Naming collision of hidden variables when mixing for-of and spread operator

[voltrevo]

TypeScript Version: 2.1.4

Code

function baz(x: any) {
  return [[x, x]];
}

function foo(set: any) {
  for (const [value, i] of baz(set.values)) {
    const bar: any = [];
    (() => bar);

    set.values.push(...[]);
  }
};

Expected behavior:

Names of hidden/implementation variables should not clash.

Actual behavior:

Two _a hidden variables are created which would be in different scopes under ES6 let/const but are in the same scope since they are var:

function baz(x) {
    return [[x, x]];
}
function foo(set) {
    var _loop_1 = function (value, i) {
        var bar = [];
        (function () { return bar; });
        (_a = set.values).push.apply(_a, []);
    };
    for (var _i = 0, _a = baz(set.values); _i < _a.length; _i++) {
        var _b = _a[_i], value = _b[0], i = _b[1];
        _loop_1(value, i);
    }
    var _a;
}
;

Note:

After pruning my code to isolate the issue, I'm not sure what the impact of the collision is anymore, but in my original code I was getting an infinite loop due to the loop index being part of the collision. I should be able to produce a short example like that if needed.

[voltrevo]

Wow that was fast @vladima 🎉