emacs.pkgs.overrodeScope' examples do not work

[neosimsim]

Describe the bug
I had to change an emacs package (update spinner from elpa). While trying to figure out how this is done I found
2 examples, which both did not work.

• nixpkgs/pkgs/build-support/emacs/wrapper.nix
• last code in snippet in https://nixos.org/manual/nixpkgs/stable/#sec-emacs

To Reproduce
Steps to reproduce the behavior:
Run

   $ cat >emacs.nix <<EOF
   with import <nixpkgs> {};
   let customEmacsPackages =
     emacs.pkgs.overrideScope' (self: super: {
       emacs = emacs-nox;
       foo = self.magit;
     });
   in customEmacsPackages.emacs.pkgs.withPackages (epkgs: [ epkgs.foo ])
   EOF

and

   $ nix-build emacs.nix

which reports

   error: attribute 'foo' missing

         at emacs.nix:7:58:

              6|   });
              7| in customEmacsPackages.emacs.pkgs.withPackages (epkgs: [ epkgs.foo ])
               |                                                          ^
   (use '--show-trace' to show detailed location information)

Expected behavior
Builds emacs with magit aliased to foo.

Notify maintainers
@tteggel @adisbladis

[neosimsim]

Seems that emacs.pkgs still contains the old packages set after calling overrideScope'.
I don't know if that is the actual bug?