Using --downlevelIteration with nested loops results in incorrect error handling code

[tsznuk]

TypeScript Version: 3.4.0-dev.20190223

Search Terms:

downlevelIteration, nested loop

Code

for (const i of [0, 1, 2, 3, 4]) {
  try {
    for (const j of [1, 2, 3]) {
      if (i === 2) {
        throw new Error('ERR');
      }
    }
    console.log(i);
  } catch (err) {
    console.log('E %s %s', i, err);
  }
}

Expected behavior:

When compiled with --downlevelIteration and ran the code should output:

0
1
E 2 Error: ERR
3
4

Actual behavior:

Instead the output is:

0
1
E 2 Error: ERR
E 3 Error: ERR
E 4 Error: ERR

This can be explained by looking at the generated JS code:

var __values = (this && this.__values) || function (o) {
    var m = typeof Symbol === "function" && o[Symbol.iterator], i = 0;
    if (m) return m.call(o);
    return {
        next: function () {
            if (o && i >= o.length) o = void 0;
            return { value: o && o[i++], done: !o };
        }
    };
};
var e_1, _a, e_2, _b;
try {
    for (var _c = __values([0, 1, 2, 3, 4]), _d = _c.next(); !_d.done; _d = _c.next()) {
        var i = _d.value;
        try {
            try {
                for (var _e = __values([1, 2, 3]), _f = _e.next(); !_f.done; _f = _e.next()) {
                    var j = _f.value;
                    if (i === 2) {
                        throw new Error('ERR');
                    }
                }
            }
            catch (e_2_1) { e_2 = { error: e_2_1 }; }
            finally {
                try {
                    if (_f && !_f.done && (_b = _e["return"])) _b.call(_e);
                }
                finally { if (e_2) throw e_2.error; }
            }
            console.log(i);
        }
        catch (err) {
            console.log('E %s %s', i, err);
        }
    }
}
catch (e_1_1) { e_1 = { error: e_1_1 }; }
finally {
    try {
        if (_d && !_d.done && (_a = _c["return"])) _a.call(_c);
    }
    finally { if (e_1) throw e_1.error; }
}

The e_2 variable is not cleared at the start of a loop, so the exception is rethrown in each subsequent iteration.

Note that compilation without --downlevelIteration woks as expected and generated correct code.

Playground Link:

Cannot provide a playground link since --downlevelIteration cannot be enabled in the playground (see #18209)

[NN---]

Use this playground: typescript-play.js.org/?downlevelIteration=true