From 5c3772e62e426237b4471a26591e4df295ad170a Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Wed, 4 Feb 2026 16:23:55 -0500
Subject: [PATCH] xo-facet: + to_facet() convenience method on obj<A,D>

---
 xo-facet/cmake/xo_facetConfig.cmake.in         |  1 +
 xo-facet/include/xo/facet/FacetRegistry.hpp    | 18 +++++++++++++++++-
 xo-facet/include/xo/facet/OObject.hpp          |  4 ++++
 .../include/xo/facet/facet_implementation.hpp  |  8 ++++++++
 xo-facet/include/xo/facet/obj.hpp              | 10 ++++++++++
 5 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/xo-facet/cmake/xo_facetConfig.cmake.in b/xo-facet/cmake/xo_facetConfig.cmake.in
index 88709824..23483949 100644
--- a/xo-facet/cmake/xo_facetConfig.cmake.in
+++ b/xo-facet/cmake/xo_facetConfig.cmake.in
@@ -2,6 +2,7 @@
 
 include(CMakeFindDependencyMacro)
 # note: dependencies here must coordinate with xo-facet/CMakeLists.txt
+find_dependency(xo_arena)
 find_dependency(xo_reflectutil)
 include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
 check_required_components("@PROJECT_NAME@")
diff --git a/xo-facet/include/xo/facet/FacetRegistry.hpp b/xo-facet/include/xo/facet/FacetRegistry.hpp
index ad19cf53..d4b15188 100644
--- a/xo-facet/include/xo/facet/FacetRegistry.hpp
+++ b/xo-facet/include/xo/facet/FacetRegistry.hpp
@@ -236,9 +236,25 @@ namespace xo {
 
             /** runtime lookup table (AFacet,DRepr) -> impl **/
             xo::map::DArenaHashMap<key_type, const void *, KeyHash> registry_;
-            //std::unordered_map<key_type, const void *, KeyHash> registry_;
         };
 
+        // Deferred definitioon of obj<AFacet,DRepr>::to_facet(),
+        // since implementation requires FacetRegistry
+        //
+        template <typename AFacet, typename DRepr>
+        template <typename AOther>
+        obj<AOther,DRepr>
+        obj<AFacet,DRepr>::to_facet()
+        {
+            if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
+                // return type has type-erased data
+                return FacetRegistry::instance().variant<AOther,AFacet>(*this);
+            } else {
+                // return type has known data
+                return obj<AOther,DRepr>(this->data());
+            }
+        }
+
     } /*namespace facet*/
 } /*namespace xo*/
 
diff --git a/xo-facet/include/xo/facet/OObject.hpp b/xo-facet/include/xo/facet/OObject.hpp
index e6eb0f6d..234a84df 100644
--- a/xo-facet/include/xo/facet/OObject.hpp
+++ b/xo-facet/include/xo/facet/OObject.hpp
@@ -59,6 +59,10 @@ namespace xo {
          **/
         template <typename AFacet, typename DRepr = DVariantPlaceholder>
         struct OObject {
+            static_assert(has_facet_impl<AFacet, DRepr>,
+                          "Missing FacetImplementation<AFacet,DRepr> specialization. "
+                          "Did you include IFacet_DRepr.hpp (via the convenience header)?");
+
             using FacetType = AFacet;
             using ISpecific = FacetImplType<AFacet, DRepr>;
             using DataType = DRepr;
diff --git a/xo-facet/include/xo/facet/facet_implementation.hpp b/xo-facet/include/xo/facet/facet_implementation.hpp
index 025645e8..7ce2ed5a 100644
--- a/xo-facet/include/xo/facet/facet_implementation.hpp
+++ b/xo-facet/include/xo/facet/facet_implementation.hpp
@@ -92,6 +92,14 @@ namespace xo {
             //static_assert(false && "expect specialization <AFacet,DRepr> which should provide ImplType trait");
         };
 
+        /** true iff FacetImplementation<AFacet,DRepr> has been specialized with ImplType.
+         *  False when specialization header (IFacet_DRepr.hpp) not included.
+         **/
+        template <typename AFacet, typename DRepr>
+        concept has_facet_impl = requires {
+            typename FacetImplementation<AFacet, DRepr>::ImplType;
+        };
+
         /** Retrieve facet implementation for a (facet,datatype) pair **/
         template <typename AFacet, typename DRepr>
         using FacetImplType = FacetImplementation<AFacet, DRepr>::ImplType;
diff --git a/xo-facet/include/xo/facet/obj.hpp b/xo-facet/include/xo/facet/obj.hpp
index 850ef7bf..3619ae3e 100644
--- a/xo-facet/include/xo/facet/obj.hpp
+++ b/xo-facet/include/xo/facet/obj.hpp
@@ -6,6 +6,7 @@
 #pragma once
 
 #include "RRouter.hpp"
+//#include "FacetRegistry.hpp"  // nope, would create include cycle
 #include <utility>
 #include <cassert>
 
@@ -127,6 +128,15 @@ namespace xo {
                     return obj(iface, data);
                 }
 
+            /** - runtime polymorphism (DRepr == DVariantPlaceholder)
+             *      (requires FacetRegistry for lookup)
+             *  - comptime polymorphism (DRepr != DVariantPlaceholder)
+             *
+             *  Definition in FacetRegistry.hpp, to avoid #include dependency
+             **/
+            template <typename AOther>
+            obj<AOther,DRepr> to_facet();
+
             /** enabled when RRouter<AFacet> provides _preincrement.
              *  Note we don't need this trick for comparison operators,
              *  since return type is fixed.