The intention is that "\\<\\>" will never match at all, hence that
match-end will never be executed. "\\<" means "beginning of word" and
"\\>" means "end of word". Could there be something in your environment
whereby an end of word might be at the same position as the beginning of
a word? If this is the case, then I will need to come up with an actual
regexp which can never match anything.


It matchs when cursor is between a Chinese char and an ASCII char like “你hao”( cursor before ‘h’).


Is the bug reproducible?
Yes. See https://github.com/joaotavora/yasnippet/issues/923#issuecomment-376569337

If so, could you possibly patch that bit of
code to add (not (equal c-opt-cpp-prefix "\\<\\>")) before the
(looking-at ...) form, and see if that stops the error.

No. the value of c-opt-cpp-prefix printed as "\s *#\s *”.