lib.makeOverridable: fix functionArgs on returned function

lib/customisation.nix

@@ -69,8 +69,8 @@ rec {
      "<pkg>.overrideDerivation" to learn about `overrideDerivation` and caveats
      related to its use.
   */
-  makeOverridable = f: origArgs:
-    let
+  makeOverridable = f: lib.setFunctionArgs
+    (origArgs: let
       result = f origArgs;
 
       # Creates a functor with the same arguments as f
@@ -95,7 +95,8 @@ rec {
         lib.setFunctionArgs result (lib.functionArgs result) // {
           override = overrideArgs;
         }
-      else result;
+      else result)
+    (lib.functionArgs f);
 
 
   /* Call the package function in the file `fn` with the required

lib/tests/misc.nix

@@ -43,6 +43,18 @@ in
 
 runTests {
 
+# CUSTOMIZATION
+
+  testFunctionArgsMakeOverridable = {
+    expr = functionArgs (makeOverridable ({ a, b, c ? null}: {}));
+    expected = { a = false; b = false; c = true; };
+  };
+
+  testFunctionArgsMakeOverridableOverride = {
+    expr = functionArgs (makeOverridable ({ a, b, c ? null }: {}) { a = 1; b = 2; }).override;
+    expected = { a = false; b = false; c = true; };
+  };
+
 # TRIVIAL
 
   testId = {