mkPackages: automagically wrapping with hello as a test package

packages/hello/call.nix

@@ -0,0 +1,7 @@
+{ callPackage }:
+{
+  callPackage = f: x: callPackage f ({ bar = "foo"; } // x);
+  args = {
+    foo = "bar";
+  };
+}

packages/hello/package.nix

@@ -0,0 +1,19 @@
+{
+  stdenv,
+  fetchurl,
+  foo,
+  bar,
+}:
+
+stdenv.mkDerivation (finalAttrs: {
+  pname = "hello";
+  version = "2.12.1";
+
+  src = fetchurl {
+    url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz";
+    hash = "sha256-jZkUKv2SV28wsM18tCqNxoCZmLxdYH2Idh9RLibH2yA=";
+  };
+  passthru = {
+    inherit foo bar;
+  };
+})

packages/hello/wrapper.nix

@@ -0,0 +1,17 @@
+{
+  symlinkJoin,
+  foo,
+  bar,
+  hello-unwrapped,
+}:
+symlinkJoin {
+  name = "hello-wrapped";
+  paths = [ hello-unwrapped ];
+  postBuild = ''
+    ln -s $out/bin/hello $out/bin/hello-wrapped
+  '';
+
+  passthru = {
+    inherit foo bar;
+  };
+}