the four options:

// common prefix
if (map.has(key)) return key;
let value = callback(key);

// option 1
if (map.has(key)) throw;
map.set(key, value);
return value;

// option 2
map.set(key, value);
return value;

// option 3 (not proposed)
if (map.has(key)) return map.get(key);
map.set(key, value);
return value;

// option 4 (not proposed): change `Map.prototype.set` to check if we're currently in a `getOrInsertComputed` for this map

sounds like people like option 2 (which I'm also happy with)

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

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

The feature has sufficient testing and appropriate pre-implementation experience