From 5cfdec6e94574e8e08068e2de5e8ef2a375bb894 Mon Sep 17 00:00:00 2001
From: Silvan Mosberger <infinisil@icloud.com>
Date: Fri, 7 Sep 2018 20:59:12 +0200
Subject: [PATCH 1/3] lib: Improve overrideExisting implementation

---
 lib/attrsets.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 1e4142562fa6..9bac69af34f3 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -440,7 +440,7 @@ rec {
       => { a = { b = 6; d = 2; }; }
   */
   overrideExisting = old: new:
-    old // listToAttrs (map (attr: nameValuePair attr (attrByPath [attr] old.${attr} new)) (attrNames old));
+    mapAttrs (name: value: new.${name} or value) old;
 
   /* Get a package output.
      If no output is found, fallback to `.out` and then to the default.

From afd86206210e468173ec0d2a0e135843d5ab2ee7 Mon Sep 17 00:00:00 2001
From: Silvan Mosberger <infinisil@icloud.com>
Date: Mon, 17 Sep 2018 22:40:08 +0200
Subject: [PATCH 2/3] lib/tests: Add overrideExisting tests

---
 lib/tests/misc.nix | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index d3bd7746d89c..d89bcfde4819 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -236,6 +236,20 @@ runTests {
     };
   };
 
+  testOverrideExistingEmpty = {
+    expr = overrideExisting {} { a = 1; };
+    expected = {};
+  };
+
+  testOverrideExistingDisjoint = {
+    expr = overrideExisting { b = 2; } { a = 1; };
+    expected = { b = 2; };
+  };
+
+  testOverrideExistingOverride = {
+    expr = overrideExisting { a = 3; b = 2; } { a = 1; };
+    expected = { a = 1; b = 2; };
+  };
 
 # GENERATORS
 # these tests assume attributes are converted to lists

From c7104d97c87316a6062c88696d80a45918fab7a6 Mon Sep 17 00:00:00 2001
From: Silvan Mosberger <infinisil@icloud.com>
Date: Mon, 17 Sep 2018 23:28:47 +0200
Subject: [PATCH 3/3] lib.overrideExisting: Better example

---
 lib/attrsets.nix | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 9bac69af34f3..2a1b866dbc5e 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -435,9 +435,12 @@ rec {
     useful for deep-overriding.
 
     Example:
-      x = { a = { b = 4; c = 3; }; }
-      overrideExisting x { a = { b = 6; d = 2; }; }
-      => { a = { b = 6; d = 2; }; }
+      overrideExisting {} { a = 1; }
+      => {}
+      overrideExisting { b = 2; } { a = 1; }
+      => { b = 2; }
+      overrideExisting { a = 3; b = 2; } { a = 1; }
+      => { a = 1; b = 2; }
   */
   overrideExisting = old: new:
     mapAttrs (name: value: new.${name} or value) old;