Assert and crash in qcalendar.cpp when the first call to QDateTime::toString is done from two simultaneous threads

#include <QDateTime>
#include <atomic>
#include <thread>

static std::atomic_bool g_startThreads = false;

static void thread_func() {
	for (; !g_startThreads; );
	const auto date = QDateTime::currentDateTimeUtc();
	date.toString("yyyyMMdd_hhmmss");
}

int main(int, char*[]) {
	std::vector<std::thread> threads;
	for (int i = 0; i < 2; ++i)
		threads.emplace_back(&thread_func);

	// The following line fixes the problem (presumably, by performing orderly first initialization of the internal Qt structures used for QDateTime::toString()
	//QDateTime::currentDateTimeUtc().toString("yyyyMMdd_hhmmss");

	g_startThreads = true;

	for (auto& th: threads)
		th.join();

	return 0;
}