You can define a struct subtype hierarchy by adding alias this declarations to each struct. Like this:

    S3 <: S2 <: S1

    struct S3 { auto toS2() { return S2(); } alias toS2 this; }
    etc.

You can also define a class subtype hierarchy by using class inheritance:

    C3 <: C2 <: C1

    class C3 : C2 {}
    etc.

But apparently the mechanisms don't mix. This works:

    C1 <: S3 <: S2 <: S1

    class C1 { auto toS3() { return S3(); } alias toS3 this; }

But this doesn't work:

    C2 <: C1 <: S3 <: S2 <: S1

    class C2 : C1 {}

I think that should work.

I know I can put an alias this in C2. But it seems like inheritance should be enough to bridge the gap, since subtyping should be transitive.