Some changes from vaibhavsagar

default.nix

@@ -41,6 +41,8 @@ let
     # Should this be self?
     ghcHEAD = ghc super;
 
+    bootstrapHaskellPackages = self.haskell.packages.ghc865;
+
     haskellPackages =
       let patchesOverrides = self.callPackage patches {};
           patches = self.callPackage (import ./scripts/overrides.nix) { patches = ./patches; };
@@ -54,7 +56,7 @@ let
               inherit (self) lib;
               pkgs = self;
             };
-            buildHaskellPackages = self.buildPackages.haskell.packages.ghc865;
+            buildHaskellPackages = haskellPackages;
             ghc = ghcHEAD;
             compilerConfig = self1: super1: {
               # Packages included in GHC's global package database
@@ -91,11 +93,13 @@ let
               doctest = haskellPackages.callHackage "doctest" "0.16.0" {};
               http-api-data = haskellPackages.callPackage ./http-api-data.nix {};
 
-              jailbreak-cabal = buildHaskellPackages.jailbreak-cabal;
-              cabal2nix = buildHaskellPackages.cabal2nix;
+              # These are things we don't want to build with HEAD
+              inherit (bootstrapHaskellPackages) jailbreak-cabal cabal2nix Unixutils;
             };
           };
-      in baseHaskellPackages.extend overrides;
+      in baseHaskellPackages.override (old: {
+        overrides = self.lib.composeExtensions (old.overrides or (_: _: {})) overrides;
+      });
 
     headHackageScripts = self.stdenv.mkDerivation {
       name = "head-hackage-scripts";