diff --git a/source/mijin/util/annot.hpp b/source/mijin/util/annot.hpp
index d1dd3e9..6ee18e4 100644
--- a/source/mijin/util/annot.hpp
+++ b/source/mijin/util/annot.hpp
@@ -35,6 +35,10 @@ concept nullable_type = !std::is_same_v<T, std::nullptr_t> && requires(T t) { t
 template<nullable_type T>
 class NotNullable
 {
+public:
+    using wrapped_t = T;
+    using element_type = std::remove_reference_t<decltype(*std::declval<const wrapped_t>())>;
+    using pointer = element_type*;
 private:
     T base_;
 public:
@@ -55,6 +59,12 @@ public:
     {
         MIJIN_ASSERT(base_ != nullptr, "Constructed non-nullable type with nullptr.");
     }
+    template<typename TOther> requires(std::is_base_of_v<typename NotNullable<TOther>::element_type, element_type> && std::is_constructible_v<T, pointer>)
+    explicit constexpr NotNullable(const NotNullable<TOther>& other) MIJIN_NOEXCEPT_IF((std::is_nothrow_constructible_v<T, pointer>))
+            : base_(static_cast<pointer>(&*other))
+    {
+        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))