Adding polymorphic boxing helpers.

Fortune for libexopt's current commit: Small blessing − 小吉

include/boxed.h

@@ -34,7 +34,9 @@ namespace exopt::types { namespace boxed {
 #endif
 	return std::make_unique<T>(*c); }
 
-
+	template<typename From, typename To>
+	concept binary_convertible = std::is_convertible_v<From, To> and std::is_convertible_v<To, From>;
+	
 	template<typename T>
 	struct Box {
 		typedef boxable_value_type<T>::type type;
@@ -102,6 +104,21 @@ namespace exopt::types { namespace boxed {
 		std::unique_ptr<T> m_ptr; // Unique<T> m_ptr;
 	};
 
+#define _EO_ADD_RV std::add_rvalue_reference_v
+
+	template<typename Base, typename T = Base>
+	constexpr inline bool is_boxable_v = binary_convertible<_EO_ADD_RV<T>, _EO_ADD_RV<Base> > and requires(T&& value) { 
+		typename Box<Base>;
+		typename Box<Base>::type;
+		{ std::make_unique<T>(std::move(value)) } -> std::same_as<std::unique_ptr<T>>;
+	};
+	template<typename T> concept is_boxable = is_boxable_v<T>;
+
+	template<typename T>
+	constexpr inline bool is_nothrow_boxable_v = is_boxable_v<T> && std::is_nothrow_constructible_v<Box<T>, T>;
+
+#undef _EO_ADD_RV
+
 	template<typename T, typename U> requires(requires(T&& o) { static_cast<U&&>(o); })
 	constexpr Box<U> static_box_cast(Box<T>&& b) noexcept { return Box<U>::from_raw_ptr(static_cast<U*>(b.release())); }
 
@@ -115,6 +132,21 @@ namespace exopt::types { namespace boxed {
 			throw;
 		}
 	} //TODO: Overload for `const&` that does the type check *before* the copy allocation.
+
+	template<typename T, typename R /*XXX: Maybe remove this, and put the requires after the definition, using `auto&& value` instead of `R&& value`*/> requires(std::derived_from<R, T>)
+	constexpr Box<T> box(R&& value, util::comptime<bool> auto Check = _EO_CONSTANT_VALUE(false)) noexcept(std::is_nothrow_invocable_v< Box<T>::new_dynamic<R, bool(Check)>) //XXX: This seems illegal...
+	{ return Box<T>::template new_dynamic<R, Check>(std::move(value)); }
+
+	template<typename T>
+	constexpr Box<T> box(std::convertible_to<T> auto&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, std::add_rvalue_reference_v<T>>)
+	{
+		/*if constexpr(requires(decltype(value) v) { static_cast<T&&>(std::move(v)); }) {
+			return Box<T>::template new_dynamic<T>(std::move(value));
+		} else */return { static_cast<T>(std::move(value)) };
+	}
+	template<typename T>
+	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, T>)
+	{ return { std::move(value) }; }
 }
 using boxed::Box;
 }