[libcxx-commits] [libcxx] [libc++] Fix incorrect down cast in __tree::operator= (PR #152285)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/152285

This has been introduced by #151304. This problem is diagnosed by UBSan
with optimizations enabled. Since we run UBSan only with optimizations
disabled currently, this isn't caught in our CI. We should look into
enabling UBSan with optimizations enabled to catch these sorts of issues
before landing a patch.


>From 18532a7e816015679561667c7ccd18f83478a17a Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Wed, 6 Aug 2025 12:09:38 +0200
Subject: [PATCH] [libc++] Fix incorrect down cast in __tree::operator=

This has been introduced by #151304. This problem is diagnosed by UBSan
with optimizations enabled. Since we run UBSan only with optimizations
disabled currently, this isn't caught in our CI. We should look into
enabling UBSan with optimizations enabled to catch these sorts of issues
before landing a patch.
---
 libcxx/include/__tree | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libcxx/include/__tree b/libcxx/include/__tree
index 6ca1a623536f2..74b20d5b6f814 100644
--- a/libcxx/include/__tree
+++ b/libcxx/include/__tree
@@ -1388,8 +1388,9 @@ __tree<_Tp, _Compare, _Allocator>& __tree<_Tp, _Compare, _Allocator>::operator=(
     if (__root())
       __root()->__parent_ = __end_node();
   }
-  __begin_node_ = static_cast<__end_node_pointer>(std::__tree_min(static_cast<__node_base_pointer>(__end_node())));
-  __size_       = __t.size();
+  __begin_node_ =
+      __end_node()->__left_ ? static_cast<__end_node_pointer>(std::__tree_min(__end_node()->__left_)) : __end_node();
+  __size_ = __t.size();
 
   return *this;
 }