D interface bug?

[Alex]

On Saturday, 30 March 2019 at 00:06:23 UTC, H. S. Teoh wrote:
> On Fri, Mar 29, 2019 at 11:44:35PM +0000, Alex via 
> Digitalmars-d-learn wrote:
>> interface iBase
>> {
>> 	iBase fooBase(iBase);
>> }
>> 
>> 
>> class cBase : iBase
>> {
>> 	cBase fooBase(cBase c) { return c; }
>> 
>> }
>> 
>> cBase.fooBase should be a valid override of iBase.fooBase 
>> because they are the same type! cBase is a super type so it 
>> contains everything iBase contains and maybe more.
>
> No, that's wrong. Consider this:
>
> 	class cBase : iBase
> 	{
> 		int x;
> 		cBase fooBase(cBase c) { return (x==1) ? c : null; }
> 	}
>
> 	class dBase : iBase
> 	{
> 		string y;
> 		dBase fooBase(dBase c) { return (y=="a") ? c : null; }
> 	}
>
> 	iBase intf = new cBase;
> 	dBase dobj = new dBase;
> 	dobj.fooBase(intf); // oops: intf.y doesn't exist!
>
> I.e., it's invalid for dBase.fooBase to override the interface 
> method.
>
> The parameter type of fooBase must be the interface type or a 
> super-interface thereof.  For a class C to inherit from an 
> interface X means that C contains a subset of all possible 
> objects that X might refer to.  Therefore, if a method takes a 
> parameter of type C, it *cannot* be passed an argument of type 
> X, since the actual object might be outside the subset that C 
> includes. IOW, such a method cannot be covariant with a method 
> that takes X as a parameter.
>
>
>> There should be no reason why the compiler can't figure this 
>> out. It's a very simple rule.
>> 
>> Any time the user calls iBase.fooBase it can be replaced with 
>> cBase.fooBase so it should not compromise any code to go ahead 
>> and accept it as a proper override.
> [...]
>
> Nope.  The user can call iBase.fooBase, passing it an instance 
> of a different class that also implements iBase but does not 
> inherit from cBase.  Then cBase.fooBase would receive an 
> argument of incompatible type.
>
>
> T


Ok. In my use case, which is what I was thinking of, there will 
never be a dBase. There will never be any other class that 
inherits from the interface. I have to use an interface ONLY 
because D does not allow for multiple inheritance.

class X;
class C;

class Q : X, C;


Which can't be done, so I want to do

interface iC;
class C : iC;

class Q : X, iC;


which now works. The problem now is that I have to then still 
follow these rules which are very restrictive. It's true that 
someone could come along and create an new class D : iC and cause 
problems, but that should never happen in my case. Ideally, if 
they did, they would use the same pattern as above:

interface iD;
class D : C, iD;

and this then also alleviates the problem.

In your case it is
    iC
   /  \
  /    \
C      D

but in my case it should never happen, or if it would, it is 
better to do


    iC
   /
  /
C     iD
  \   /
   \ /
    D


I'm only using interfaces because I have to, not because I want 
to. But then that forces me to do strange things in D and it 
causes many problems. Since one can't have fields in an interface 
it requires using properties and all that code bloat that comes 
with them, along with the casting issues, and overloading, etc.