Type guard do not narrow array element accessed inline

[caesarsol]

TypeScript Version: 2.2.1

A type guard in a loop does not narrow an array element to its type inside the conditional.

On the opposite, the same works by accessing the array element using a variable in between.

This could be by design, but it's quite strange to me...

Code

declare function isNum(a: any): a is number
declare function acceptNum(c: number): void

let children: Array<string | number> = [3, 4, 5]

for (let i = 0; i < children.length; ++i) {
    if (isNum(children[i])) {
        acceptNum(children[i]); // error: Type string is not assignatable to type number 
    }
}

for (let i = 0; i < children.length; ++i) {
    const child = children[i];
    if (isNum(child)) {
        acceptNum(child); // no error
    }
}

TS play demo here

Expected behavior:
children[i] type narrowed to number inside the if body.

Actual behavior:
children[i] type is still string | number inside the if body.

[mhegazy]

Duplicate of #10530

[caesarsol]

Sorry about that, I searched with various keywords and nothing popped out. Very few of the phrasing in common!