With fully incomplete types, `unsafe_cast` is used instead of `try_cast` in `operator [const] T*()`.

Fortune for opaque_handle's current commit: Curse − 凶

include/opaque.hh

@@ -15,6 +15,9 @@ struct opaque_bad_cast : $_oe("bad opaque_handle pointer cast");
 struct opaque_handle;
 struct opaque_handle_impl;
 
+template<typename T>
+constexpr inline bool opaque_is_incomplete = !requires { { sizeof(T) }; };
+
 struct opaque_handle_impl {
 	friend class opaque_handle;
 
@@ -36,10 +39,14 @@ struct opaque_handle_impl {
 
 
 	template<typename To>
-	inline constexpr const To* try_cast() const noexcept { return is_type(typeid(To)) ? static_cast<const To*>(as_raw_ptr()) : nullptr; }
+	inline constexpr const To* try_cast() const noexcept { return is_type<To>() ? static_cast<const To*>(as_raw_ptr()) : nullptr; }
 	template<typename To>
-	inline constexpr To* try_cast() noexcept { return is_type(typeid(To)) ? static_cast<To*>(as_raw_ptr()) : nullptr; }
+	inline constexpr To* try_cast() noexcept { return is_type<To>() ? static_cast<To*>(as_raw_ptr()) : nullptr; }
 
+	template<typename T>
+	constexpr bool is_type() const noexcept {
+		if constexpr(requires { { sizeof(T) };  }) return is_type(typeid(T)); else return true;
+	}
 	constexpr virtual bool is_type(const std::type_info& a) const noexcept {
 		if(const auto* p = get_type_info()) return *p == a;
 		return false; // Types are uncomparable.
@@ -87,6 +94,7 @@ struct opaque_handle final {
 	template<typename T, bool Reinterpret=false>
 	constexpr T* unsafe_cast() noexcept { return _impl ? _impl->unsafe_cast<T, Reinterpret>() : nullptr; }
 
+	/// WARNING: try_cast<T>(), where T is an incomplete type, will always succeed, which may lead to undefined behaviour. Make sure to account for this
 	template<typename T>
 	constexpr const T* try_cast() const noexcept { return _impl ? _impl->try_cast<T>() : nullptr; }
 	template<typename T>
@@ -138,9 +146,15 @@ struct opaque_handle final {
 	inline auto operator*() noexcept { return *(try_cast<T>() ?: throw opaque_bad_cast{}); }
 */
 	template<typename T>
-	constexpr operator T*() noexcept { return try_cast<T>(); }
+	constexpr operator T*() noexcept { 
+		if constexpr(opaque_is_incomplete<T>) return unsafe_cast<T>();
+		else return try_cast<T>(); 
+	}
 	template<typename T>
-	constexpr operator const T*() const noexcept { return try_cast<T>(); }
+	constexpr operator const T*() const noexcept { 
+		if constexpr(opaque_is_incomplete<T>) return unsafe_cast<T>();
+		else return try_cast<T>(); 
+	}
 
 private:
 	struct _impl_deref_const {
@@ -248,7 +262,11 @@ constexpr inline opaque_handle make_opaque_handle(T* data, const HandleF& handle
 		{ return const_cast<T*>(data); }
 		constexpr opaque_handle_impl* try_clone() const override final 
 		{ return data ? new object_handler(*this) : nullptr; }
-		inline constexpr const std::type_info* get_type_info() const noexcept override final { return &typeid(T); }
+		inline constexpr const std::type_info* get_type_info() const noexcept override final {
+			if constexpr(requires { { sizeof(T) }; })
+				return &typeid(T);
+			else return nullptr;
+		}
 		
 		T* data;
 		const HandleF& handler;
@@ -294,7 +312,11 @@ constexpr inline opaque_handle make_opaque_handle(T* data, HandleF&& handler) no
 				return data ? new object_handler(*this) : nullptr; 
 			} else return nullptr;
 		}
-		inline constexpr const std::type_info* get_type_info() const noexcept override final { return &typeid(T); }
+		inline constexpr const std::type_info* get_type_info() const noexcept override final {
+			if constexpr(requires { { sizeof(T) }; })
+				return &typeid(T);
+			else return nullptr;
+		}
 
 		T* data;
 		HandleF handler;
@@ -334,7 +356,11 @@ constexpr inline opaque_handle make_opaque_handle(T* data) noexcept
 		{ return const_cast<T*>(data); }
 		constexpr opaque_handle_impl* try_clone() const override final 
 		{ return data ? new object_handler(*this) : nullptr; }
-		inline constexpr const std::type_info* get_type_info() const noexcept override final { return &typeid(T); }
+		inline constexpr const std::type_info* get_type_info() const noexcept override final {
+			if constexpr(requires { { sizeof(T) }; })
+				return &typeid(T);
+			else return nullptr;
+		}
 
 		T* data;
 	};

src/main.cpp

@@ -101,6 +101,9 @@ static void use_moh()
 // Testing a hypothetical C interface
 extern "C" {
 	struct c_struct_example;
+	struct opaque;
+
+	void black_box(const opaque* __restrict__){}
 	
 	c_struct_example* cs_create(const c_struct_example* __restrict__ from);
 	void cs_free(c_struct_example* p);
@@ -113,6 +116,10 @@ static void use_c_struct()
 	using type = c_struct_example;
 	auto* allocp = cs_create(nullptr);
 
+	opaque_handle impossible{make_opaque_handle<opaque>(nullptr, [](auto*, auto) noexcept { return static_cast<opaque*>(nullptr); })};
+
+	black_box(impossible);
+
 	const opaque_handle v{make_opaque_handle<type>(allocp, [](auto* ptr, auto op) noexcept {
 		if(ptr) {
 			switch(op) {