I'm wondering whether the result of comparing collation sort keys (= already pre-processed strings for faster collation-compatible sorting/searching) is a strong or weak ordering. The implementation doesn't seem to be available and the documentation (https://developer.apple.com/documentation/coreservices/1390378-uccomparecollationkeys) very carefully talks about equivalence (indicating a weak ordering) and not of equality (which would indicate a strong ordering).

So which one is it?

All other platform-specific collation APIs produce an octet-stream whose ordering is compatible with strcmp(), so definitely strong. OSX is the odd one out.