Infer more specific type from a union of tuple types when using string literals

[fongandrew]

TypeScript Version: 2.2.1

Code

declare var x: { type: "A", value: number; } | { type: "B", value: string; };
if (x.type === "A") {
    x.value; // Correctly inferred to be number
}

declare var y: ["A", number] | ["B", string];
if (y[0] === "A") {
    y[1]; // Not inferred as number but as string | number
}

Expected behavior: y[1] should be inferred as a number based on the value of y[0]

Actual behavior: y[1] is inferred to be string|number.

[mhegazy]

Duplicate of #10530