Restriction on interface function types

[Steven Schveighoffer]

On Wed, 12 Mar 2014 09:05:05 -0400, Steve Teale  
<steve.teale at britseyeview.com> wrote:

> On Wednesday, 12 March 2014 at 11:13:00 UTC, monarch_dodra wrote:
>>
>> That said, I'd expect this to work:
>>
>> //----
>> interface I
>> {
>>    I myType();
>> }
>>
>> class A: I
>> {
>>    //auto myType() { return cast(A) null; } //Nope
>>    A myType() { return cast(A) null; } //OK
>> }
>> //----
>>
>> Yup, covariance desired, but
>
> import std.stdio;
>
> interface I
> {
>     I myType();
> }
>
> class A: I
> {
>     A myType() { return cast(A) null; }
>     void foo() { writeln("foo"); }
> }
>
> void main()
> {
>     I a = new A();
>     writeln(typeof(a.myType()).stringof);
> }
>
> returns I
>
> Seems like a bug to me.

No, not a bug. What you want is actually not possible.

To demonstrate further:

void bad(I i)
{
    typeof(i.myType()) x;
}

class A : I
{
    A myType() { return cast(A)null;}
}

class B : I
{
    B myType() {return cast(B) null;}
}

void main()
{
    I[] arr = [new A, new B];
    foreach(i; arr) {bad(i);}
}

How is the compiler to build it's one copy of bad? Should x be typed as A  
or B? Or something not even seen in this module that could derive from I?

-Steve