QDate::FromString breaks when accessed from multiple threads using default calendar parameter

After updating from Qt 5.12.9 to 5.15.1, part of our multithreaded code which accesses QDate::FromString started firing a Q_ASSERT in qcalendar.cpp non determinstically, line 103.  After debugging into Qt source code, the problem is with the global static calendar registry:

Q_GLOBAL_STATIC(Registry, calendarRegistry);

This registry is accessed concurrently when QDate::FromString is called, and the assert  Q_ASSERT(byId[size_t(id)] == nullptr)  fails whenever QDate::FromString is called using default calendar parameter which makes sense, since there is already a gregorian default calendar in registry.

Sample code to reproduce:

 
class HelloWorldTask : public QRunnable
{
	void run() override
	{
		QDate::fromString("2017-12-21", "yyyy-MM-dd");
	}
};

for(int i = 0; i < 40 ; ++i)
{
	HelloWorldTask *hello = new HelloWorldTask();
	QThreadPool::globalInstance()->start(hello);
}
QThreadPool::globalInstance()->waitForDone();

We worked around the problem using a constant global static QCalendar object which is passed as the last parameter in QDate::FromString

Thanks