Is std::to_string thread safe?
std::to_string behaves as if it calls sprintf ([string.conversions]/7), and the behavior of sprintf depends on the global locale, which can be modified by setlocale (or by std::locale::global, which internally calls setlocale).
The wording in [clocale.syn]/2 seems to imply that std::to_string is thread safe, because it does not allow setlocale to introduce a data race with std::to_string or sprintf.