Shadow-catcher stays noisy even with Denoising - also very dark I am facing some issues with the shadow-catcher and denoiser in Blender 2.79.
Denoising doesn't seem to have any effect on shadow catcher shadow. If I turn off shadow catcher on ground plane the desnoising works well. But if it's turned on - the shadow stays noisy. 

Also the shadow become much darker with shadow catcher turned on.

Am I doing something wrong?
How do I get noise free contact shadow and lighter shadow like the left render?


Here is the .blend file of the scene in case someone wants to inspect it:
http://pasteall.org/blend/index.php?id=48592



Will really appreciate any help.
It's because denoising works only with RGB-channels, not alpha-channel.
You can try to denoise it in compose nodes.

I use Bilateral Blur to denoise alpha channel in render, then set it in result picture by Set Alpha node. Extract ground plane by object ID mask: