overlay not working

default.nix

let
  gcOverlay = self: super: {
    # jumper = telomare_jumper;
    gc = self.boehmgc;
  };
in

{ # Fetch the latest haskell.nix and import its default.nix
  # haskellNix ? import (builtins.fetchTarball "https://github.com/input-output-hk/haskell.nix/archive/dc7acfeb7628032d5109747a8b330d58817b953f.tar.gz") {}
  haskellNix ? import (builtins.fetchTarball "https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz") { overlays = [ gcOverlay ]; }

# # For LLVM
# , enableLLVMAssertions ? true # TODO: Fix

, compiler ? "ghc865"
}:
let
  # haskell.nix provides access to the nixpkgs pins which are used by our CI,
  # hence you will be more likely to get cache hits when using these.
  # But you can also just use your own, e.g. '<nixpkgs>'.
  nixpkgsSrc = haskellNix.sources.nixpkgs-2003;

  # haskell.nix provides some arguments to be passed to nixpkgs, including some
  # patches and also the haskell.nix functionality itself as an overlay.
  nixpkgsArgs = haskellNix.nixpkgsArgs;

  # import nixpkgs with overlays
  pkgs = (import nixpkgsSrc nixpkgsArgs);
         # // { overlays = [ gcOverlay ]; } // { gc = pkgs.boehmgc; };

  # gcOverlay = self: super: {
  #   jumper = telomare_jumper;
  #   gc = self.boehmgc;
  # };
  # overlayedPkgs = pkgs // { overlays = [ gcOverlay ]; };
  telomare_jumper = pkgs.stdenv.mkDerivation {
    name = "telomareJumper";
    src = ./cbits;
    buildInputs = [ pkgs.boehmgc ];
  };

in
pkgs.haskell-nix.cabalProject {
  src = pkgs.haskell-nix.cleanSourceHaskell {
    src = ./.;
    name = "telomare";
  };
  compiler-nix-name = compiler;
  # pkg-def-extras = [ (hkg : { packages = { gc = pkgs.boehmgc; }; }) ];
  pkg-def-extras = [ (hkg : { gc = pkgs.gc; }) ];
  modules = [ ];
}