From TDPL (page 411)
--------------------------------------------------
The global definition
  shared uint threadsCount;

introduces a value of type shared (uint),
which corresponds to a global unsigned int in a C program.
Such a variable is visible to all threads in the system.
The annotation helps the compiler a great deal: the language "knows" that
threadsCount is freely accessible from multiple threads and
forbids naive access to it. For example:
void bumpThreadsCount(){
  ++threadsCount; // Error! Cannot increment a shared int!
}
--------------------------------------------------

But if we try the program with dmd 2.064.2 it will compile and run.
Even more the example below shows that shared(int) increment and decrement are not atomic:

import std.stdio, std.parallelism;

shared int i;

void inc(){
  foreach(n; 0 .. 1000_000)
    ++i;
}

void dec(){
  foreach(n; 0 .. 1000_000)
    --i;
}

void main(){
  taskPool.put(task(&inc));
  taskPool.put(task(&dec));
  taskPool.finish(true);
  writefln("i = %s", i);
}

https://d.puremagic.com/issues/show_bug.cgi?id=3672

On Thursday, 6 February 2014 at 15:35:31 UTC, Daniel Murphy wrote:
> https://d.puremagic.com/issues/show_bug.cgi?id=3672

Oh. Thank you. Looks like my googling skills are not good enough :)