The question from @Sergey Sharybin (sergey) in T47212 was to verify if this node still works equally well on actual green screen footage, like the one from Tears of Steel. Did you try that?

How about we just close this and rename the node to "Doesn't do what the node label is supposed to do" node, then we won't need to commit the three line patch.

@Brecht Van Lommel (brecht) I am a little confused by this because that is not what this node is designed for.
It is not a keying node but a matte node used to remove high or low contrast pieces of an image.
Also, the fact that it might look different should not matter. All this patch is fixing something that
was missed in rBdd38dce7f0ae604396d1e96bc49500369fdedf29

If you look the example file I posted above you can see the difference in weights between the current
the RGBtoBW node and the luminance matte node. So yes it is going to be different but it will be a
correct difference to match the rest of Blender. Besides this code will not go into a release until 2.8.