Your ReadOne function takes three type parameters: S, Input, and Output. Since S has a constraint, Input and Output also need a constraint (which can just be interface{}).
> func ReadOne(type S Stream(Input, Output), Input interface{}, Output interface{})(...) (...)
I also had to add explicit type arguments to the call to ReadOne since the compiler complains that the type of Output can't be inferred.
> res, err := ReadOne(*dummyStream, int, int)(nil, getStream, 13)
Cheers,
Andrew Ekstedt