Auto return type inference issue?

[Matthew Dudley via Digitalmars-d-learn]

Here's the gist of what I'm trying to do.

https://gist.github.com/pontifechs/11169069

I'm getting an error I don't understand:

tinker.d(42): Error: mismatched function return type inference of 
tinker.B and tinker.A
tinker.d(55): Error: template instance tinker.DynamicTuple!(A, 
B).DynamicTuple.notopDispatch!"one" error instantiating
Failed: 'dmd' '-v' '-o-' 'tinker.d' '-I.'

Also, as an aside, why can't tuples be indexed dynamically? Most 
of my problems with this have been because you apparently can't.