I'd like this to work:

int main()
{
   static class Foo { } // Class derived from Object

   void nf(Object o) { } // Nested function (delegate) taking an Object instead of Foo

   void delegate(Foo) dg = &nf; // test.d(10): cannot implicitly convert expression (&nf) of type void delegate(Object o) to void delegate(Foo)

   return 0;
}

where the parameters are at least base class types. This will let my event library handle derived classes better. e.g. code that knows about the more derived class won't need to cast the parameter, and code that doesn't can stick with the base class.


Return covariance could also be added:

int main()
{
   static class Foo { } // Class derived from Object

   Foo nf() { return new Foo; } // Nested function (delegate) returning Foo instead of Object

   Object delegate() dg = &nf; // test.d(10): cannot implicitly convert expression (&nf) of type Foo delegate() to Object delegate()

   return 0;
}