Initial commit - restart from existing code

lib/All/entt/test/example/signal_less.cpp

@@ -0,0 +1,43 @@
+#include <array>
+#include <type_traits>
+#include <gtest/gtest.h>
+#include <entt/entity/registry.hpp>
+#include <entt/entity/storage.hpp>
+
+template<typename Type, typename Entity>
+struct entt::storage_type<Type, Entity> {
+    // no signal regardless of element type ...
+    using type = basic_storage<Type, Entity>;
+};
+
+template<typename Entity>
+struct entt::storage_type<char, Entity> {
+    // ... unless it's char, because yes.
+    using type = sigh_mixin<basic_storage<char, Entity>>;
+};
+
+template<typename, typename, typename = void>
+struct has_on_construct: std::false_type {};
+
+template<typename Entity, typename Type>
+struct has_on_construct<Entity, Type, std::void_t<decltype(&entt::storage_type_t<Type>::on_construct)>>: std::true_type {};
+
+template<typename Entity, typename Type>
+inline constexpr auto has_on_construct_v = has_on_construct<Entity, Type>::value;
+
+TEST(Example, SignalLess) {
+    // invoking registry::on_construct<int> is a compile-time error
+    ASSERT_FALSE((has_on_construct_v<entt::entity, int>));
+    ASSERT_TRUE((has_on_construct_v<entt::entity, char>));
+
+    entt::registry registry;
+    const std::array entity{registry.create()};
+
+    // literally a test for storage_adapter_mixin
+    registry.emplace<int>(entity[0], 0);
+    registry.erase<int>(entity[0]);
+    registry.insert<int>(entity.begin(), entity.end(), 3);
+    registry.patch<int>(entity[0], [](auto &value) { value = 2; });
+
+    ASSERT_EQ(registry.get<int>(entity[0]), 2);
+}