diff --git a/source/mijin/util/annot.hpp b/source/mijin/util/annot.hpp
index 822a53e..c6061df 100644
--- a/source/mijin/util/annot.hpp
+++ b/source/mijin/util/annot.hpp
@@ -29,7 +29,10 @@
 
 namespace mijin
 {
-template<typename T> requires(!std::is_same_v<T, std::nullptr_t>) && requires(T t) { t == nullptr; }
+template<typename T>
+concept nullable_type = !std::is_same_v<T, std::nullptr_t> && requires(T t) { t == nullptr; };
+
+template<nullable_type T>
 class NotNullable
 {
 private:
@@ -41,6 +44,18 @@ public:
     {
         MIJIN_ASSERT(base_ != nullptr, "Constructed non-nullable type with nullptr.");
     }
+    template<typename TOther> requires(std::is_constructible_v<T, const TOther&>)
+    constexpr NotNullable(const NotNullable<TOther>& other) MIJIN_NOEXCEPT_IF((std::is_nothrow_constructible_v<T, const TOther&>))
+            : base_(other.base_)
+    {
+        MIJIN_ASSERT(base_ != nullptr, "Constructed non-nullable type with nullptr.");
+    }
+    template<typename TOther> requires(std::is_constructible_v<T, TOther&&>)
+    constexpr NotNullable(NotNullable<TOther>&& other) MIJIN_NOEXCEPT_IF((std::is_nothrow_constructible_v<T, TOther&&>))
+            : base_(std::exchange(other.base_, nullptr))
+    {
+        MIJIN_ASSERT(base_ != nullptr, "Constructed non-nullable type with nullptr.");
+    }
     template<typename TArg, typename... TArgs> requires(!std::is_same_v<TArg, std::nullptr_t>
                                                       && (!std::is_same_v<TArg, T> && sizeof...(TArgs) == 0)
                                                       && std::is_constructible_v<T, TArg&&, TArgs&&...>)
@@ -129,6 +144,9 @@ public:
 
     [[nodiscard]]
     constexpr const T& get() const MIJIN_NOEXCEPT { return base_; }
+
+    template<nullable_type TOther>
+    friend class NotNullable;
 };
 
 #if MIJIN_USE_GSL