overlay: fix bug where prev should have been final

(cherry picked from commit 81d9ff97c9)

flake.lock

@@ -5,11 +5,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1710146030,
-        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "lastModified": 1726560853,
+        "narHash": "sha256-X6rJYSESBVr3hBoH0WbKE5KvhPU5bloyZ2L4K60/fPQ=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "rev": "c1dfcf08411b08f6b8615f7d8971a2bfa81d5e8a",
         "type": "github"
       },
       "original": {
@@ -36,24 +36,24 @@
     "lix": {
       "flake": false,
       "locked": {
-        "lastModified": 1723503926,
-        "narHash": "sha256-Rosl9iA9MybF5Bud4BTAQ9adbY81aGmPfV8dDBGl34s=",
-        "rev": "bcaeb6388b8916ac6d1736e3aa2b13313e6a6bd2",
+        "lastModified": 1729298361,
+        "narHash": "sha256-hiGtfzxFkDc9TSYsb96Whg0vnqBVV7CUxyscZNhed0U=",
+        "rev": "ad9d06f7838a25beec425ff406fe68721fef73be",
         "type": "tarball",
-        "url": "https://git.lix.systems/api/v1/repos/lix-project/lix/archive/bcaeb6388b8916ac6d1736e3aa2b13313e6a6bd2.tar.gz?rev=bcaeb6388b8916ac6d1736e3aa2b13313e6a6bd2"
+        "url": "https://git.lix.systems/api/v1/repos/lix-project/lix/archive/ad9d06f7838a25beec425ff406fe68721fef73be.tar.gz?rev=ad9d06f7838a25beec425ff406fe68721fef73be"
       },
       "original": {
         "type": "tarball",
-        "url": "https://git.lix.systems/lix-project/lix/archive/2.91.0.tar.gz"
+        "url": "https://git.lix.systems/lix-project/lix/archive/2.91.1.tar.gz"
       }
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1723175592,
-        "narHash": "sha256-M0xJ3FbDUc4fRZ84dPGx5VvgFsOzds77KiBMW/mMTnI=",
+        "lastModified": 1729070438,
+        "narHash": "sha256-KOTTUfPkugH52avUvXGxvWy8ibKKj4genodIYUED+Kc=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "5e0ca22929f3342b19569b21b2f3462f053e497b",
+        "rev": "5785b6bb5eaae44e627d541023034e1601455827",
         "type": "github"
       },
       "original": {

flake.nix

@@ -1,7 +1,7 @@
 {
   inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
   inputs.lix = {
-    url = "https://git.lix.systems/lix-project/lix/archive/2.91.0.tar.gz";
+    url = "https://git.lix.systems/lix-project/lix/archive/2.91.1.tar.gz";
     flake = false;
   };
   inputs.flake-utils.url = "github:numtide/flake-utils";
@@ -47,7 +47,7 @@
       {
         inherit pkgs;
         packages = {
-          default = pkgs.nixVersions.nix_2_18;
+          default = pkgs.nix;
           inherit (pkgs) nix-doc nix-eval-jobs;
         };
 
@@ -57,6 +57,7 @@
 
         checks = {
           inherit (self.packages.${system}) default nix-eval-jobs;
+          inherit (pkgs) nixos-option;
         } // lib.optionalAttrs (lib.elem system linux64BitSystems) {
           # wrongMajor intentionally not included here since it is expected to fail
           inherit (self.nixosTests.${system}) it-builds;

overlay.nix

@@ -1,16 +1,13 @@
 { lix, versionSuffix ? "" }:
 final: prev:
 let
-  # This is kind of scary to not override the nix version to pretend to be
-  # 2.18 since nixpkgs can introduce new breakage in its Nix unstable CLI
-  # usage.
-  # https://github.com/nixos/nixpkgs/blob/6afb255d976f85f3359e4929abd6f5149c323a02/nixos/modules/config/nix.nix#L121
   lixPackageFromSource = final.callPackage (lix + "/package.nix") ({
     inherit versionSuffix;
   });
 
-  # These packages depend on Nix features that Lix does not support
-  overridelist_2_18 = [
+  # These packages should receive CppNix since they may link to it or otherwise
+  # cause problems (or even just silly mass-rebuilds) if we give them Lix
+  overridelist_upstream = [
     "attic-client"
     "devenv"
     "nix-du"
@@ -20,11 +17,6 @@ let
     "nurl"
     "prefetch-yarn-deps" # force these onto upstream so we are not regularly rebuilding electron
   ];
-  override_2_18 = prev.lib.genAttrs overridelist_2_18 (
-    name: prev.${name}.override {
-      nix = final.nixVersions.nix_2_18_upstream;
-    }
-  );
 
   inherit (prev) lib;
 
@@ -64,7 +56,37 @@ let
   # errors. This is a simple safeguard to put in at least something that might be seen.
   maybeWarnWrongMajor = x: if !(lib.hasPrefix supportedLixMajor lixPackageToUse.version) then builtins.trace wrongMajorWarning x else x;
 
-  overlay = override_2_18 // {
+  # It is not enough to *just* throw whatever the default nix version is at
+  # anything in the "don't give lix" list, we have to *also* ensure that we
+  # give whatever upstream version as specified in the callPackage invocation.
+  #
+  # Unfortunately I don't think there is any actual way to directly query that,
+  # so we instead do something extremely evil and guess which version it
+  # probably was. This code is not generalizable to arbitrary derivations, so
+  # it will hopefully not make us cry, at least.
+  useCppNixOverlay =
+    lib.genAttrs overridelist_upstream (
+      name:
+      if (lib.functionArgs prev.${name}.override ? "nix") then
+        let
+          # Get the two common inputs of a derivation/package.
+          inputs = prev.${name}.buildInputs ++ prev.${name}.nativeBuildInputs;
+          nixDependency = lib.findFirst
+            (drv: (drv.pname or "") == "nix")
+            final.nixVersions.stable_upstream # default to stable nix if nix is not an input
+            inputs;
+          nixMajor = lib.versions.major (nixDependency.version or "");
+          nixMinor = lib.versions.minor (nixDependency.version or "");
+          nixAttr = "nix_${nixMajor}_${nixMinor}";
+          finalNix = final.nixVersions.${nixAttr};
+        in
+        prev.${name}.override {
+          nix = finalNix;
+        }
+      else prev.${name}
+    );
+
+  overlay = useCppNixOverlay // {
     lix-overlay-present = 1;
     # used for things that one wouldn't necessarily want to update, but we
     # nevertheless shove it in the overlay and fixed-point it in case one *does*
@@ -73,15 +95,14 @@ let
 
     lix = maybeWarnWrongMajor (maybeWarnDuplicate lixPackageToUse);
 
-    nixVersions = prev.nixVersions // rec {
-      nix_2_18 = final.lix;
-      stable = nix_2_18;
-      nix_2_18_upstream = prev.nixVersions.nix_2_18;
+    nixVersions = prev.nixVersions // {
+      stable = final.lix;
+      stable_upstream = prev.nixVersions.stable;
     };
 
     nix-eval-jobs = (prev.nix-eval-jobs.override {
       # lix
-      nix = final.nixVersions.nix_2_18;
+      nix = final.lix;
     }).overrideAttrs (old:
       let src = final.lix-sources.nix-eval-jobs;
       in {