It is not a stretch to envision someone writing:

public interface DoubleFunction extends Function<Double, Double> {}

And while it is good practice to declare the functional method Double apply(Double in);, I don't know that we should require it.

At the very least, we should ensure that the matcher throws an informative, suitable error when asked for a DoubleFunction. Currently, an IndexOutOfBoundsException is thrown here. At the very least, we should throw some IllegalArgumentException. It would be even cooler if we could just find the functional interface within Function.