std::async won’t spawn a new thread when return value is not stored

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From just::thread documentation:

If policy is std::launch::async then runs INVOKE(fff,xyz...) on its own thread. The returned std::future will become ready when this thread is complete, and will hold either the return value or exception thrown by the function invocation. The destructor of the last future object associated with the asynchronous state of the returned std::future shall block until the future is ready.

In

std::async(std::launch::async, foo, arg1, arg2);

The returned future is not assigned anywhere and its destructor blocks until foo finishes.

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