Contravariance (is this a bug)

[Tomek Sowiński]

Jesse Phillips wrote:

> Contravariance allows derived classes to return from a function any
> derived class of what the parent class returns. I have discovered that if
> you claim to return a derived class, you can still return the parent
> class. In the example below, A_Parser is returning Report[] from a
> function that claims to return A_Report[]. I'm thinking this is a bug, is
> there any reason it wouldn't be?
> 
> import std.stdio;
> 
> class A_Parser : Parser {
>    Report[string] stuff;
> 
>    this() {
>       stuff = ["hi": new A_Report("Bye")];
>    }
> 
>    override A_Report get(string uuid) {
>       return stuff[uuid];
>    }
> }
> 
> class A_Report : Report {
>    string value;
>    this(string val) {
>       value = val;
>    }
> 
>    void print() {
>       writeln(value);
>    }
> }
> 
> abstract class Parser {
>    abstract Report get(string);
> }
> 
> class Report {
> }
> 
> void main() {
>    auto a = new A_Parser();
> 
>    a.get("hi").print;
> 
> }


Yep, it's a bug. File it (but with a _minimal_ test-case).


Tomek