Aliased Conditions with explicit type definition results in the "old" behavior

Bug Report

🔎 Search Terms

Aliased Conditions

🕗 Version & Regression Information

V4.4.0-beta / nightly

• I was unable to test this on prior versions because it is a new feature

⏯ Playground Link

Playground link with relevant code

💻 Code

type Shape =
    | { kind: "circle", radius: number }
    | { kind: "square", sideLength: number };

function area(shape: Shape): number {
    const isCircle: boolean = shape.kind === "circle";
    if (isCircle) {
        // We know we have a circle here!
        return Math.PI * shape.radius ** 2;
    }
    else {
        // We know we're left with a square here!
        return shape.sideLength ** 2;
    }
}

🙁 Actual behavior

Adding the explicit type to "isCircle" results in that the sample breaks

Property 'radius' does not exist on type 'Shape'.
  Property 'radius' does not exist on type '{ kind: "square"; sideLength: number; }'.
Property 'sideLength' does not exist on type 'Shape'.
  Property 'sideLength' does not exist on type '{ kind: "circle"; radius: number; }'.

🙂 Expected behavior

Making the implicit type explicit, does not result in the behavior to change.

Using the sample provided by the TS 4.4 RC announcement, getting the implicit type of "isCircle" (by hovering over it) yields boolean

. By then explicitly defining the type (boolean), the Aliased Conditions seems to break/revert back to pre 4.4 behavior.

Beginne mit dem verlinkten TypeScript Playground und reproduziere den Unterschied zwischen den inferierten und explizit annotierten isCircle-Bedingungen. Untersuche das Control-Flow-Narrowing des Compilers für aliasierte Bedingungen; abgeschlossen ist die Untersuchung, wenn die explizite boolean-Annotation das Narrowing in beiden Zweigen ohne die gemeldeten diagnostics beibehält.