shared Duration math

[knutaf via Digitalmars-d]

Hello,

My question probably reveals a dangerous lack of understanding of 
the language, but perhaps someone can help me understand why the 
following code works and doesn't work in the indicated places.

Secondarily, does anyone know a really good article that explains 
how "shared" works, especially shared as a qualifier on member 
functions? And what it even means to be a shared local variable?

import std.stdio;
import core.time;
import std.datetime;

void main()
{
     shared Duration sd1 = dur!"seconds"(5);
     shared Duration sd2 = dur!"seconds"(6);
     Duration d3 = dur!"seconds"(7);

     //Duration d4 = sd1 + sd2; // error, incompatible types
     //Duration d4 = sd1 + d3; // error, incompatible types
     //Duration d4 = (cast(Duration)sd1) + (cast(Duration)sd2); // 
error, template deduction

     // compiles, but how is the implicit cast allowed, when 
explicit fails above?
     Duration d5 = sd1;
     Duration d6 = sd2;
     Duration d7 = d5 + d6;
}