Does an HDR environment texture affect the Shadow pass? I am compositing a scene using a standard process in Cycles, and it worked correctly as long as I had my lighting setup in the scene, however when I switched to HDR, my Shadow pass seems to become a solid black. Why is this, and how can it be avoided?

My node setup:
The HDR environment does not affect the shadow pass. At all. The problem originated with the removal of the lighting setup.

In other words, the shadow pass only supports lamps. Mesh lights and world lighting will not affect it. See Why does the shadow pass not account for mesh lights?