Override additional GHC base packages missed by stack2nix

input-output-hk/stack2nix#134
input-output-hk/stack2nix#84

Author: michaelpj

nix/overlays/required.nix

@@ -26,4 +26,14 @@ self: super: {
     } // pkgs.lib.optionalAttrs (args ? src) {
       src = cleanSourceHaskell args.src;
     });
+
+    # stack2nix doesn't have the right set of GHC base packages nulled out for 8.4, as
+    # per https://github.com/input-output-hk/stack2nix/issues/84, which means
+    # we can hit https://github.com/input-output-hk/stack2nix/issues/134 unless
+    # we do it oursevles.
+    mtl = null;
+    parsec = null;
+    stm = null;
+    text = null;
+    xhtml = null;
   }