It would be in the case for converting shape layers into solids for the purpose of masking the shape. AE has this behavior where creating shape layers won't allow you to apply a mask over it. Pre-composing will solve it but their isn't a solution for making the shape layer respond to masks.
Hi William, are you aware you can actually add masks to Shape Layers?
When you create a shape layer after effects defaults the Pen and Rectangle tools to create more shapes within the layer but to the right of the main tool bar, when you have the pen or rectangle tool selected you can choose 'Tool Cretes Shape' or 'Tool Creates Masks.'
From what it sounds like, this is all you need to do.
However, if you still need the Shape converted to a Solid for some other reason then select the shape and go:
Layer / Auto-Trace, this will create a Solid layer that uses a Mask to recreate the Shape layers shape.