutObservableStatic {
<T>(value: T): KnockoutObservable<T>;
}
interface KnockoutObservableArrayStatic {
<T>(value: T[]): KnockoutObservable<T>;
}
interface KnockoutStatic {
observable: KnockoutObservableStatic;
observableArray: KnockoutObservableArrayStatic;
}
declare var ko: KnockoutStatic;
ko.observableArray([
{ someProp: 1 },
{ someProp: 2, _destroy: "evals to true" },
{ someProp: 3 },
{ someProp: 4, _destroy: ko.observable(false) },
]);At the moment, the inner ko.observable has a resolving signature - that is temporarily set on it during inference. Requesting its type here changes that prematurely or smth like that. I haven't yet had time to investigate this further to see what I could do to mitigate this problem though.
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fixes #58508
fixes #61095
I think the added
tests/cases/compiler/contextuallyTypedByDiscriminableUnion2.tstest case tells it all (TS playground). Currently both property assignments that reference external values and shorthand property assignments are capable of narrowing this case but a property assignment with an "inline" initializer isn't.