Iterators in structs

[repr-man]

As an extension of my previous thread 
(https://forum.dlang.org/thread/nupwljahyutnyxdvpxob@forum.dlang.org), I wanted to ask about iterators in structs.  I will start with the following code:

void main()
{
     int[5] a = [0, 1, 2, 3, 4];
     int[5] b = [5, 6, 7, 8, 9];

     auto x = ChunksOf(chain(a[], b[]), 2);
}

struct ChunksOf
{
     this(R)(R r, size_t width)
     {
         auto iter = r.chunks(width);
         assert(is(typeof(iter) == Chunks!R));
     }
}

This code works as expected.  However, if the struct changes to:

struct ChunksOf
{
     Chunks!R iter;

     this(R)(R r, size_t width)
     {
         this.iter = r.chunks(width);
         assert(is(typeof(iter) == Chunks!R));
     }
}

it doesn't compile because R is only in the scope of the 
constructor.  This leads me to change the struct to:

struct ChunksOf(R)
{
     Chunks!R iter;

     this(R r, size_t width)
     {
         this.iter = r.chunks(width);
         assert(is(typeof(iter) == Chunks!R));
     }
}

This works, only if I change the declaration of x in main() to:

     auto x = ChunksOf!(chain(a[], b[]))(chain(a[], b[]), 2);

This requires me to pass the iterator as a template parameter and 
a regular parameter.  Since this is a bit redundant, and I wanted 
to know if there was a better way to do it.

Thanks for the help!