Replying to GerdP:
Please clarify the description. I use bicycle=use_sidepath often and I see no need to add a tag like cycleway=separate. I agree that a combination like cycleway=both + bicycle=use_sidepath should produce a warning.
Hope the description is a bit better, now.
Replying to Famlam:
Probably good to exclude any highway that contains bicycle:conditional, bicycle:forward/backward/both_ways:conditional for this check.
I would expect cycleway:conditional or similar then.
There might be some more combinations with oneway=yes.
Be additionally aware of oneway:bicycle, oneway:conditional, oneway:bicycle:conditional in this case :)
Yes, I am aware of oneway:bicycle. For *:conditional again, I would expect all tags to carry the suffix.