It's better to put guards as template constraints at the std.typecons.Tuple, so erroneous Tuple template instantiations fail at the instantiation point, giving a more useful error message in the user code instead giving an error inside the std.typecons module (this is one of the main purposes of template constraints).

This code guards against duplicated field names:


private template Iota(int stop) { // this is useful in general
    static if (stop <= 0)
        alias TypeTuple!() Iota;
    else
        alias TypeTuple!(Iota!(stop-1), stop-1) Iota;
}

private bool distinctFieldNames(T...)() {
    enum int tlen = T.length; // can't move this below, probably DMD bug
    foreach (i1; Iota!(tlen))
        static if (is(typeof(T[i1]) : string))
            foreach (i2; Iota!(tlen))
                static if (i1 != i2 && is(typeof(T[i2]) : string))
                    if (T[i1] == T[i2])
                        return false;
    return true;
}

// ...............

struct Tuple(T...) if (distinctFieldNames!(T)())
{