Mixin a function into a struct only if no member with that name already exists

[Tobias Pankrath]

I am trying to implement the builder pattern for some structs. 
Currently I am auto implementing all methods by mixing them into 
the builder, but now I need some custom logic for some of the 
fields and I figured I just write them down by hand and mixin the 
rest.

I tried checking if it is contained in the FieldNameTuple, but 
that (understandably) creates a circular reference: 
https://run.dlang.io/is/Xih7yP

How do I mixin a function only if it is not already present?