It's a regression because it breaks Lissio, which used to work.

The issue seems to be caused by explicit calling block() in String#scan. Changing it to Opal.yield1/Opal.yieldX and wrapping arguments of Opal.yieldX into array makes it working correctly.

But I don't understand why there are some many block.call() invocations in corelib/stdlib. All of them don't care about args auto-expanding and trailing comma after arguments.

@elia @meh Do you have any ideas why is it written this way? Performance? atm corelib has 52 invocations of block(arg) or block.call(arg), all of them are affected by this issue.

@iliabylich I'm 99% sure it's just historical.

Could this be caused by commit b0f239b (sorry my attempt at a link did no work) where in corelib/array.rb

  def <<(object)
    _back-tick_
      self.push(object);
      return self;
    _back-tick_
  end

ended up as

  def <<(object)
    _back-tick_
      self.push(object);
    _back-tick_
      self
  end

and possibly should be

  def <<(object)
    _back-tick_
      self.push(object);
      return self;
    _back-tick_
  end

Sorry I cannot verify this hypothesis but it looks suspicious to me.