reference to delegates and garbage collection

[AnFive via Digitalmars-d-learn]

Hello everybody. I am new to D, and I am trying to familiarize
with all the new (to me) features.
I have a question about a strange behavior I cannot understand.
(In case it matters, I am using gdc 2.065 for Windows, but the
same happens with dmd).

Example code :

class A {       
        void delegate() del;
                
        ~this() { writeln("A Destructor called"); }   
}

class B {       
        void fun() {}   
}

A makeA() {     
        auto a = new A();
        a.del = &(new B().fun);
        return a;
}

void main() {
                
        auto a = makeA();
         /* Magic line goes here, see below */
        a = null;
        
        foreach(i; 0..10) {
                writeln("Collecting");
                GC.collect();           
        }
        
        writeln("End");       
}

If I run this code, the instance of A is of course collected at
the first call to GC.collect().
But if I call the delegate (i.e. a.del() ) in the "magic line",
the instance is not destroyed until the end of the main. If I
"wrap" the call to a.del() in another function or method, the
instance is collected.

Can somebody explain why this happens? Is it just a strangeness
of the GC? Does calling the delegate keep a reference to a in the
current scope? Am I going crazy? I spent about half an hour
looking for a non-existent memory leak in my program because of
this behavior, so I'd like to have an explanation.

Thanks in advance!