From cc5ca5512bd38051cb8838f3d526b49727aa2a83 Mon Sep 17 00:00:00 2001
From: Alex Guteniev <gutenev@gmail.com>
Date: Tue, 16 Sep 2025 22:13:45 +0300
Subject: [PATCH] Fix UB in vector<bool> by adding missing special case

---
 stl/inc/vector | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stl/inc/vector b/stl/inc/vector
index 3e469e3d15e..093b0c96acf 100644
--- a/stl/inc/vector
+++ b/stl/inc/vector
@@ -3859,7 +3859,7 @@ _CONSTEXPR20 _OutIt _Copy_vbool(_VbIt _First, _VbIt _Last, _OutIt _Dest) {
 
         const auto _DestMask = _FirstDestMask | (_DestEnd._Myoff == 0 ? 0 : _LastDestMask);
         if (_Last._Myoff != 0) {
-            const auto _LastShift     = _DestEnd._Myoff - _Last._Myoff;
+            const auto _LastShift     = _DestEnd._Myoff != 0 ? _DestEnd._Myoff - _Last._Myoff : _VBITS - _Last._Myoff;
             const auto _LastSourceVal = (*_VbLast & _LastSourceMask) << _LastShift;
             *_VbDest                  = (*_VbDest & _DestMask) | _SourceVal | _LastSourceVal;
         } else {