I have a question.
My model was using a plane as a shadow catcher in blender 2.79.
I turned off glossy and difuse for the plane.
Set the render settings to transparent and enabled the environment pass.
I am using and environment texture as a background.
The problem comes in compositing when I try to blur the env using the alpha over node.
The entire plane shows up as a dark area and the shadow is gone.
The image and shadow plane work as designed without trying to blur the background.