Optional delegates in C#

This is a simple example of two extension methods overloads

public static class Extended 
{
    public static IEnumerable<int> Even(this List<int> numbers)
    {
        return numbers.Where(num=> num % 2 == 0);
    }

    public static IEnumerable<int> Even(this List<int> numbers, Predicate<int> predicate)
    {
        return numbers.Where(num=> num % 2 == 0 && predicate(num));
    }
}

I'd like to be able to merge them into one, by setting a delegate to be optional:

public static class Extended 
{
    public static IEnumerable<int> Even(this List<int> numbers, Predicate<in> predicate = alwaysTrue)
    {
        return numbers.Where(num=> num % 2 == 0 && predicate(num));
    }

    public static bool alwaysTrue(int a) { return true; }
}

However, compiler throws an error:

Default parameter value for 'predicate' must be a compile-time constant

I don't see how my alwaysTrue function is not constant, but hey, compiler knows better :)

Is there any way to make the delegate parameter optional?