Using `future<>::detach()` is a possible memory leak

[sean-parent]

{
auto p = package<int()>(immediate_executor, []{ return 42; });
p.second.then([a = annotate()](int x){ std::cout << x << '\n'; }).detach();
}

outputs:

annotate ctor
annotate move-ctor
annotate move-ctor
annotate move-ctor
annotate move-ctor
annotate dtor
annotate dtor
annotate dtor
annotate dtor

Notice we are one dtor short... detach() creates a retain cycle - this needs to be broken in the event of a broken promise exception (or any exception).

[FelixPetriconi]

Fixed in 1.3.0