The following code does not compile:

struct S { }

class A
{
  S s;
  alias s this;
}

class B : A
{
}

void main()
{
  A asA = new B;
  B asB = cast(B)asA;
}

I would expect the last line to successfully cast the B instance I created back into type B, however this seems to be preempted by the alias this:

Error: cannot cast expression asA.s of type S to app.B

Is there a way to force this cast to operate on the object of type A instead of automatically using A.s?

Thanks,
Aurelien

On Sunday, 5 November 2017 at 07:07:43 UTC, Aurelien Fredouelle wrote:
> struct S { }
>
> class A
> {
>   S s;
>   alias s this;
> }
>
> class B : A
> {
> }
>
> void main()
> {
>   A asA = new B;
>   B asB = cast(B)asA;
> }
>
> I would expect the last line to successfully cast the B instance I created back into type B, however this seems to be preempted by the alias this:
>
> Error: cannot cast expression asA.s of type S to app.B
>
> Is there a way to force this cast to operate on the object of type A instead of automatically using A.s?

Known issue: https://issues.dlang.org/show_bug.cgi?id=6777

You can work around by casting to Object first:

    B asB = cast(B) cast(Object) asA;