[Issue 3180] New: Need delegate covariance and contravariance

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=3180

           Summary: Need delegate covariance and contravariance
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: rayerd.wiz at gmail.com


import std.stdio;

class A {}
class B : A {}

class X
{
    A foo() { return new A; }
}
class Y : X
{
    B foo() { return new B; }
}

class V
{
    void foo(B){}
}

void main()
{
    // Class Covariance (supported already)
    {
        X x = new X;
        Y y = new Y;
        A r = x.foo();
        A s = y.foo();
        B t = y.foo();
        writeln(r); // A
        writeln(s); // B
        writeln(t); // B
    }
    // Delegate Covariance
    {
        A delegate() f = delegate A() { return new A; }; // Of course, OK.
        writeln(f()); // A

        //A delegate() g = delegate B() { return new B; }; // Need support the
covariance
        //writeln(g()); // This should be B.
    }


    // Delegate Contravariance
    {
        V v = new V;
        void delegate(B) g = &v.foo; // Of course, OK.
        //void delegate(A) f = &v.foo; // Need suport the contravariance.
    }
}

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