add is_virtual_base_of trait

develop/singles/headers/meta.hpp/meta_all.hpp

@@ -1180,6 +1180,38 @@ namespace meta_hpp::detail
     inline constexpr bool is_in_place_type_v = is_in_place_type<T>::value;
 }
 
+namespace meta_hpp::detail
+{
+    namespace impl
+    {
+        template < typename From, typename To >
+        constexpr bool is_virtual_base_of_impl(...) noexcept {
+            return false;
+        }
+
+        template <
+            typename From,
+            typename To,
+            decltype(static_cast<const volatile To*>(std::declval<const volatile From*>())) = nullptr >
+        constexpr bool is_virtual_base_of_impl(int) noexcept {
+            return true;
+        }
+    }
+
+    // clang-format off
+
+    template < typename Base, typename Derived >
+    struct is_virtual_base_of : std::integral_constant<bool,
+          std::is_base_of_v<Base, Derived> &&
+          impl::is_virtual_base_of_impl<Derived, Base>(0) &&
+          !impl::is_virtual_base_of_impl<Base, Derived>(0)> {};
+
+    // clang-format on
+
+    template < typename Base, typename Derived >
+    inline constexpr bool is_virtual_base_of_v = is_virtual_base_of<Base, Derived>::value;
+}
+
 namespace meta_hpp::detail
 {
     class memory_buffer final {

develop/untests/meta_base/is_virtual_base_of_tests.cpp

@@ -0,0 +1,77 @@
+/*******************************************************************************
+ * This file is part of the "https://github.com/blackmatov/meta.hpp"
+ * For conditions of distribution and use, see copyright notice in LICENSE.md
+ * Copyright (C) 2021-2023, by Matvey Cherevko (blackmatov@gmail.com)
+ ******************************************************************************/
+
+#include <meta.hpp/meta_all.hpp>
+#include <doctest/doctest.h>
+
+namespace
+{
+    struct A {};
+    struct B : virtual A {};
+    struct C : virtual A {};
+    struct D : B, C {};
+    struct E {};
+
+    struct I {};
+    struct J : virtual I {};
+    struct K : J {};
+}
+
+TEST_CASE("meta/meta_base/is_virtual_base_of") {
+    namespace meta = meta_hpp;
+    using meta::detail::is_virtual_base_of;
+    using meta::detail::is_virtual_base_of_v;
+
+    {
+        static_assert(!is_virtual_base_of_v<A, A>);
+        static_assert(is_virtual_base_of_v<A, B>);
+        static_assert(is_virtual_base_of_v<A, C>);
+
+        static_assert(is_virtual_base_of_v<A, D>);
+        static_assert(!is_virtual_base_of_v<B, D>);
+        static_assert(!is_virtual_base_of_v<C, D>);
+
+        static_assert(!is_virtual_base_of_v<A, E>);
+        static_assert(!is_virtual_base_of_v<E, A>);
+
+        static_assert(!is_virtual_base_of_v<E, D>);
+        static_assert(!is_virtual_base_of_v<D, E>);
+    }
+
+    {
+        static_assert(!is_virtual_base_of_v<I, I>);
+        static_assert(is_virtual_base_of_v<I, J>);
+        static_assert(is_virtual_base_of_v<I, K>);
+
+        static_assert(!is_virtual_base_of_v<J, I>);
+        static_assert(!is_virtual_base_of_v<J, J>);
+        static_assert(!is_virtual_base_of_v<J, K>);
+
+        static_assert(!is_virtual_base_of_v<K, I>);
+        static_assert(!is_virtual_base_of_v<K, J>);
+        static_assert(!is_virtual_base_of_v<K, K>);
+    }
+
+    {
+        static_assert(is_virtual_base_of_v<I, J>);
+
+        static_assert(is_virtual_base_of_v<const I, J>);
+        static_assert(is_virtual_base_of_v<volatile I, J>);
+        static_assert(is_virtual_base_of_v<const volatile I, J>);
+
+        static_assert(is_virtual_base_of_v<const I, const J>);
+        static_assert(is_virtual_base_of_v<volatile I, const J>);
+        static_assert(is_virtual_base_of_v<const volatile I, const J>);
+
+        static_assert(is_virtual_base_of_v<const I, volatile J>);
+        static_assert(is_virtual_base_of_v<volatile I, volatile J>);
+        static_assert(is_virtual_base_of_v<const volatile I, volatile J>);
+
+        static_assert(is_virtual_base_of_v<const I, const volatile J>);
+        static_assert(is_virtual_base_of_v<volatile I, const volatile J>);
+        static_assert(is_virtual_base_of_v<const volatile I, const volatile J>);
+    }
+}