Error for nested type guard

[andy-hanson]

TypeScript Version: nightly (Version 2.3.0-dev.20170227)

Code

interface Base {
    kind: string;
}

interface A extends Base {
    kind: "a";
}

interface B extends Base {
    kind: "b";
    b: number;
}

declare function isAOrB(x: Base): x is A | B;

function f(x: A | B): void {
    if (x.kind === "b") {
        x.b; // works
    }
}

function g(x: Base): void {
    if (isAOrB(x)) {
        f(x); // works
        if (x.kind === "b") {
            x.b; // error
        }
    }
}

Expected behavior:

No error

Actual behavior:

Error when a type guard is nested inside of another type guard.

[krryan]

These kinds of issues relating to narrowing being not-quite-clever-enough have caused a number of headaches for me. Would definitely like to see as many cases, at least the easy ones, handled.

[andy-hanson]

I suspect this is related:

interface Base {
    name?: string;
}

interface A extends Base {
    kind: "A";
}

interface B extends Base {
    kind: "B";
}

function f(x: A | B): void {
    if (x.name && x.kind !== "B") {
        x.name.length; // Error: Object is possibly 'undefined'.
    }
}

[Andarist]

This one was fixed in TS 4.0. The issue can be closed, cc @jakebailey