snapshot: refine the check in the constructor of the loader class - close #1106

src/entt/entity/snapshot.hpp

@@ -164,7 +164,7 @@ public:
     basic_snapshot_loader(registry_type &source) noexcept
         : reg{&source} {
         // restoring a snapshot as a whole requires a clean registry
-        ENTT_ASSERT(reg->template storage<entity_type>().empty() && (reg->storage().begin() == reg->storage().end()), "Registry must be empty");
+        ENTT_ASSERT(reg->template storage<entity_type>().free_list() == 0u, "Registry must be empty");
     }
 
     /*! @brief Default move constructor. */

test/entt/entity/snapshot.cpp

@@ -222,6 +222,11 @@ TEST(BasicSnapshotLoader, Constructors) {
     static_assert(std::is_move_assignable_v<entt::basic_snapshot_loader<entt::registry>>, "Move assignable type required");
 
     entt::registry registry;
+
+    // helps stress the check in the constructor
+    registry.emplace<int>(registry.create(), 0);
+    registry.clear();
+
     entt::basic_snapshot_loader loader{registry};
     entt::basic_snapshot_loader other{std::move(loader)};