Inference of auto storage classes for interface function return type

[Nicholas Wilson]

On Wednesday, 4 July 2018 at 14:07:35 UTC, Timoses wrote:
> How can I return inferred storage class from interface 
> functions?
> I can't use auto as return value in interface. Neither can I 
> use "inout" as I don't pass a parameter.
>
> 	// Ref Type
> 	interface IRef
> 	{
> 		Ref opIndex(size_t idx) const;
> 	}
> 	class CRef : IRef
> 	{
> 		Ref[] a;
> 		this() immutable
> 		{ this.a = [new Ref()]; }
> 		Ref opIndex(size_t idx) const
> 		{ return a[idx]; } // Error: cannot implicitly convert 
> expression this.a[idx] of type const(Ref) to app.Ref
> 	}
> 	class Ref{}
>
> 	void main()
> 	{
> 		auto a = new immutable(CRef)();
> 		auto s = a[3];
> 	}
>
> For value types it works, I presume since they are passed by 
> value, so the instance returned is an actual copy of the stored 
> value.
>
>         // Value Type
> 	interface IValue
> 	{
> 		Value opIndex(size_t idx) const;
> 	}
> 	class CValue : IValue
> 	{
> 		this() immutable { i = [Value()]; }
> 		Value[] i;
> 		Value opIndex(size_t idx) const
> 		{ return i[idx]; }
> 	}
> 	struct Value{}
>
> However, for ref types this doesn't work.
>
> Do I have to define two `opIndex` in the interface? One 
> mutable, one for immutable type instances?

IIRC to apply inout to the this pointer:

Ref opIndex(size_t idx) inout;

or
inout(Ref) opIndex(size_t idx) inout;

not sure off the top of my head,