xo-alloc2 scaffold for ACollector + AGCObject facets. [WIP]

xo-alloc2/include/xo/alloc2/IGCObject_Any.hpp

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+/** @file IGCObject_Any.hpp
+ *
+ *  @author Roland Conybeare, Dec 2025
+ **/
+
+#pragma once
+
+#include "AGCObject.hpp"
+#include <new>
+
+namespace xo {
+    namespace mm { struct IGCObject_Any; }
+
+    namespace facet {
+        template <>
+        struct FacetImplementation<xo::mm::AGCObject, DVariantPlaceholder> {
+            using ImplType = xo::mm::IGCObject_Any;
+        };
+    }
+
+    namespace mm {
+        /** @class IGCObject_Any
+         *  @brief AGCObject implementation for empty variant instance
+         **/
+        struct IGCObject_Any : public AGCObject {
+            using size_type = std::size_t;
+
+            const AGCObject * iface() const { return std::launder(this); }
+
+            // from AGCObject
+            int32_t _typeseq() const noexcept override { return s_typeseq; }
+
+            [[noreturn]] size_type shallow_size() const noexcept override { _fatal(); }
+            [[noreturn]] Opaque * shallow_copy() const noexcept override { _fatal(); }
+            [[noreturn]] size_type forward_children() const noexcept override { _fatal(); }
+
+        private:
+            [[noreturn]] static void _fatal();
+
+        public:
+            static int32_t s_typeseq;
+            static bool _valid;
+        };
+    } /*namespace mm*/
+} /*namespace xo*/
+
+/* end IGCObject_Any.hpp */