You're right, it's a bug. I'd fix that with only making company-css-value-classes recursive (but not refer back to company-css-property-alist there).
To put it differently, I don't like that there's conflation of property values and property names: if there appears a value sometimes that is the same as some property's name (unlikely, I know), it would be hard to represent in the proposed structure.
Anyway, I've taken more than enough of your time. Please go ahead with whichever version you prefer.
Don't worry Dmitry, I very much appreciate your through reviews.
You are right that there are some cases where property names and value
class names are the same (there are currently four), and I found a bug
in my handling of one of those. I remedied that by going with your
suggestion of making value classes symbols again, and only referring to
other value classes from those.
For properties, I made them also refer to value classes by default, but
made it possible to refer back to other properties in the cases where
the reference isn't found in the value class alist.
I attempted to document this thoroughly in the code and added some tests
that should cover the tricky cases.
I've installed the patch with the changes mentioned above.
Thanks again!
-- Simen