Delegate Covariance?

bearophile bearophileHUGS at
Sat Mar 20 17:30:54 PDT 2010

This page about C#:
contains this C# code:

class Mammals {}
class Dogs : Mammals {}

class Program {
    public delegate Mammals HandlerMethod();
    public static Mammals FirstHandler() { return null; }
    public static Dogs SecondHandler() { return null; }

    static void Main() {
        HandlerMethod handler1 = FirstHandler;
        HandlerMethod handler2 = SecondHandler;

That works:

So I have tried to translate it to D (using an 'HandlerMethod' alias and a function pointer 'handler'):

class Mammals {}
class Dogs : Mammals {}
alias Mammals function() HandlerMethod;
Mammals function() handler;
Mammals FirstHandler() { return null; }
Dogs SecondHandler() { return null; }
void main() {
    HandlerMethod handler1 = &FirstHandler;
    HandlerMethod handler2 = &SecondHandler; // Err
    handler = &SecondHandler; // err

But that D2 code doesn't compile, it gives two errors. Can you help me understand a little this situation for the D compiler? Can the D compiler change the way it works here?

Thank you and bye,

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