Union of tuple types destructures incorrectly

[osdiab]

TypeScript Version: 3.2.0-dev.20181004
Search Terms: union tuple array destructuring
Code

type Unioned = [string] | [string, number]
const ex: Unioned = ["hi"] as Unioned

// unexpected: y is typed as string | number, instead of number | undefined
const [x, y] = ex;

Expected behavior:
y typed as number | undefined

Actual behavior:
y typed as string | number

Playground Link: Playground

Related Issues: Not really

[ulrichb]

@ahejlsberg

I have a similar situation with function args tuples (and get TS2556: Expected 1 arguments, but got 2 or more). Is this the same issue?

type RequiredArg = string;
type ArgsTuple = [RequiredArg, ...string[]];

interface FuncHolderA {
    readonly func: (...args: ArgsTuple) => void;
}

interface FuncHolderB {
    readonly func: (arg: RequiredArg) => void;
}

//

type FuncHolder = FuncHolderA | FuncHolderB;

function invoke(fh: FuncHolder) {
    const args: ArgsTuple = ["x", "y", "z"];

    fh.func(...args); // TS2556: Expected 1 arguments, but got 2 or more.
    fh.func(args[0]); // compiles, but isn't that what we want
}

//

const fhA: FuncHolderA = { func: (x, y, z) => console.log(x, y, z) };
invoke(fhA);

const fhB: FuncHolderB = { func: (x) => console.log(x) };
invoke(fhB);

Playground