[libc++] Fix incorrect down cast in __tree::operator=

[philnik777]

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.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/152285.diff

1 Files Affected:

• (modified) libcxx/include/__tree (+3-2)

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;
 }