lib/customization: propagate function arguments in callPackagesWith

makeOverridable is very careful to ensure the arguments to the
overridden function are the same as the input function. As a result,
the arguments of hello.override are exactly the same as the original
arguments of the hello function that produced the derivation.

However, callPackagesWith calls makeOverridable with a lambda that
does not propagate the arguments. The override function for a package
instantiated with callPackagesWith will not have the original
arguments.

For example:

    nix-repl> lib.functionArgs hello.override
    { callPackage = false; fetchurl = false; hello = false; lib = false; nixos = false; stdenv = false; testers = false; }

    nix-repl> lib.functionArgs openssl.override
    { }

By copying the arguments onto the inner lambda before passing it to
makeOverridable, we can make callPackage and callPackages behave the
same.

    nix-repl> lib.functionArgs openssl.override
    { buildPackages = false; coreutils = false; cryptodev = false; enableSSL2 = true; enableSSL3 = true; fetchurl = false; lib = false; perl = false; removeReferencesTo = false; static = true; stdenv = false; withCryptodev = true; withPerl = true; }

lib/customisation.nix

@@ -221,9 +221,10 @@ rec {
     let
       f = if isFunction fn then fn else import fn;
       auto = intersectAttrs (functionArgs f) autoArgs;
+      mirrorArgs = mirrorFunctionArgs f;
       origArgs = auto // args;
       pkgs = f origArgs;
-      mkAttrOverridable = name: _: makeOverridable (newArgs: (f newArgs).${name}) origArgs;
+      mkAttrOverridable = name: _: makeOverridable (mirrorArgs (newArgs: (f newArgs).${name})) origArgs;
     in
       if isDerivation pkgs then throw
         ("function `callPackages` was called on a *single* derivation "

lib/tests/misc.nix

@@ -55,6 +55,24 @@ runTests {
     expected = { a = false; b = false; c = true; };
   };
 
+  testCallPackageWithOverridePreservesArguments =
+    let
+      f = { a ? 0, b }: {};
+      f' = callPackageWith { a = 1; b = 2; } f {};
+    in {
+      expr = functionArgs f'.override;
+      expected = functionArgs f;
+    };
+
+  testCallPackagesWithOverridePreservesArguments =
+    let
+      f = { a ? 0, b }: { nested = {}; };
+      f' = callPackagesWith { a = 1; b = 2; } f {};
+    in {
+      expr = functionArgs f'.nested.override;
+      expected = functionArgs f;
+    };
+
 # TRIVIAL
 
   testId = {