component: reintroduce utility ignore_as_empty_v (it's actually useful sometimes)

src/entt/entity/component.hpp

@@ -49,6 +49,13 @@ struct component_traits {
     static constexpr std::size_t page_size = internal::page_size<Type>::value;
 };
 
+/**
+ * @brief Helper variable template.
+ * @tparam Type Type of component.
+ */
+template<class Type>
+inline constexpr bool ignore_as_empty_v = (component_traits<Type>::page_size == 0u);
+
 } // namespace entt
 
 #endif

src/entt/entity/storage.hpp

@@ -743,7 +743,7 @@ private:
 
 /*! @copydoc basic_storage */
 template<typename Entity, typename Type, typename Allocator>
-class basic_storage<Entity, Type, Allocator, std::enable_if_t<component_traits<Type>::page_size == 0u>>
+class basic_storage<Entity, Type, Allocator, std::enable_if_t<ignore_as_empty_v<Type>>>
     : public basic_sparse_set<Entity, typename std::allocator_traits<Allocator>::template rebind_alloc<Entity>> {
     using alloc_traits = std::allocator_traits<Allocator>;
     static_assert(std::is_same_v<typename alloc_traits::value_type, Type>);

test/entt/entity/component.cpp

@@ -24,6 +24,7 @@ TEST(Component, DefaultParamsEmpty) {
 
     static_assert(!traits::in_place_delete);
     static_assert(traits::page_size == 0u);
+    static_assert(entt::ignore_as_empty_v<default_params_empty>);
 }
 
 TEST(Component, DefaultParamsNonEmpty) {
@@ -31,6 +32,7 @@ TEST(Component, DefaultParamsNonEmpty) {
 
     static_assert(!traits::in_place_delete);
     static_assert(traits::page_size == ENTT_PACKED_PAGE);
+    static_assert(!entt::ignore_as_empty_v<default_params_non_empty>);
 }
 
 TEST(Component, SelfContained) {
@@ -38,6 +40,7 @@ TEST(Component, SelfContained) {
 
     static_assert(traits::in_place_delete);
     static_assert(traits::page_size == 4u);
+    static_assert(!entt::ignore_as_empty_v<self_contained>);
 }
 
 TEST(Component, TraitsBased) {
@@ -45,4 +48,5 @@ TEST(Component, TraitsBased) {
 
     static_assert(!traits::in_place_delete);
     static_assert(traits::page_size == 8u);
+    static_assert(!entt::ignore_as_empty_v<traits_based>);
 }