Cartesian product of immutable ranges

[matovitch]

I posted a question on stackoverflow about the cartesian product 
of immutable ranges :

http://stackoverflow.com/questions/21368501/cartesian-product-of-immutable-ranges
There are a lot of various thing I don't understand.

This code compiles and run :

import std.stdio;
import std.range;
import std.algorithm;

void main() {

    immutable int[] B = [ 1, 2, 3 ];
    int[] C = [ 4, 5, 6 ];
    auto BC = cartesianProduct(B, C);
    writeln(BC);
}

This one doesn't :

import std.stdio;
import std.range;
import std.algorithm;

void main() {

    int[] B = [ 1, 2, 3 ];
    immutable int[] C = [ 4, 5, 6 ];
    auto BC = cartesianProduct(B, C);
    writeln(BC);
}

And this one does :

import std.stdio;
import std.range;
import std.algorithm;

void main() {

    immutable int[] B = [ 1, 2, 3 ];
    immutable int[] C = [ 4, 5, 6 ];
    auto BC = zip(B, C);
    writeln(BC);
}

Here B and C aren't inputRange thought acording to the template 
constraint of zip there should be. How can it compiles ? How to 
compute the cartesian product of two immutable ranges ?