[dhall-nix] merge operator on Optional not transpiled correctly

[DarkKirb]

I’m currently working on creating bindings for nix library functions in dhall so that you can write more of your NixOS configuration in dhall and I have run into some friction surrounding the merge operator, as it is not translated correctly.

let Optional/default =
      ./default.dhall
        sha256:5bd665b0d6605c374b3c4a7e2e2bd3b9c1e39323d41441149ed5e30d86e889ad

in  λ(v : Optional Natural) → Optional/default Natural 0 v

is translated as

v:
  v { None = 0; Some = _: _; }

and not as the working

v: if v != null then (_: _) v else 0

It generates the same code as normal unions, which is inappropriate because Some v is mapped to v and None t is mapped to null.