Discriminated union types lose their narrowed type inside nested function scopes

[ryanatkn]

TypeScript Version:

nightly (1.9.0-dev.20160619-1.0)

Code

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

interface Circle {
    kind: "circle";
    radius: number;
}

type Shape = Rectangle | Circle;

function test1(s: Shape) {
    if (s.kind === "rectangle") {
        s;  // Rectangle
        setTimeout(() => {
          s; // Rectangle | Circle
        });
    }
}

Expected behavior: both s usages to narrow to type Rectangle

Actual behavior: the s in the nested function scope has type Rectangle | Circle

[yortus]

I believe this is by design, and applies to all type guards, not just those for discriminated unions. The problem is that the compiler cannot in general be sure when the nested function will be called, so it's not safe to assume the narrowed type will still apply by the time the function is actually called. The variable may have been re-assigned by then.

There are a couple of special cases involving IIFEs and consts where the narrowing can safely cross into the nested function body, and the compiler handles these cases correctly. See #8849.

[ryanatkn]

Makes sense, thank you!

So the workaround would look like this:

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

interface Circle {
    kind: "circle";
    radius: number;
}

type Shape = Rectangle | Circle;

function test1(s: Shape) {
    if (s.kind === "rectangle") {
        const rect = s;
        s;  // Rectangle
        setTimeout(() => {
          s; // Rectangle | Circle
          rect; // Rectangle
        });
    }
}