How compiler detects forward reference errors

[Adam D. Ruppe via Digitalmars-d-learn]

On Saturday, 3 September 2016 at 14:06:06 UTC, Igor wrote:
> 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:

Via bugs, it does

if(would_be_annoying_to_user)
   trigger_error();
else
   let_it_work();


If a user complains they say "report to bugzilla" but bugzilla is 
conveniently redirected to "wait forever; fix not coming".


It is all random bugs