How compiler detects forward reference errors

[Igor via Digitalmars-d-learn]

Can anyone explain in plain English how does compiler process and 
detect a "test.d(6) Error: forward reference of variable a" in 
following code:

import std.stdio;

enum a = 1 + b;
enum d = 5 + a; // No error here
enum b = 12 + c;
enum c = 10 + a; // error here

void main()
{
     writeln("Hello World!", b);
}