From 40aabe76c7211e503290a44a6ea6407df5c81fec Mon Sep 17 00:00:00 2001
From: Michele Caini <michele.caini@gmail.com>
Date: Sun, 26 Dec 2021 16:36:30 +0100
Subject: [PATCH] sparse_set: fixed a throwing case that could leave the set in
 an inconsistent state

---
 src/entt/entity/sparse_set.hpp  | 2 +-
 test/entt/entity/sparse_set.cpp | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/entt/entity/sparse_set.hpp b/src/entt/entity/sparse_set.hpp
index 6face2a49..17155e805 100644
--- a/src/entt/entity/sparse_set.hpp
+++ b/src/entt/entity/sparse_set.hpp
@@ -274,8 +274,8 @@ protected:
         ENTT_ASSERT(entity_traits::to_version(elem) == entity_traits::to_version(tombstone), "Slot not available");
 
         if(free_list == null) {
-            elem = entity_traits::combine(static_cast<typename entity_traits::entity_type>(packed.size()), entity_traits::to_integral(entt));
             packed.push_back(entt);
+            elem = entity_traits::combine(static_cast<typename entity_traits::entity_type>(packed.size() - 1u), entity_traits::to_integral(entt));
         } else {
             elem = entity_traits::combine(entity_traits::to_integral(free_list), entity_traits::to_integral(entt));
             free_list = std::exchange(packed[static_cast<size_type>(entity_traits::to_entity(free_list))], entt);
diff --git a/test/entt/entity/sparse_set.cpp b/test/entt/entity/sparse_set.cpp
index c36aee458..f1732c4d4 100644
--- a/test/entt/entity/sparse_set.cpp
+++ b/test/entt/entity/sparse_set.cpp
@@ -1246,6 +1246,14 @@ TEST(SparseSet, ThrowingAllocator) {
     ASSERT_TRUE(set.contains(entt::entity{0}));
     ASSERT_EQ(set.capacity(), 1u);
 
+    test::throwing_allocator<entt::entity>::trigger_on_allocate = true;
+
+    ASSERT_THROW(set.emplace(entt::entity{1}), test::throwing_allocator<entt::entity>::exception_type);
+    ASSERT_EQ(set.extent(), ENTT_SPARSE_PAGE);
+    ASSERT_TRUE(set.contains(entt::entity{0}));
+    ASSERT_FALSE(set.contains(entt::entity{1}));
+    ASSERT_EQ(set.capacity(), 1u);
+
     entt::entity entities[2u]{entt::entity{1}, entt::entity{ENTT_SPARSE_PAGE}};
     test::throwing_allocator<entt::entity>::trigger_after_allocate = true;