Shared and race conditions

[Wanderer via Digitalmars-d-learn]

I'm trying to simulate a race condition in D with the following 
code:

(https://run.dlang.io/is/RfOX0I)
```
import std.stdio;
import core.thread;
import std.concurrency;

shared struct IdGen(T)
{
    T id;

    this(T start)
    {
        id = start;
    }

    T next()
    {
        id = id.next();
        return id;
    }
}

struct MyId
{
    uint num;

    shared MyId next()
    {
        return MyId(num + 1);
    }
}

static void getId(shared IdGen!(MyId)* g)
{
    writeln("next: ", g.next());
    writeln("next: ", g.next());
}

void main()
{
    MyId start = {0};
    auto g = IdGen!(MyId)(start);

    auto num = 12;
    for (auto i = 0; i < num; i++)
    {
        spawn(&getId, &g);
    }

    thread_joinAll();
    writeln("Done");
}
```

But all I get is correct IDs without any sign of a race (i.e. 
duplicate ids).
Does compiler add synchronization for `shared` data?