The mongoc_log_func_t typedef is used in the parameters of exported function symbols declared with __cdecl (including mongoc_log_default_handler) but is not declared with __cdecl, meaning a different default calling convention (e.g. __vectorcall with /Gv) changes the calling convention of the mongoc_log_func_t type and dependent exported function symbols. This means the following program fails to link with /Gv:

void broken(void) {
  // error: mongoc_log_default_handler is declared with __cdecl, but due to /Gv,
  //        mongoc_log_set_handler requires a function pointer with __vectorcall
  //        because mongoc_log_func_t is missing __cdecl, so mongoc_log_default_handler
  //        is not considered a valid argument (incompatible pointer types).
  mongoc_log_set_handler(&mongoc_log_default_handler, NULL);
}