From f8c3d6301619ddef8c5cc62e7714dd67dfa957a3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ion=20Gazta=C3=B1aga?= <igaztanaga@gmail.com>
Date: Mon, 30 Mar 2026 11:18:26 +0200
Subject: [PATCH] Fix "compose" in segmented_iterator_traits, the returned
 deque iterator must be always valid.

---
 include/boost/container/detail/deque_impl.hpp | 26 +++++++++++++++----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/boost/container/detail/deque_impl.hpp b/include/boost/container/detail/deque_impl.hpp
index 35e5bfa..70ef06c 100644
--- a/include/boost/container/detail/deque_impl.hpp
+++ b/include/boost/container/detail/deque_impl.hpp
@@ -388,16 +388,32 @@ struct segmented_iterator_traits< deque_iterator<Pointer, IsConst, BlockBytes, B
    typedef typename deque_iterator_type::val_alloc_ptr local_iterator;
    typedef typename deque_iterator_type::index_pointer segment_iterator;
 
-   BOOST_CONTAINER_FORCEINLINE static segment_iterator segment(deque_iterator_type it) { return it.get_node(); }
+   BOOST_CONTAINER_FORCEINLINE static segment_iterator segment(deque_iterator_type it)
+   { return it.get_node(); }
 
-   BOOST_CONTAINER_FORCEINLINE static local_iterator   local(deque_iterator_type it)   { return it.get_cur(); }
+   BOOST_CONTAINER_FORCEINLINE static local_iterator   local(deque_iterator_type it)
+   { return it.get_cur(); }
 
    BOOST_CONTAINER_FORCEINLINE static deque_iterator_type compose(segment_iterator s, local_iterator l)
-   { return deque_iterator_type(l, s); }
+   {
+      if (BOOST_UNLIKELY(s && l == (end)(s))) {
+         ++s;
+         l = *s;
+      }
+      return deque_iterator_type(l, s);
+   }
 
-   BOOST_CONTAINER_FORCEINLINE static local_iterator begin(segment_iterator s) { return *s; }
+   BOOST_CONTAINER_FORCEINLINE static local_iterator begin(segment_iterator s)
+   {
+      BOOST_ASSERT(s != segment_iterator());
+      return *s;
+   }
 
-   BOOST_CONTAINER_FORCEINLINE static local_iterator end(segment_iterator s)   { return *s + deque_iterator_type::get_block_size(); }
+   BOOST_CONTAINER_FORCEINLINE static local_iterator end(segment_iterator s)
+   {
+      BOOST_ASSERT(s != segment_iterator());
+      return *s + deque_iterator_type::get_block_size();
+   }
 };
 
 ////////////////////////////////////////////////////////////////////////////