Expected: ReferenceError thrown when calling the various test* functions Actual: No ReferenceError thrown function testForOfRhs() { let x = "outer"; for (let x of [x]) print(x); } testForOfRhs(); function testForOfComputed() { let x = "outer"; for (let {[x]: x} of [{outer: 123}]) { print(x) } } testForOfComputed(); function testForInRhs() { let x = "outer"; for (let x in {key: x}) print(x); } testForInRhs(); function testForInComputed() { let x = "0"; for (let {[x]: x} in {key: 123}) { print(x) } } testForInComputed(); function testForRhs() { let x = "outer"; for (let x = x;;) { print(x); break; } } testForRhs() function testForComputed() { let x = "outer"; for (let {[x]: x} = {outer: 123};;) { print(x); break; } } testForComputed();

Another fun addition I seem to have discovered in reading the semantics for this stuff just now: var item; for (let x of [function() { x; }]) item = x; // This is a TDZ violation as far as I can tell: |x;| refers to // an x in the loop-head, not an x for an iteration of the loop. item(); I don't believe this changes the implementability of anything (or at least it seems like it shouldn't, in advance of implementation) -- but it is a rather odd bit of behavior.

Will see if I can make any progress on this.

I have a PR against the Github repo/branch where bug 1263355 is being worked on, that I believe fixes this. Don't anybody bother testing it, tho, because there are known cases (e.g. any for-in/of loop with lexical declaration whose body captures one of those lexical declarations) where the PR falls down, because the underlying branch isn't fully completed yet. https://github.com/syg/gecko-dev/pull/1

Fixed by bug 1263355.