`if` statement in loop changes type of variable outside the `if`

TypeScript Version: 2.7.0-dev.20171112

Code

type U = { kind: "A", xs?: number[] } | { kind: "B", xs?: number[] };
function f(u: U) {
	if (u.xs) {
		for (const b of u.xs) {
			if (u.kind === "A") {}
		}
	}
}

Expected behavior:

No error.

Actual behavior:

src/a.ts(4,19): error TS2532: Object is possibly 'undefined'.

[ahejlsberg]

Duplicate of #18840.

[ahejlsberg]

Actually, I'll flip it back to a bug since it is not exactly the same as #18840. However the root cause is similar: A narrowing by a discriminant property (such as u.kind === "A") resets any previous narrowing in effect for the variable. Here, the back edge of the for loop causes the narrowing of u.xs to disappear. This is unfortunately not a simple thing to fix.

Another case:

function containsItself(arr: {}): boolean {
    if (!Array.isArray(arr)) {
        return false;
    }
    for (const x of arr) {
        if (x === arr) {
            return true;
        }
    }
    return false;
}

src/a.ts(5,16): error TS7022: 'x' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

And another:

function f(u: { type: "A"; x: number; } | { type: "B"; x?: number; }) {
    if (u.type === "A") {
        u.x; //number
    }
    if (u.type === "B" && u.x !== undefined) {
        u.x; //number
    }
    if (u.type === "A" || u.type === "B" && u.x !== undefined) {
        u.x; // number | undefined
    }
}