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 Vladimir Matveev (@vladima) 🎉