Why isn't intended class constructor called?

[Zak via Digitalmars-d-learn]

I have defined a class that's meant to represent a data series, 
which has an index and a set of values.  Sometimes the user wants 
to specify a particular index of custom type, other times they 
don't care and we want to default to an array of contiguous "int" 
starting from 0.

I have attempted to create a class where the index type is a 
parameter, but defaults to int.  I also tried to create two 
constructors:  one for if the index values are not specified (in 
which the constructor makes the array of ints); and one where the 
user passes in a literal of values that match the specified type.

However, it seems that only the first constructor is getting 
called, even though I am passing in two parameters instead of 
one.  Why isn't the call matching the second constructor and 
behaving as intended?


 import std.stdio;

 class MyClass(T, U = int) {

    T[] values;
    U[] index;

    this(T[] values) {
        this.values = values;
        // Default index of contiguous ints
        for (int i; i < values.length; i++) {
            index ~= i;
        }
    }

    this(T[] values, U[] index) {
        this.values = values;
        this.index = index;
    }
}

void main() {
    auto myc1 = new MyClass!(int)([1,2,-3]);

    auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b", 
"c"]); // Error: cannot append type int to type string[]

}