reactor-core/src/main/java/reactor/core/publisher/FluxFlatMap.java
@SuppressWarnings("unchecked")
	static <T, R> boolean trySubscribeScalarMap(Publisher<? extends T> source,
			CoreSubscriber<? super R> s,
			Function<? super T, ? extends Publisher<? extends R>> mapper,
			boolean fuseableExpected) {
		if (source instanceof Callable) {
			T t;

			try {
				t = ((Callable<? extends T>) source).call();
			}
			catch (Throwable e) {
				Operators.error(s, Operators.onOperatorError(e, s.currentContext()));
				return true;
			}

			if (t == null) {
				Operators.complete(s);
				return true;
			}

			Publisher<? extends R> p;

			try {
				p = Objects.requireNonNull(mapper.apply(t),
						"The mapper returned a null Publisher");
			}
			catch (Throwable e) {
				Operators.error(s, Operators.onOperatorError(null, e, t, s.currentContext()));
				return true;
			}

			if (p instanceof Callable) {
				R v;

				try {
					v = ((Callable<R>) p).call();
				}
				catch (Throwable e) {
					Operators.error(s, Operators.onOperatorError(null, e, t, s.currentContext()));
					return true;
				}

				if (v != null) {
					s.onSubscribe(Operators.scalarSubscription(s, v));
				}
				else {
					Operators.complete(s);
				}
			}
			else {
				if (!fuseableExpected || p instanceof Fuseable) {
					p.subscribe(s);
				}
				else {
					p.subscribe(new FluxHide.SuppressFuseableSubscriber<>(s));
				}
			}

			return true;
		}

		return false;
	}
@SuppressWarnings("unchecked")
	static <T, R> boolean trySubscribeScalarMap(Publisher<? extends T> source,
			CoreSubscriber<? super R> s,
			Function<? super T, ? extends Publisher<? extends R>> mapper,
			boolean fuseableExpected) {
		if (source instanceof Callable) {
			T t;

			try {
				t = ((Callable<? extends T>) source).call();
			}
			catch (Throwable e) {
				Operators.error(s, Operators.onOperatorError(e, s.currentContext()));
				return true;
			}

			if (t == null) {
				Operators.complete(s);
				return true;
			}

			Publisher<? extends R> p;

			try {
				p = Objects.requireNonNull(mapper.apply(t),
						"The mapper returned a null Publisher");
			}
			catch (Throwable e) {
				Operators.error(s, Operators.onOperatorError(null, e, t, s.currentContext()));
				return true;
			}

			if (p instanceof Callable) {
				R v;

				try {
					v = ((Callable<R>) p).call();
				}
				catch (Throwable e) {
					Operators.error(s, Operators.onOperatorError(null, e, t, s.currentContext()));
					return true;
				}

				if (v != null) {
					s.onSubscribe(Operators.scalarSubscription(s, v));
				}
				else {
					Operators.complete(s);
				}
			}
			else {
				if (!fuseableExpected || p instanceof Fuseable) {
					p.subscribe(s);
				}
				else {
					p.subscribe(new FluxHide.SuppressFuseableSubscriber<>(s));
				}
			}

			return true;
		}

		return false;
	}
Checks if the source is a Supplier and if the mapper's publisher output is also
a supplier, thus avoiding subscribing to any of them.

@param source the source publisher
@param s the end consumer
@param mapper the mapper function
@param fuseableExpected if true, the parent class was marked Fuseable thus the
mapping output has to signal onSubscribe with a QueueSubscription

@return true if the optimization worked