fixup! squash into 1st commit

Co-authored-by: Sergei Zimmerman <[email protected]>

Author: Ericson2314

src/libutil/include/nix/util/topo-sort.hh

@@ -17,9 +17,8 @@ struct Cycle
 template<typename T>
 using TopoSortResult = std::variant<std::vector<T>, Cycle<T>>;
 
-template<typename T, typename Compare, typename F>
-    requires std::invocable<F, const T &>
-             && std::same_as<std::remove_cvref_t<std::invoke_result_t<F, const T &>>, std::set<T, Compare>>
+template<typename T, typename Compare, std::invocable<const T &> F>
+    requires std::same_as<std::remove_cvref_t<std::invoke_result_t<F, const T &>>, std::set<T, Compare>>
 TopoSortResult<T> topoSort(std::set<T, Compare> items, F && getChildren)
 {
     std::vector<T> sorted;
@@ -37,7 +36,7 @@ TopoSortResult<T> topoSort(std::set<T, Compare> items, F && getChildren)
         }
         parents.insert(path);
 
-        auto && references = getChildren(path);
+        auto && references = std::invoke(getChildren, path);
 
         for (auto & i : references)
             /* Don't traverse into items that don't exist in our starting set. */