Return completions when yield* is used

[anba]

var it = Iterator.concat({
  [Symbol.iterator]() { return this; },
  next() { console.log("next"); return {done: false}; },
  return() { console.log("return"); return {done: false}; },
});

Before #18:

js> it.next()
next
({value:(void 0), done:false})
js> it.return()
return
({value:(void 0), done:true})
js> it.return()
({value:(void 0), done:true})

When using yield* semantics:

js> it.next()
next
({done:false})
js> it.return()
return
({done:false})
js> it.return()
return
({done:false})

But the current spec requires something in between, because per spec return completions must always close the iterator, whereas yield* allows to continue after return completions.

After #18:

s> it.next()
next
({done:false})
js> it.return()
return
({value:(void 0), done:true})
js> it.return()
({value:(void 0), done:true})

[anba]

SpiderMonkey implementation before #18:

for (var innerValue of allowContentIterWith(item, method)) {
  yield innerValue;
}

Implementation after #18:

// Step 3.a.i.
var iter = callContentFunction(method, item);

// Step 3.a.ii.
if (!IsObject(iter)) {
  ThrowTypeError(JSMSG_GET_ITER_RETURNED_PRIMITIVE);
}

// Step 3.a.iii.
var iterNext = iter.next;

// Steps 3.a.iv-v.
var wrapped = {
  next() {
    return callContentFunction(iterNext, iter);
  },
  return() {
    IteratorClose(item);
    return {done: true, value: undefined};
  },
};
yield* allowContentIterWithNext(wrapped, wrapped.next);

[michaelficarra]

After #18:

s> it.next()
next
({done:false})
js> it.return()
return
({value:(void 0), done:true})
js> it.return()
({value:(void 0), done:true})

This seems like the correct behaviour. What are you saying is undesirable about it? That it can't be implemented as literally

function* concat(...iterables) {
  for (const it of iterables) yield* it;
}

?

[anba]

Yes, it seems undesirable to add another wrapper to be able to use yield*.