Merge "libutil: fix args assert being thrown on Darwin in nix-eval-jobs" into main

src/libutil/args.cc

@@ -64,7 +64,7 @@ RootArgs & Args::getRoot()
     while (p->parent)
         p = p->parent;
 
-    auto * res = dynamic_cast<RootArgs *>(p);
+    auto res = p->asRootArgs();
     assert(res);
     return *res;
 }

src/libutil/args.hh

@@ -244,6 +244,13 @@ protected:
      */
     virtual void initialFlagsProcessed() {}
 
+    /**
+     * Returns this Args as a RootArgs if it is one, or \ref std::nullopt otherwise.
+     */
+    virtual std::optional<std::reference_wrapper<RootArgs>> asRootArgs() {
+        return std::nullopt;
+    }
+
 public:
 
     void addFlag(Flag && flag);

src/libutil/args/root.hh

@@ -65,6 +65,10 @@ protected:
      */
     std::set<ExperimentalFeature> flagExperimentalFeatures;
 
+    virtual std::optional<std::reference_wrapper<RootArgs>> asRootArgs() override {
+        return *this;
+    }
+
 private:
 
     std::optional<std::string> needsCompletion(std::string_view s);