Started `optional::null_optimise<Box<T>>` implementation. (Incomplete.)

Fortune for libexopt's current commit: Middle blessing − 中吉

include/boxed.h

@@ -10,7 +10,6 @@
 #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>
@@ -31,7 +30,7 @@ namespace exopt::types { namespace boxed {
 		requires(std::is_copy_constructible_v<T>)
 	{ if(__builtin_expect(!bool(c), false)) 
 #ifdef DEBUG
-		throw ptr::null_exception<T>{}; 
+		throw ptr::NullDerefException<T>{};
 #else
 		__builtin_unreachable(); 
 #endif
@@ -63,17 +62,8 @@ namespace exopt::types { namespace boxed {
 		{ static_cast<std::decay_t<To> &&>(from) } -> std::convertible_to<To>;
 	};
 
-	struct ErasedTypeDeleter { 
-		constexpr virtual ~ErasedTypeDeleter() = default;
-		constexpr ErasedTypeDeleter() noexcept = default;
-
-		constexpr virtual void apply_delete() = 0;
-		//TODO: See below...
-	};
-
 	template<polymorphic_castable<void> T>
 	constexpr auto uniq_erase_type(std::unique_ptr<T>&& from) noexcept {
-		//TODO: Overload case for `unique_ptr<T, D>` for `deleter` to apply (and contain) `D()` on back_cast()ed pointer T* instead of assuming default `delete`.
 		struct deleter final /*: public ErasedTypeDeleter XXX <- Is this useful, for unerasing the type, maybe?*/ {
 			typedef T type;
 
@@ -94,18 +84,26 @@ namespace exopt::types { namespace boxed {
 				delete static_cast<T*>(p);
 			}
 			constexpr T* back_cast(void *p) noexcept { return static_cast<T*>(p); }
-			/*constexpr deleter() noexcept = default;
+			constexpr deleter() noexcept = default;
-			constexpr ~deleter() noexcept = default;*/
+			constexpr deleter(deleter const&) noexcept = default;
+			constexpr deleter(deleter &&) noexcept = default;
+			constexpr deleter& operator=(deleter const&) noexcept = default;
+			constexpr deleter& operator=(deleter &&) noexcept = default;
+
+			constexpr ~deleter() noexcept = default;
 		};
 		return std::unique_ptr<void, deleter> {
-			//XXX: Cannot retain type information of most-derived class from this: \
+			// Cannot retain type information of most-derived class from this: \
 			dynamic_cast<void*>	
 			static_cast<void*>
 			(std::move(from).release())
 			
 		};
 	}
-	template<polymorphic_castable<void> T, typename D> requires(requires{ typename std::unique_ptr<T, D>; })
+	template<polymorphic_castable<void> T, typename D> requires(requires{ 
+		typename std::unique_ptr<T, D>;
+		requires(!std::is_same_v<std::unique_ptr<T, D>::deleter_type, std::unique_ptr<T>::deleter_type);
+	})
 	constexpr auto uniq_erase_type(std::unique_ptr<T, D>&& from) noexcept {
 		class deleter final {
 			D m_del;
@@ -127,6 +125,7 @@ namespace exopt::types { namespace boxed {
 			constexpr deleter& operator=(const deleter&) noexcept(std::is_nothrow_copy_assignable_v<D>) = default;
 			constexpr deleter& operator=(deleter&&) noexcept(std::is_nothrow_move_assignable_v<D>) = default;
 
+			constexpr ~deleter() noexcept(std::is_nothrow_destructible_v<D>) requires(std::is_trivially_destructible_v<D>) = default;
 			constexpr ~deleter() noexcept(std::is_nothrow_destructible_v<D>) {}
 		};
 		if constexpr(std::is_default_constructible_v<D>)
@@ -166,72 +165,8 @@ namespace exopt::types { namespace boxed {
 		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
-	/// Base class which is used to store the layout of Box<T>'s allocated memory. Intended for use to determine if a certain `T`'s `this` pointer is inside a `Box<T>` or not.
-	class dynamic_boxed_layout_base{
-		using Self = dynamic_boxed_layout_base;
-	public:
-		constexpr dynamic_boxed_layout_base() noexcept = default;
-		constexpr ~dynamic_boxed_layout_base() = default;
-
-		typedef void* (dynamic_cast_t)(Self*);
-		typedef const void* (dynamic_const_cast_t)(Self const*);
-
-		constexpr virtual size_t value_byte_offset() const noexcept =0;
-
-		[[gnu::returns_nonnull]]
-		constexpr void* extract_untyped_pointer() noexcept {
-			return reinterpret_cast<void*>(
-				reinterpret_cast<this>(unsigned char*) + value_byte_offset()
-			);
-		}
-		[[gnu::returns_nonnull]]
-		constexpr const void* extract_untyped_pointer() const noexcept {
-			return reinterpret_cast<void const*>(
-				reinterpret_cast<this>(const unsigned char*) + value_byte_offset()
-			);
-		}
-
-		constexpr virtual void* invoke_casting_shim(dynamic_cast_t const*) const noexcept =0;
-
-		template<typename T>
-		constexpr bool is_typeof() const noexcept { return invoke_casting_shim([] (Self* self) { return dynamic_cast<T*>(self)
-
-		template<typename T>
-		[[gnu::returns_nonnull]]
-		inline T* extract_pointer_unsafe() noexcept {
-			return reinterpret_cast<T*>(extract_untyped_pointer());
-		}
-		template<typename T>
-		[[gnu::returns_nonnull]]
-		inline const T* extract_pointer_unsafe() const noexcept {
-			return reinterpret_cast<const T*>(extract_untyped_pointer());
-		}
-
-		template<typename T>	
-		[[gnu::returns_nonnull]]
-		inline T* try_extract_pointer() noexcept {
-			return is_typeof<T>() ? extract_pointer_unsafe<T>() : nullptr;
-		}
-	};
-
 	template<typename T>
-	struct Box final {
+	struct alignas(std::unique_ptr<T>) Box {
 		typedef boxable_value_type<T>::type type;
 
 		template<std::derived_from<T> U, bool RuntimeCheck=false>
@@ -277,15 +212,24 @@ namespace exopt::types { namespace boxed {
 		}
 
 		[[gnu::nonnull(1)]]
-		constexpr static Box<T> from_raw_ptr(T* ptr) noexcept {
+		constexpr static Box<T> from_raw_ptr(T* ptr) noexcept(!_EO(DEBUG)) {
+#if DEBUG
+			ptr::throw_if_null(ptr);
+#endif
 			return Box<T>{ std::unique_ptr<T>{ ptr } };
 		}
-		constexpr static Box<T> from_raw_ptr(std::unique_ptr<T>&& uniq) 
+
+		constexpr static Box<T> from_raw_ptr(std::unique_ptr<T>&& uniq) {
+			if(__builtin_expect(!bool(uniq), false)) ptr::throw_null_exception<T>();
+			_EO_ASSUME(uniq.get() != nullptr);
+			return Box<T> { std::move(uniq) };
+		}
+
+		constexpr static Box<T> from_raw_ptr_unchecked(std::unique_ptr<T>&& uniq) noexcept(!_EO(DEBUG)) {
 #if DEBUG
-			{ if(__builtin_expect(!uniq, false)) throw ptr::NullException{};
+			if(__builtin_expect(!uniq, false)) ptr::throw_null_exception<T>(); 
 #else
 
-			noexcept {
 			_EO_ASSUME(uniq.get() != nullptr);
 #endif
 			return Box<T>{ std::move(uniq) };
@@ -300,49 +244,63 @@ namespace exopt::types { namespace boxed {
 	//TODO: Accessors, `(explicit?) operator std::unique_ptr<T> const&[&]`?, etc.
 
 		constexpr ~Box() = default;
-	private:
+	protected:
-		constexpr explicit Box(std::unique_ptr<T>&& ptr)
+		// For types that may need to invalidate the Box<T> guarantee without exposing it in API, they can inherit (non-virtual). 
-			: m_ptr(std::move(ptr)) {
-			_EO_ASSUME(m_ptr.get());
-		}
-
-// TODO: Identifying if a value `this` pointer is boxed
-		struct alignas(type) inner_layout_t final : public dynamic_boxed_layout_base {
-			using boxed_value_t = type;
 
-			constexpr inner_layout_t(type&& v) noexcept(std::is_nothrow_move_constructible_v<type>)
+		// For unsafe operations, wether or not to apply checks.
-				: value(std::move(v)) {}
+		enum unsafe_t {
-			//TODO: All the ctors, assigns, and operators that can make this newtype wrapper more conveniently useable as a deref-target for accessing `value`.
+			UNSAFE,
+		};
+		enum maybe_unsafe_t {
+			CHECKED,
+			DEBUG_CHECKED,
+		};
 
-			constexpr /*virtual*/ ~inner_layout_t() = default;
+		constexpr Box(util::exact_type<unsafe_t> auto u, std::unique_ptr<T>&& ptr) noexcept 
+			: m_ptr(std::move(ptr)) { (void)u; }
 
-			[[gnu::const]]
+		constexpr static Box<T> from_raw_ptr(unsafe_t u, std::unique_ptr<T>&& uniq) noexcept {
-			constexpr size_t value_byte_offset() const noexcept override { return offsetof(inner_layout_t, value); }
+			return Box{u, std::move(uniq)};
+		}
 
-			type value;
+		constexpr std::unique_ptr<T>& as_unique(unsafe_t) & noexcept { return m_ptr; }
-		};
+		constexpr std::unique_ptr<T> const & as_unique(unsafe_t) const& noexcept { return m_ptr; }
 
-		struct inner_unique_ptr_t final {
+		constexpr std::unique_ptr<T>&& as_unique(unsafe_t) && noexcept { return std::move(m_ptr); }
-			std::unique_ptr<inner_layout_t>
+		constexpr std::unique_ptr<T> const&& as_unique(unsafe_t) const&& noexcept { return std::move(m_ptr); }
-		};
 
-		inner_unique_ptr_t m_ptr; // Unique<T> m_ptr;
+		constexpr T* as_unsafe_ptr(unsafe_t) const noexcept { return m_ptr.get(); }
-	};
+		constexpr std::unique_ptr<T> swap_unsafe_ptr(unsafe_t u, T* raw) noexcept 
+		{ return (void)u; std::exchange(std::move(m_ptr), std::unique_ptr<T>{ raw }); }
+		constexpr void set_unsafe_ptr(unsafe_t 8, T* raw) noexcept { 
+			(void)u;
+			m_ptr.reset(raw);
+		}
+		constexpr decltype(auto) set_unsafe(unsafe_t u, std::unique_ptr<T>&& ptr) noexcept 
+		{ (void)u; return std::exchange(std::move(m_ptr), std::move(ptr)); }
+		constexpr auto& swap_unsafe(unsafe_t, std::unique_ptr<T>& ptr) noexcept { 
+			using std::swap;
+			swap(m_ptr, ptr);
+			return m_ptr;
+		}
 
-#define _EO_ADD_RV std::add_rvalue_reference_v
+		constexpr T* release_unsafe(unsafe_t) noexcept { return m_ptr.release(); }
+	private:
+		constexpr explicit Box(std::unique_ptr<T>&& ptr) noexcept
+			: m_ptr(std::move(ptr)) {
+			_EO_ASSUME(m_ptr.get());
+		}
 
-	template<typename Base, typename T = Base>
+		std::unique_ptr<T> m_ptr; // Unique<T> m_ptr;
-	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>>;
 	};
+	static_assert(util::shares_layout<Box<_Complex double>, _Complex double*>, "Invalid Box<T> memory layout: More than just T*");
+	static_assert(util::shares_layout<Box<_Complex double>, std::unique_ptr<_Complex double>>, "Invalid Box<T> memory layout: More than just 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())); }
@@ -356,7 +314,12 @@ namespace exopt::types { namespace boxed {
 			delete rel;
 			throw;
 		}
-	} //TODO: Overload for `const&` that does the type check *before* the copy allocation.
+	} 
+
+	template<typename T, typename U> requires(requires(T const& o) { static_cast<U const&>(o); })
+	constexpr Box<U> dynamic_box_cast(Box<T> const& b) {
+		return Box<U>::from_raw_ptr(std::addressof(dynamic_cast<U const&>(*b)));
+	} 
 
 #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>)
@@ -399,6 +362,8 @@ namespace exopt::types { namespace boxed {
 using boxed::Box;
 }
 
+//TODO: See phone notes on correctly implementing the nullopt for Box<T>
+#if 0
 namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
 	template<typename T>
 	struct null_optimise<boxed::Box<T> > { constexpr static inline bool value = true;
@@ -413,3 +378,4 @@ namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
 
 
 }
+#endif

include/optional.h

@@ -6,6 +6,8 @@
 
 #include "pointer.h"
 
+#include "exopt.h"
+
 /// A better std::optional
 namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
 	template<typename T>
@@ -21,6 +23,8 @@ namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
 		constexpr static inline auto sentinel = nullptr;
 	};
 
+	
+
 	template<typename T>
 	struct null_optimise<std::reference_wrapper<T> > { constexpr static inline bool value = true;
 		using held_type = T*;
@@ -31,7 +35,30 @@ namespace exopt::types { namespace optional [[gnu::visibility("internal")]] {
 		constexpr static inline auto sentinel = nullptr;
 	};
 
+	template<typename T>
+	class null_optimise<boxed::Box<T> > {
+		using boxed::Box;
+		using box_t = Box<T>;
+		struct /*XXX: Shouldn't be needed alignas(box_t)*/ invariant : box_t {
+			//TODO: See phone notes.
+			constexpr ~invariant() = default;
+		};
+		static_assert(util::shares_layout<invariant, box_t>, "invariant (held_type) does not share layout with viewed type (Box<T>)");
+	public:
+		typedef invariant held_type;	// invariant ^: Held internal type.
+		using type = box_t;		// Box<T>: API seen & operated on type
+
+		//TODO: See phone notes.
 
+		constexpr static inline auto sentinel = nullptr;
+	};
+#ifdef DEBUG /* We don't want to instantiate `null_optimise<>` *at all* if we don't have to outside of actual real use in `Option<T>`. This check is for development only since Option<Box<T>> is a bit of an awkward developmental case.
+		 Save compile time for release builds, the `static_assert` inside the null-opt partial spec for `..::invariant`'s layout will be checked if/when `Option<Box<T>>` is ever instantiated. This check is the same just assuring it's valid for *if* Option<Box<T>> is ever instantiated */
+	static_assert(!requires(requires{ typename boxed::Box<int>; }) or // Skip check if `boxed.h` is not included.
+		 requires(requires(null_optimise<boxed::Box<int>>::held_type const& held) { 
+			{ static_cast<boxed::Box<int> const&>(held) };
+		}), "Nullopt held_type for Box<T> either has a unneeded and unwanted generated vtable or has invalid alignment, check into that");
+#endif
 
 	template<typename T>
 	struct null_optimise<ptr::Unique<T> > { constexpr static inline bool value = true;

include/pointer.h

@@ -104,6 +104,33 @@ namespace exopt::ptr {
 		return std::forward<decltype(ptr)>(ptr);
 	}
 
+	template<typename E, typename... Args> requires(std::is_constructible_v<E, Args...>)
+	constexpr decltype(auto) throw_if_null(PointerEqCmp auto const& ptr, Args&&... error) {
+		auto _throw = [&] [[noreturn, gnu::noinline, gnu::cold]] () {
+			throw E{std::forward<Args>(error)...};
+		};
+		if(ptr == nullptr) _throw();
+		return std::forward<decltype(ptr)>(ptr);
+	}
+
+	
+	[[gnu::always_inline]]
+	constexpr decltype(auto) throw_if_null(PointerEqCmp auto const& ptr) {
+		if(__builtin_expect(ptr == nullptr, false)) throw_null_exception<decltype(ptr)>();
+		return std::forward<decltype(ptr)>(ptr);
+	}
+
+	// Throws NullDerefException<?> for first type of `ptr` that is null (if any are).
+	// If: `All == true`:  Instead will always throw `NullDerefException<T...> (all types) if any are, and will not assume every pointer is likely to be non-null.
+	template<bool All = false>
+	[[gnu::always_inline]]
+	constexpr void throw_if_null(PointerEqCmp auto const&... ptr) requires(sizeof...(decltype(ptr)) > 1) {
+		if constexpr(All) {
+			using E = NullDerefException<decltype(ptr)...>;
+			((void)throw_if_null<E>(std::forward<decltype(ptr)>(ptr)),...);
+		} else ((void)throw_if_null(std::forward<decltype(ptr)>(ptr)),...);
+	}
+
 	template<typename E, typename... Args> requires(std::is_constructible_v<E, Args...>)
 	constexpr decltype(auto) deref_or_throw(PointerDeref auto&& ptr, Args&&... error)
 	{
@@ -190,6 +217,13 @@ namespace exopt::ptr {
 
 	consteval NullException/*&&*/ null_exception() noexcept { return {}; }
 
+	template<typename... Types>
+	[[noreturn, gnu::noinline, gnu::cold]]
+	constexpr void throw_null_exception() { throw null_exception<Types...>(); }
+
+	[[noreturn, gnu::noinline, gnu::cold]]
+	constexpr void throw_null_exception() { throw null_exception(); }
+
 	template<typename T> requires(!std::is_reference_v<T>)
 	struct NonNull {
 

include/util.hh

@@ -68,6 +68,12 @@ namespace exopt { namespace util [[gnu::visibility("internal")]] {
 	static_assert(_EO_CONSTANT_VALUE(std::string_view{"hello world"}) == std::string_view{"hello world"}, "Bad CONSTANT_VALUE()");
 
 
+	template<typename T, typename U>
+	concept shares_layout = sizeof(T) == sizeof(U) && alignof(T) == alignof(U);
+
+	template<typename T, typename U>
+	concept exact_type = std::is_same_v<T, U>;
+
 	template<typename T, size_t N, typename Array= std::array<T, N>, typename _I = std::make_index_sequence<N>>
 	constexpr auto array_literal(const T (&a)[N]) noexcept 
 		-> Array
@@ -178,17 +184,90 @@ namespace exopt { namespace util [[gnu::visibility("internal")]] {
 	static_assert(concat_str_v< std::string_view{"hello"}, std::string_view{" "}, std::string_view{"view"} >
 			== std::string_view{"hello view"}, "concat_str_v<>: Concatenated string_view failed");
 
+	template<template<typename> P, typename... Values>
+	struct _EO(internal) is_all_match {
+		constexpr static inline auto value = (P<Values> && true && ...);
+
+		consteval operator auto() const noexcept { return value; }
+	};
+
+	template<template<typename> P, typename... Values>
+	struct _EO(internal) is_any_match {
+		constexpr static inline auto value = (P<Values> || false || ...);
+
+		consteval operator auto() const noexcept { return value; }
+	};
+
+	//XXX: Idk if template<template<s are allowed in this context...
+	template<template<typename> P, typename... Values> requires(requires{ typename is_all_match<P, Values>; })
+	constexpr inline auto is_all_match_v = is_all_match<P, Values...>::value;
+
+	template<template<typename> P, typename... Values> requires(requires{ typename is_any_match<P, Values>; })
+	constexpr inline auto is_any_match_v = is_any_match<P, Values...>::value;
+
+	template<auto... Values>
+	struct _EO(internal) is_all {
+		constexpr static inline auto value = (Values && true && ...);
+	};
+
+	template<auto... Values>
+	struct _EO(internal) is_any {
+		constexpr static inline auto value = (Values || false || ...);
+	};
+
+	template<auto... Values>
+	constexpr inline auto is_all_v = is_all<Values...>::value;
+	template<auto... Values>
+	constexpr inline auto is_any_v = is_any<Values...>::value;
+
+	template<typename... Args>
+	constexpr inline bool is_empty_v = sizeof...(Args) == 0;
+
+	template<auto... Values> // auto and typename aren't compatible unfortunately
+	constexpr inline bool is_empty_vv = sizeof...(Values) == 0;
+
+	// Convenience immediate function overloads for checking if a parameter pack *or* a value pack is empty without different names.
+	template<typename... A>
+	consteval bool is_empty_pack() noexcept { return sizeof...(A) == 0; }
+
+	template<auto... A>
+	consteval bool is_empty_pack() noexcept { return sizeof...(A) == 0; }
+
+	///  Deducable version of `is_empty_pack<typename...>()`
+	///
+	/// Can be used as `is_empty_pack(pack...)` instead of the slightly more ugly `is_empty_pack<decltype(pack)...>()` though with (very unlikely, but possible) implications of actually "evaluating" the pack (despite the values themselves are `const&` and never used, they're still passed.) 
+	/// HOWEVER: The latter should likely be preferred, since this function takes possible-nonconstexpr types as arguments, it cannot be immediate like the non-argument taking overload. See below comment for more information on that.
+	template<typename... A>
+	[[gnu::const]] //TODO: XXX: This mostly useless function won't mess up overload resolution for `is_empty_pack<types...>()`, right? If it will, either find a way to make sure it comes LAST, or remove it, it's not worth the hassle just so users (i.e. me) can avoid typeing `decltype` for once in my fuckin life... what a stupud keyword name srsl (see static_assert() below.)
+	constexpr auto is_empty_pack(A const&...) noexcept { return is_empty_pack<A...>(); } // This overload exists for: e.g usage for index_sequence: \
+		`template<size_t N> f(const auto (&v)[N]) -> \
+		template<size_t Is...> _f(auto const& v, std::index_sequence<Is...>);`\
+		Can be used like: \
+		_f(const auto&v, std::convertible_to<size_t> auto&&... is) \
+			{ if constexpr(is_empty_pack(is...)) { ... } ... }` \
+		i.e: for unknown-typed non-type parameter packs `auto...`s packs can be `forward<decltype(pack)>(pack)...`'d (or just passed, as the taken value is always `const&`) to is_empty_pack(...) and `A...` can therefor be deduced. This is not possible for the non-value taking overloads, and also must be non-consteval since any `A` might not be constexpr. \
+\
+		It is a convenience function that is essentially equivelent to `is_empty_pack<decltype(pack)...>()`, which should be preferred, since it is consteval and will always be guaranteed to be consteval regardless of the contents of `pack` itself, but I highly doubt any compiler will actually generate any argument-passing code for this overload either.
+
+	static_assert(!is_empty_pack<int, long, float>() 
+		and is_empty_pack<>() //XXX: This might also be ambiguous between the type and value one... Eh.... That's actually useful though, so removing this explicit (and pointless) *usage* of the function from the test would be fine if they don't clash in other ways too.
+		and !is_empty_pack<0, 1, 2>()
+		and !is_empty_pack(0u, 1.0, 2l)
+		and is_empty_pack(), "`is_empty_pack()` overload issues.");
+
 	template<typename R, typename... Args>
-	constexpr auto map(auto const& fun, Args&&... values) noexcept((std::is_nothrow_invocable_v<decltype(fun), Args> && ...))
+	constexpr auto map(auto const& fun, Args&&... values) noexcept(is_empty_v<Args...> or is_all_v<std::is_nothrow_invocable_v<decltype(fun), Args>...>)
-								requires((std::is_invocable_v<decltype(fun), Args> && ...) and (
+								requires(is_empty_v<Args...> or (is_all_v<std::is_invocable_v<decltype(fun), Args>...> and (
-			(std::is_void_v<std::invoke_result_t<decltype(fun), Args>> || ...) ||
+			is_any_v<std::is_void_v<std::invoke_result_t<decltype(fun), Args>>...> ||
-			 std::is_constructible_v<R, std::invoke_result_t<decltype(fun), Args>...>))
+			 std::is_constructible_v<R, std::invoke_result_t<decltype(fun), Args>...>)))
-		-> std::conditional_t< (std::is_void_v<std::invoke_result_t<decltype(fun), Args>> || ...)
+		-> std::conditional_t< is_empty_v<Args...> || is_any_v<std::is_void_v<std::invoke_result_t<decltype(fun), Args>>...>
-					, void
+					, std::void_t<std::invoke_result_t<decltype(fun), Args>...>
 					, R>
 	{
-		if constexpr( (std::is_void_v<std::invoke_result_t<decltype(fun), Args>> || ...) ) {
+		if constexpr( is_any_v<std::is_void_v<std::invoke_result_t<decltype(fun), Args>>...> ) {
 			((void)std::invoke(fun, values),...);
+		else if constexpr(sizeof...(Args) == 0)
+			(void)0;
 		} else return { std::invoke(fun, values)... };
 	}
 
@@ -197,6 +276,13 @@ namespace exopt { namespace util [[gnu::visibility("internal")]] {
 	template<typename... Args>
 	using map_tuple = map<std::tuple<Args...>, Args...>;
 
+	//TODO: template< is_valid_metafunction_with_two_unknown_type_arguments_and_a_possibly_variable_return_type<R(T1, T2)> Op, typename... Args> \
+		auto reduce(auto const& fun, Op const& comb, Args&&... values) noexcept(...) \
+		requires(???) -> std::invoke_result_t<??????> { // TODO: Reframe the call to `comb` as an operator overload in a shim class, and use binary fold to make the call, like: ` \
+		shim s{comb}; return (s + std::invoke(fun, values) + ...); }`
+
+
+
 	template<typename Fn, typename... Args> requires((std::is_invocable_v<Fn, Args> && ...))
 	constexpr void apply(const Fn& fun, Args&&... values) noexcept((std::is_nothrow_invocable_v<Fn, Args> && ...))
 		/*-> std::conditional_t<	(std::is_void_v<std::invoke_result_t<Fn, Args>> || ...)