Improving error interface: Begin working on `Box<T>` to allow any `T` to tell if it is owned by a `Box<T>`.

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

include/boxed.h

@@ -10,12 +10,15 @@
 #include "exopt.h"
 
 namespace exopt::types { namespace boxed {
+
 	template<typename T>
 	struct [[gnu::visibility("internal")]]  boxable_value_type { using type = std::conditional_t
 			<std::is_reference_v<T>
 			,std::reference_wrapper<std::remove_cvref_t<T> >
 			,T>; };
 
+	namespace { 
+
 	template<typename T>
 	constexpr std::unique_ptr<T> uniq_clone(std::unique_ptr<T> const& c) noexcept(std::is_nothrow_copy_constructible_v<T>) requires(std::is_copy_constructible_v<T>)
 	{ if(__builtin_expect(bool(c), true)) return std::make_unique<T>(*c); return nullptr; }
@@ -34,11 +37,74 @@ namespace exopt::types { namespace boxed {
 #endif
 	return std::make_unique<T>(*c); }
 
+
+	template<typename Error = ptr::NullException>
+	constexpr decltype(auto) force_ok(auto&& mv) requires(std::is_convertible_v<decltype(mv), bool> && std::is_default_constructible_v<Error>)
+	{
+		struct err {
+			[[gnu::noinline, gnu::cold, noreturn]]
+			static constexpr void _throw() { throw Error{}; }
+		};
+		if( __builtin_expect(!bool(mv), false)) err::_throw();
+		return std::forward<decltype(mv)>(mv);
+	}
+
+	} // _impl (anon)
+	
+
 	template<typename From, typename To>
 	concept binary_convertible = std::is_convertible_v<From, To> and std::is_convertible_v<To, From>;
 
+	template<typename From, typename To>
+	concept polymorphic_castable = std::derived_from<std::decay_t<To>, std::decay_t<From>> 
+					or std::derived_from<std::decay_t<From>, std::decay_t<To>>
+					or std::is_same_v<std::decay_t<From>, std::decay_t<To>>
+					or requires(std::decay_t<From> && from) {
+		{ static_cast<std::decay_t<To> &&>(from) } -> std::convertible_to<To>;
+	};
+
+	template<typename To, typename From> requires(polymorphic_castable<From, To>)
+	constexpr std::unique_ptr<To> static_uniq_cast(std::unique_ptr<From>&& from) noexcept
+	{
+		return std::unique_ptr<To> { 
+			static_cast<To*>( std::move(from).release() )
+		};
+	}
+
+	template<typename To, typename From> requires(polymorphic_castable<From, To>)
+	constexpr std::unique_ptr<To> dynamic_uniq_cast(std::unique_ptr<From>&& from) noexcept
+	{
+		return std::unique_ptr<To> { 
+			dynamic_cast<To*>( std::move(from).release() )
+		};
+	}
+
+	template<typename To, typename From, bool Check = false> requires(polymorphic_castable<From, To>)
+	constexpr std::unique_ptr<To> uniq_cast(std::unique_ptr<From>&& from) noexcept
+	{
+		if constexpr(Check) return dynamic_uniq_cast(std::move(from));
+		else return static_uniq_cast(std::move(from));
+	}
+
+#if 0
+//TODO: Cannot be implemented with this method
+	constexpr bool is_boxed_value(auto const& value) noexcept {
+		return bool(dynamic_cast<ObjectBase const*>(std::addressof(value)));
+	}
+	template<typename T>
+	constexpr bool is_boxed_type() noexcept {
+		using type = std::remove_reference_t<T>;
+		constexpr type const* VALUE = nullptr;
+		return bool(dynamic_cast<ObjectBase const*>(VALUE));
+	}
+
+// We need either: Tagged pointers (XOR, bit-mangled, manual heap, or GOT (external allocation) kind), \
+	or (preferred): layout of Box<T> to be `{ aligned<T>(T, box_type_tag) } unique*` \
+		box_type_tag should be: One pointer width, starting at offset *T+1 (aligned to T within internal tuple struct), provides a way to access RTTI for `*T`, a pointer to static (non-allocated) type information or dynamic_cast used lambda would be ideal.
+#endif
+
 	template<typename T>
-	struct Box {
+	struct Box final {
 		typedef boxable_value_type<T>::type type;
 
 		template<std::derived_from<T> U, bool RuntimeCheck=false>
@@ -50,6 +116,18 @@ namespace exopt::types { namespace boxed {
 			return Box<T>{std::make_unique<T>(dynamic_cast<T&&>(std::move(v)))};
 		}
 
+		template<std::derived_from<T> U, bool Check = false>
+		constexpr Box<U> upcast() && noexcept(!Check) 
+		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }
+
+		template<typename U, bool Check = true> requires(std::derived_from<T, U>)
+		constexpr Box<U> downcast() && noexcept(!Check)
+		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }
+
+		template<polymorphic_castable<T> U, bool Check = !std::derived_from<U, T> > // Default dynamic check set to false only if we can statically determine that T is derived from U and the conversion is therfore infallible
+		constexpr Box<U> polycast() && noexcept(!Check)
+		{ return Box<U> { force_ok(uniq_cast<U, T, Check>(std::move(m_ptr))) }; }
+
 		constexpr Box() noexcept(std::is_nothrow_default_constructible_v<T>) requires(std::is_default_constructible_v<T>)
 			: m_ptr{std::make_unique<T>()} {}
 
@@ -101,6 +179,17 @@ namespace exopt::types { namespace boxed {
 			_EO_ASSUME(m_ptr.get());
 		}
 
+#if 0
+// TODO: Identifying if a value `this` pointer is boxed
+		struct alignas(type) inner_layout {
+			constexpr inner_layout() noexcept = default;
+			constexpr inner_layout(const inner_layout&) noexcept = default;
+			constexpr ~inner_layout() noexcept = default;
+
+			type value;
+			box_type_tag identifier{box_tag_of<T>()};
+		};
+#endif
 		std::unique_ptr<T> m_ptr; // Unique<T> m_ptr;
 	};
 
@@ -133,20 +222,43 @@ namespace exopt::types { namespace boxed {
 		}
 	} //TODO: Overload for `const&` that does the type check *before* the copy allocation.
 
+#if 0
 	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...
+	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, util::comptime_eval_v<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>>)
+	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, T>)
+	{ return { std::move(value) }; }
+#endif
+	template<typename T>
+	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, decltype(value)>)
 	{
 		/*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)) };
+		} else */return Box<T> { std::move(value) };
 	}
+
 	template<typename T>
-	constexpr Box<T> box(T&& value) noexcept(std::is_nothrow_constructible_v<Box<T>, T>)
+	constexpr Box<T> box(std::unique_ptr<T>&& value) noexcept { return Box<T> { std::move(value) }; }
-	{ return { std::move(value) }; }
+
+	template<typename Base, typename T> /*requires(polymorphic_castable<Base, T>)
+	constexpr Box<Base> polybox(T&& value) noexcept
+	{
+		return Box<T>{ std::move(value) }.polycast<Base>()
+	}*/ requires(std::derived_from<T, Base>)
+	constexpr Box<Base> polybox(T&& value) noexcept {
+		return Box<T>{ std::move(value) }.polycast<Base, false>(); // Convertion from derived to base infallible, no need for check.
+	}
+
+	template<typename Base, typename T> /*requires(polymorphic_castable<Base, T>)
+	constexpr Box<Base> polybox(T&& value) noexcept
+	{
+		return Box<T>{ std::move(value) }.polycast<Base>()
+	}*/ requires(std::derived_from<T, Base>)
+	constexpr Box<Base> polybox(std::unique_ptr<T>&& value) noexcept {
+		return Box<T>{ std::move(value) }.polycast<Base, false>(); // Convertion from derived to base infallible, no need for check.
+	}
 }
 using boxed::Box;
 }

include/error.hh

@@ -88,7 +88,7 @@ namespace exopt {
 
 	private:
 		inline types::Box<TracedError> into_traced(std::stacktrace trace&&) &&noexcept override final// = std::stacktrace::current()) && noexcept;
-		{ return typed::Box<TracedError> { std::move(*this) }; } //TODO: XXX: How to check if `this` is *already* boxed? This dynamic double-boxing is inefficient, but also can probably not be checked unless relying on enabling RTTI by default
+		{ return typed::Box<TracedError> { std::move(*this) }; } //TODO: use `boxed::is_boxed_value()` to check if *this is boxed... Uhh, actually... XXX: No that doesn't work, since `Box<T>` doesn't make `T` visible to `boxed::ObjectBase`.... Eh...
 		inline typed::Box<TracedError> into_traced(std::stacktrace trace = std::stacktrace::current()) && noexcept override final
 		{ return typed::Box<TracedError> { std::move(*this) }; } 
 		std::stacktrace m_trace; //TODO: Maybe box this? idk how big it is...

include/util.hh

@@ -12,6 +12,7 @@
 #define _EO_CONSTANT_VALUE(X) ([]() { \
 	struct { \
 		typedef decltype(X) comptime_constant_t; \
+		constexpr static inline auto VALUE = (X); \
 		consteval operator comptime_constant_t() const noexcept { return (X); } \
 	} inner; \
 	return inner; \
@@ -28,7 +29,7 @@ namespace exopt { namespace util [[gnu::visibility("internal")]] {
 
 	constexpr auto comptime_value(auto value) noexcept {
 		using U = decltype(value);
-		struct inner {
+		struct inner final {
 			typedef U comptime_constant_t;
 			consteval operator comptime_constant_t() const noexcept { return std::move(value); }
 			U value;
@@ -39,6 +40,30 @@ namespace exopt { namespace util [[gnu::visibility("internal")]] {
 		return inner{ std::move(value) };
 	}
 	
+	template<typename T, auto value> requires(comptime<decltype(value), T>)
+	consteval T comptime_eval() noexcept {
+		if constexpr(requires { 
+			{ decltype(value)::VALUE } -> std::convertible_to<decltype(value)::comptime_constant_t>;
+		}) {
+			return decltype(value)::VALUE;
+		} else  return value;
+	
+	}
+
+	template<typename T, auto value> requires(comptime<decltype(value), T>)
+	constexpr inline T comptime_eval_v = comptime_eval<T, value>(); 
+
+	template<typename T>
+	consteval T comptime_eval(comptime<T> auto value) noexcept {
+		return comptime_eval<T, value>();
+	}
+
+	
+
+	static_assert(comptime_eval(comptime_value(5.0)) == 5.0, "Bad comptime_eval() consteval");
+	static_assert(comptime_eval(_EO_CONSTANT_VALUE(3.f)) == 3.f, "Bad comptime_eval() macro");
+	static_assert(std::is_same_v<decltype(comptime_eval(comptime_value(4.f))), float>, "Bad comptime_eval() return type consteval");
+	static_assert(std::is_same_v<decltype(comptime_eval(_EO_CONSTANT_VALUE(4.0))), double>, "Bad comptime_eval() return type macro");
 	static_assert(comptime_value(std::string_view{"hello"}.size()) == 5, "Bad comptime_value()");
 	static_assert(_EO_CONSTANT_VALUE(std::string_view{"hello world"}) == std::string_view{"hello world"}, "Bad CONSTANT_VALUE()");