rework initialize

Author: jll63

include/boost/openmethod/initialize.hpp

@@ -34,23 +34,6 @@
 
 namespace boost::openmethod {
 
-//! N2216 ambiguity resolution.
-//!
-//! If `n2216` is present in @ref initialize\'s `Options`, additional steps are
-//! taken to select a single overrider in presence of ambiguous overriders sets,
-//! according to the rules defined in the N2216 paper. If the normal resolution
-//! procedure fails to select a single overrider, the following steps are
-//! applied, in order:
-//!
-//! - If the return types of the remaining overriders are all polymorphic and
-//!   covariant, and one of the return types is more specialized thjat all the
-//!   others, use it.
-//!
-//! - Otherwise, pick one of the overriders. Which one is used is unspecified,
-//!   but remains the same throughtout the program, and across different runs of
-//!   the same program.
-struct n2216;
-
 namespace detail {
 
 template<class Reports, class Facets, typename = void>
@@ -260,8 +243,27 @@ auto operator<<(trace_type<Registry>& trace, const spec_name& sn)
     return trace;
 }
 
+struct option_base {};
+
 } // namespace detail
 
+//! N2216 ambiguity resolution.
+//!
+//! If `n2216` is present in @ref initialize\'s `Options`, additional steps are
+//! taken to select a single overrider in presence of ambiguous overriders sets,
+//! according to the rules defined in the N2216 paper. If the normal resolution
+//! procedure fails to select a single overrider, the following steps are
+//! applied, in order:
+//!
+//! - If the return types of the remaining overriders are all polymorphic and
+//!   covariant, and one of the return types is more specialized thjat all the
+//!   others, use it.
+//!
+//! - Otherwise, pick one of the overriders. Which one is used is unspecified,
+//!   but remains the same throughtout the program, and across different runs of
+//!   the same program.
+struct n2216 : detail::option_base {};
+
 // Definition of the nested template struct outside the registry class
 template<class... Policies>
 template<class... Options>
@@ -1375,26 +1377,62 @@ void registry<Policies...>::compiler<Options...>::print(
           << " ambiguous\n";
 }
 
-template<class Registry = BOOST_OPENMETHOD_DEFAULT_REGISTRY, class... Options>
-auto initialize() {
-    if (odr_check<Registry>::count > 1) {
-        // Multiple definitions of default_registry detected.
-        // This indicates an ODR violation.
-        // Signal a final_error using the error handler, then abort.
-        if constexpr (Registry::has_error_handler) {
-            Registry::error_handler::error(odr_violation());
+namespace detail {
+template<typename T, class... Options>
+struct initialize_aux
+    : initialize_aux<T, BOOST_OPENMETHOD_DEFAULT_REGISTRY, Options...> {};
+
+template<class Registry, class... Options>
+struct initialize_aux<
+    std::void_t<typename Registry::registry_type>, Registry, Options...> {
+    static_assert(
+        (std::is_base_of_v<detail::option_base, Options> && ...),
+        "invalid option type");
+    static auto fn() {
+        if (odr_check<Registry>::count > 1) {
+            // Multiple definitions of default_registry detected.
+            // This indicates an ODR violation.
+            // Signal a final_error using the error handler, then abort.
+            if constexpr (Registry::has_error_handler) {
+                Registry::error_handler::error(odr_violation());
+            }
+
+            std::abort();
         }
 
-        std::abort();
-    }
+        typename Registry::template compiler<Options...> comp;
+        comp.initialize();
 
-    typename Registry::template compiler<Options...> comp;
-    comp.initialize();
+        return comp;
+    }
+};
+} // namespace detail
 
-    return comp;
+//! Initialize a registry.
+//!
+//! `initialize` must be called, typically at the beginning of `main`, before
+//! using any of the methods in a registry. It sets up the v-tables,
+//! multi-method dispatch tables, and any other data required by the policies.
+//!
+//! If the first template argument is a registry, initialize it. Otherwise,
+//! initialize the default registry. Additional template arguments, if any, must
+//! be option types (@ref n2216 or @ref enable_trace).
+//!
+//! @note
+//! A translation unit that contains a call to `initialize` must include the
+//! `<boost/openmethod/initialize.hpp>` header.
+//!
+//! @par Errors
+//!
+//! @li @ref missing_class: A class used in a virtual parameter was
+//! not registered.
+//!
+//! @li Errors reported by the policies.
+template<class... T>
+inline auto initialize() {
+    return detail::initialize_aux<void, T...>::fn();
 }
 
-
 namespace detail {
 
 template<class Policy, typename = void>

include/boost/openmethod/registry.hpp

@@ -92,7 +92,7 @@ struct odr_check {
     inline static std::size_t inc = count++;
 
     odr_check() {
-        [[maybe_unused]] auto _ = &inc<typename Registry::self_type>;
+        [[maybe_unused]] auto _ = &inc<typename Registry::registry_type>;
     }
 };
 
@@ -788,10 +788,10 @@ struct without_aux<Policies, Policy, MorePolicies...> {
 template<class...>
 struct use_class_aux;
 
-} // namespace detail
+template<typename, class...>
+struct initialize_aux;
 
-template<class Registry, class...>
-auto initialize();
+} // namespace detail
 
 //! A collection of methods, classes and policies.
 //!
@@ -856,31 +856,14 @@ class registry : detail::registry_base {
     inline static bool initialized;
 
   protected:
-  template<class... Options>
+    template<class... Options>
     struct compiler;
 
   public:
-    using self_type = registry;
+    using registry_type = registry;
 
-    //! Initializes the registry.
-    //!
-    //! `initialize` must be called, typically at the beginning of `main`,
-    //! before using any of the methods in the registry. It sets up the
-    //! v-tables, multi-method dispatch tables, and any other data required by
-    //! the policies.
-    //!
-    //! @note
-    //! A translation unit that contains a call to `initialize` must include the
-    //! `<boost/openmethod/initialize.hpp>` header.
-    //!
-    //! @par Errors
-    //!
-    //! @li @ref missing_class: A class used in a virtual parameter was
-    //! not registered.
-    //!
-    //! In addition, policies may encounter and report errors.
-    template<class Registry, class...>
-    friend auto initialize();
+    template<typename, class...>
+    friend struct detail::initialize_aux;
 
     //! Checks if the registry is initialized.
     //!