std.complex

[Lars T. Kyllingstad]

On Wednesday, 1 January 2014 at 12:29:35 UTC, Stewart Gordon 
wrote:
>  [...]
>
> Or even more exotically, use Complex!(Complex!real) to 
> implement hypercomplex numbers.

This is an extremely marginal use case.  Currently 
Complex!(Complex!T) folds to Complex!T.  I thought this was 
specified in the module documentation, but if it was, it's been 
removed.  It is explained in a comment in the source code, though:

/*  Makes Complex!(Complex!T) fold to Complex!T.

     The rationale for this is that just like the real line is a
     subspace of the complex plane, the complex plane is a subspace
     of itself. Example of usage:
     ---
     Complex!T addI(T)(T x)
     {
         return x + Complex!T(0.0, 1.0);
     }
     ---
     The above will work if T is both real and complex.
*/
template Complex(T) if (is(T R == Complex!R))
{
     alias T Complex;
}