Fill in missing const on Fn member functions
diff --git a/include/cxx.h b/include/cxx.h
index 8bdbc90..5de479c 100644
--- a/include/cxx.h
+++ b/include/cxx.h
@@ -154,8 +154,8 @@
 template <typename Ret, typename... Args, bool Throws>
 class Fn<Ret(Args...), Throws> {
 public:
-  Ret operator()(Args... args) noexcept(!Throws);
-  Fn operator*() noexcept;
+  Ret operator()(Args... args) const noexcept(!Throws);
+  Fn operator*() const noexcept;
 
 private:
   Ret (*trampoline)(Args..., void *fn) noexcept(!Throws);
@@ -201,12 +201,12 @@
 #endif // CXXBRIDGE02_RUST_BITCOPY
 
 template <typename Ret, typename... Args, bool Throws>
-Ret Fn<Ret(Args...), Throws>::operator()(Args... args) noexcept(!Throws) {
+Ret Fn<Ret(Args...), Throws>::operator()(Args... args) const noexcept(!Throws) {
   return (*this->trampoline)(std::move(args)..., this->fn);
 }
 
 template <typename Ret, typename... Args, bool Throws>
-Fn<Ret(Args...), Throws> Fn<Ret(Args...), Throws>::operator*() noexcept {
+Fn<Ret(Args...), Throws> Fn<Ret(Args...), Throws>::operator*() const noexcept {
   return *this;
 }