How to group similar member functions from different classes?

[cy via Digitalmars-d-learn]

On Monday, 20 June 2016 at 16:39:54 UTC, Marc Schütz wrote:
> Untested:

Seems to only work if A and B are both defined in the same file 
as Foos (defeating the purpose). Putting A and B in a.d and b.d 
respectively gives me these errors:

a.d(2): Error: undefined identifier 'Foos'
a.d(2): Error: mixin a.A.Foos!() is not defined
b.d(2): Error: undefined identifier 'Foos'
b.d(2): Error: mixin b.B.Foos!() is not defined

I also tried switching it around, like
// b.d
import foos Foos;
class B {
   mixin Foos;
}

but that of course gives the error:
foos.d(4): Error: undefined identifier 'A'
b.d(3): Error: mixin b.B.Foos!() error instantiating

since you can't do a static if(typeof(this) == A) without 
importing A from a somehow. (you can do else static 
if(typeof(this) == B) without importing B though, since it does 
the branch for A first)

I think a mixin here is just required, because you can't use an 
identifier before it's defined, even at compile time. Honestly, I 
have yet to find a use for mixin templates.