According to TDPL, this should be an ambiguous call and issue a compiler error:

import std.algorithm, std.array, std.stdio;          

@property bool empty(T)(T[] a) { return a.length == 0; }
@property ref T front(T)(T[] a) { return a[0]; }
void popFront(T)(ref T[] a) { a = a[1 .. $]; }

V reduce(alias fun, V, R)(V x, R range)
{
    for ( ; !range.empty; range.popFront()) {
        x = fun(x, range.front);
    }
    return x;
}

double average() { return 2.0; }
double average(double) { return 3.0; }

// compute the avg of a set of numbers, passable directly or via an array
double average(double[] values...)
{
    if (values.empty)
    {
        throw new Exception("Average of zero elements is undefined");
    }
    return reduce!((a, b) { return a + b; })(0.0, values) / values.length;
}

unittest {
    average();  // This should not compile, but does
    
    writeln(average());  // calls average(), returns 2.0
    writeln(average(0)); // calls average(double), returns 3.0
    
    assert(average(0) == 0);  // fails, since it calls average() which returns 2.0
    assert(average(1, 2) == 1.5);
    assert(average(1, 2, 3) == 2);
    
    // passing arrays and slices works too
    double[] v = [1, 2, 3];
    assert(average(v) == 2);
}

void main() { }