diff --git a/clap_builder/src/builder/ext.rs b/clap_builder/src/builder/ext.rs
new file mode 100644
index 00000000..66354fb2
--- /dev/null
+++ b/clap_builder/src/builder/ext.rs
@@ -0,0 +1,217 @@
+use crate::util::AnyValueId;
+use crate::util::FlatMap;
+
+#[allow(dead_code)]
+#[derive(Default, Clone, Debug)]
+pub(crate) struct Extensions {
+    extensions: FlatMap<AnyValueId, BoxedExtension>,
+}
+
+impl Extensions {
+    #[allow(dead_code)]
+    pub(crate) fn get<T: Extension>(&self) -> Option<&T> {
+        let id = AnyValueId::of::<T>();
+        self.extensions.get(&id).map(|e| e.as_ref::<T>())
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn get_mut<T: Extension>(&mut self) -> Option<&mut T> {
+        let id = AnyValueId::of::<T>();
+        self.extensions.get_mut(&id).map(|e| e.as_mut::<T>())
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn get_or_insert_default<T: Extension + Default>(&mut self) -> &mut T {
+        let id = AnyValueId::of::<T>();
+        self.extensions
+            .entry(id)
+            .or_insert_with(|| BoxedExtension::new(T::default()))
+            .as_mut::<T>()
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn set<T: Extension + Into<BoxedEntry>>(&mut self, tagged: T) -> bool {
+        let BoxedEntry { id, value } = tagged.into();
+        self.extensions.insert(id, value).is_some()
+    }
+
+    #[allow(dead_code)]
+    pub(crate) fn remove<T: Extension>(&mut self) -> Option<Box<dyn Extension>> {
+        let id = AnyValueId::of::<T>();
+        self.extensions.remove(&id).map(BoxedExtension::into_inner)
+    }
+
+    pub(crate) fn update(&mut self, other: &Self) {
+        for (key, value) in other.extensions.iter() {
+            self.extensions.insert(*key, value.clone());
+        }
+    }
+}
+
+/// Supports conversion to `Any`. Traits to be extended by `impl_downcast!` must extend `Extension`.
+pub(crate) trait Extension: std::fmt::Debug + Send + Sync + 'static {
+    /// Convert `Box<dyn Trait>` (where `Trait: Extension`) to `Box<dyn Any>`.
+    ///
+    /// `Box<dyn Any>` can /// then be further `downcast` into
+    /// `Box<ConcreteType>` where `ConcreteType` implements `Trait`.
+    fn into_any(self: Box<Self>) -> Box<dyn std::any::Any>;
+    /// Clone `&Box<dyn Trait>` (where `Trait: Extension`) to `Box<dyn Extension>`.
+    ///
+    /// `Box<dyn Any>` can /// then be further `downcast` into
+    // `Box<ConcreteType>` where `ConcreteType` implements `Trait`.
+    fn clone_extension(&self) -> Box<dyn Extension>;
+    /// Convert `&Trait` (where `Trait: Extension`) to `&Any`.
+    ///
+    /// This is needed since Rust cannot /// generate `&Any`'s vtable from
+    /// `&Trait`'s.
+    fn as_any(&self) -> &dyn std::any::Any;
+    /// Convert `&mut Trait` (where `Trait: Extension`) to `&Any`.
+    ///
+    /// This is needed since Rust cannot /// generate `&mut Any`'s vtable from
+    /// `&mut Trait`'s.
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any;
+}
+
+impl<T> Extension for T
+where
+    T: Clone + std::fmt::Debug + Send + Sync + 'static,
+{
+    fn into_any(self: Box<Self>) -> Box<dyn std::any::Any> {
+        self
+    }
+    fn clone_extension(&self) -> Box<dyn Extension> {
+        Box::new(self.clone())
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+}
+
+impl Clone for Box<dyn Extension> {
+    fn clone(&self) -> Self {
+        self.as_ref().clone_extension()
+    }
+}
+
+#[derive(Clone)]
+#[repr(transparent)]
+struct BoxedExtension(Box<dyn Extension>);
+
+impl BoxedExtension {
+    fn new<T: Extension>(inner: T) -> Self {
+        Self(Box::new(inner))
+    }
+
+    fn into_inner(self) -> Box<dyn Extension> {
+        self.0
+    }
+
+    fn as_ref<T: Extension>(&self) -> &T {
+        self.0.as_ref().as_any().downcast_ref::<T>().unwrap()
+    }
+
+    fn as_mut<T: Extension>(&mut self) -> &mut T {
+        self.0.as_mut().as_any_mut().downcast_mut::<T>().unwrap()
+    }
+}
+
+impl std::fmt::Debug for BoxedExtension {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+        self.0.fmt(f)
+    }
+}
+
+#[derive(Clone)]
+pub(crate) struct BoxedEntry {
+    id: AnyValueId,
+    value: BoxedExtension,
+}
+
+impl BoxedEntry {
+    pub(crate) fn new(r: impl Extension) -> Self {
+        let id = AnyValueId::from(&r);
+        let value = BoxedExtension::new(r);
+        BoxedEntry { id, value }
+    }
+}
+
+impl<R: Extension> From<R> for BoxedEntry {
+    fn from(inner: R) -> Self {
+        BoxedEntry::new(inner)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[derive(Default, Copy, Clone, Debug, PartialEq, Eq)]
+    struct Number(usize);
+
+    #[test]
+    fn get() {
+        let mut ext = Extensions::default();
+        ext.set(Number(10));
+        assert_eq!(ext.get::<Number>(), Some(&Number(10)));
+    }
+
+    #[test]
+    fn get_mut() {
+        let mut ext = Extensions::default();
+        ext.set(Number(10));
+        *ext.get_mut::<Number>().unwrap() = Number(20);
+        assert_eq!(ext.get::<Number>(), Some(&Number(20)));
+    }
+
+    #[test]
+    fn get_or_insert_default_empty() {
+        let mut ext = Extensions::default();
+        assert_eq!(ext.get_or_insert_default::<Number>(), &Number(0));
+    }
+
+    #[test]
+    fn get_or_insert_default_full() {
+        let mut ext = Extensions::default();
+        ext.set(Number(10));
+        assert_eq!(ext.get_or_insert_default::<Number>(), &Number(10));
+    }
+
+    #[test]
+    fn set() {
+        let mut ext = Extensions::default();
+        assert!(!ext.set(Number(10)));
+        assert_eq!(ext.get::<Number>(), Some(&Number(10)));
+        assert!(ext.set(Number(20)));
+        assert_eq!(ext.get::<Number>(), Some(&Number(20)));
+    }
+
+    #[test]
+    fn reset() {
+        let mut ext = Extensions::default();
+        assert_eq!(ext.get::<Number>(), None);
+
+        assert!(ext.remove::<Number>().is_none());
+        assert_eq!(ext.get::<Number>(), None);
+
+        assert!(!ext.set(Number(10)));
+        assert_eq!(ext.get::<Number>(), Some(&Number(10)));
+
+        assert!(ext.remove::<Number>().is_some());
+        assert_eq!(ext.get::<Number>(), None);
+    }
+
+    #[test]
+    fn update() {
+        let mut ext = Extensions::default();
+        assert_eq!(ext.get::<Number>(), None);
+
+        let mut new = Extensions::default();
+        assert!(!new.set(Number(10)));
+
+        ext.update(&new);
+        assert_eq!(ext.get::<Number>(), Some(&Number(10)));
+    }
+}
diff --git a/clap_builder/src/builder/mod.rs b/clap_builder/src/builder/mod.rs
index 098ad576..76331694 100644
--- a/clap_builder/src/builder/mod.rs
+++ b/clap_builder/src/builder/mod.rs
@@ -7,6 +7,7 @@ mod arg_group;
 mod arg_predicate;
 mod arg_settings;
 mod command;
+mod ext;
 mod os_str;
 mod possible_value;
 mod range;