I'm finding it impossible to search for C-- on Stackoverflow. It doesn't seem to have a tag either. Is this a limitation of the search system Stackoverflow uses?
There IS such a tag: c-minus-minus. It's just not used very much.
There isn't a "c--" tag because it doesn't conform to the current tag naming rules; anyone attempting to use "c--" as a tag will see it automatically converted to "c". This might explain why some C-- questions are tagged as c instead. If you do come across these, do retag them to c-minus-minus.
As for search, unfortunately punctuations and symbols are not currently accounted for and so a search for "C--" gives the same results as "C" 1.
An alternative would be to use SymbolHound, however for such a short (and ambiguous) term you may need to wade through quite a few false positives.