Fix extendModules anonymous module numbering

lib/modules.nix

@@ -114,8 +114,8 @@ rec {
                 , # This would be remove in the future, Prefer _module.check option instead.
                   check ? true
                   # Internal variable to avoid `_key` collisions regardless
-                  # of `extendModules`. Used in `submoduleWith`.
-                  # Test case: lib/tests/modules, "168767"
+                  # of `extendModules`.
+                  # Test case: lib/tests/modules, "168767", "matrix.nix"
                 , extensionOffset ? 0
                 }:
     let
@@ -345,13 +345,12 @@ rec {
         modules ? [],
         specialArgs ? {},
         prefix ? [],
-        extensionOffset ? length modules,
         }:
           evalModules (evalModulesArgs // {
             modules = regularModules ++ modules;
             specialArgs = evalModulesArgs.specialArgs or {} // specialArgs;
             prefix = extendArgs.prefix or evalModulesArgs.prefix;
-            inherit extensionOffset;
+            extensionOffset = extensionOffset + length modules;
           });
 
       type = lib.types.submoduleWith {

lib/tests/modules.sh

@@ -332,6 +332,7 @@ checkConfigOutput 'ok' config.freeformItems.foo.bar ./adhoc-freeformType-survive
 # Anonymous submodules don't get nixed by import resolution/deduplication
 # because of an `extendModules` bug, issue 168767.
 checkConfigOutput '^1$' config.sub.specialisation.value ./extendModules-168767-imports.nix
+checkConfigOutput '^{ cookie = "accept"; pill = "red"; }$' config.result ./matrix.nix
 
 cat <<EOF
 ====== module tests ======

lib/tests/modules/matrix.nix

@@ -0,0 +1,38 @@
+{ config, lib, extendModules, ... }:
+let
+  inherit (lib) mkOption types;
+
+  extendModule = module: extendModules { modules = [ module ]; };
+
+  choice = choice@{ name, ... }: {
+    options = {
+      value = mkOption {
+        type = types.lazyAttrsOf (extendModule ({ })).type;
+      };
+    };
+  };
+in
+{
+  options = {
+    matrix = mkOption {
+      type = types.lazyAttrsOf (types.submodule choice);
+      default = { };
+    };
+
+    items = mkOption {
+      type = types.attrsOf types.str;
+    };
+
+    result = mkOption {
+      default =
+        let ds = x: lib.deepSeq x x;
+        in ds config.matrix.cookie.value.accept.matrix.pill.value.red.items;
+    };
+  };
+  config = {
+    matrix.pill.value.blue.items.pill = "blue";
+    matrix.pill.value.red.items.pill = "red";
+    matrix.cookie.value.accept.items.cookie = "accept";
+    matrix.cookie.value.reject.items.cookie = "reject";
+  };
+}

lib/types.nix

@@ -627,7 +627,6 @@ rec {
           (base.extendModules {
             modules = [ { _module.args.name = last loc; } ] ++ allModules defs;
             prefix = loc;
-            extensionOffset = extensionOffset + length defs;
           }).config;
         emptyValue = { value = {}; };
         getSubOptions = prefix: (base.extendModules