From 99ab2550e7ed83597c9b753a0adbc0b5e07cd65d Mon Sep 17 00:00:00 2001
From: DavHau <hsngrmpf+github@gmail.com>
Date: Thu, 19 Mar 2026 21:43:39 +0800
Subject: [PATCH 1/2] appendOverlays: return spliced packages on empty overlay
 list

`appendOverlays []` returned `self` (the unspliced inner fixpoint)
while `import nixpkgs {}` returns `pkgs.__splicedPackages` (the
spliced version). This caused `pkgs.makeWrapper` and similar
spliced packages to lose their `__spliced` attribute after passing
through `appendOverlays`, breaking cross-compilation in NixOS
modules.

The NixOS `nixpkgs.nix` module calls `cfg.pkgs.appendOverlays
cfg.overlays`, and when no overlays are defined, the empty-list
short-circuit returned unspliced packages. This meant
`nativeBuildInputs = [ pkgs.makeWrapper ]` in NixOS modules would
evaluate the host-platform makeWrapper (which throws in cross
context) instead of being spliced to buildPackages.makeWrapper.

Fix by returning `self.__splicedPackages` instead of `self` for
the empty-list case.
---
 pkgs/top-level/stage.nix | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/pkgs/top-level/stage.nix b/pkgs/top-level/stage.nix
index be333d3bddc1..e718bda745c5 100644
--- a/pkgs/top-level/stage.nix
+++ b/pkgs/top-level/stage.nix
@@ -264,7 +264,10 @@ let
     # in one go when calling Nixpkgs, for performance and simplicity.
     appendOverlays =
       extraOverlays:
-      if extraOverlays == [ ] then self else nixpkgsFun { overlays = args.overlays ++ extraOverlays; };
+      if extraOverlays == [ ] then
+        self.__splicedPackages
+      else
+        nixpkgsFun { overlays = args.overlays ++ extraOverlays; };
 
     # NOTE: each call to extend causes a full nixpkgs rebuild, adding ~130MB
     #       of allocations. DO NOT USE THIS IN NIXPKGS.

From 514529545bab291d6351935baeb409355c608207 Mon Sep 17 00:00:00 2001
From: DavHau <hsngrmpf+github@gmail.com>
Date: Thu, 19 Mar 2026 21:48:10 +0800
Subject: [PATCH 2/2] tests.top-level: add test for appendOverlays preserving
 splicing

Ensures that appendOverlays with an empty list returns spliced
packages, so that cross-compilation in NixOS modules works correctly.
---
 pkgs/test/top-level/default.nix | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/pkgs/test/top-level/default.nix b/pkgs/test/top-level/default.nix
index e713b617f8dd..3946b363b764 100644
--- a/pkgs/test/top-level/default.nix
+++ b/pkgs/test/top-level/default.nix
@@ -64,4 +64,22 @@ lib.recurseIntoAttrs {
     assert lib.all (p: p.stdenv.buildPlatform == p.stdenv.hostPlatform) pkgsLocal;
     assert lib.all (p: p.stdenv.buildPlatform != p.stdenv.hostPlatform) pkgsCross;
     pkgs.emptyFile;
+
+  # appendOverlays must preserve splicing so that cross-compilation
+  # works in NixOS modules (which go through appendOverlays via nixpkgs.nix).
+  appendOverlaysPreservesSplicing =
+    let
+      cross = nixpkgsFun {
+        localSystem = {
+          system = "x86_64-linux";
+        };
+        crossSystem = {
+          system = "aarch64-linux";
+        };
+      };
+      appended = cross.appendOverlays [ ];
+    in
+    assert cross.makeWrapper ? __spliced;
+    assert appended.makeWrapper ? __spliced;
+    pkgs.emptyFile;
 }