diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py
index 98d2c7320003e4c..dd9f5de22c9057d 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py
@@ -27,7 +27,7 @@ def _run_test(self, defines):
 
 
 for v in [None, "ALTERNATE_LAYOUT"]:
-    for r in range(5):
+    for r in range(6):
         name = "test_r%d" % r
         defines = ["REVISION=%d" % r]
         if v:
diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/main.cpp b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/main.cpp
index b010dc25f8f804d..25dfa54bb5acbe4 100644
--- a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/main.cpp
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/main.cpp
@@ -20,7 +20,11 @@
 // Pre-D128285 layout.
 #define PACKED_ANON_STRUCT
 #endif
-// REVISION == 4: current layout
+#if REVISION <= 4
+// Pre-TODO layout.
+#define UB_PADDING
+#endif
+// REVISION == 5: current layout
 
 #ifdef PACKED_ANON_STRUCT
 #define BEGIN_PACKED_ANON_STRUCT struct __attribute__((packed)) {
@@ -34,6 +38,7 @@
 namespace std {
 namespace __lldb {
 
+#ifdef UB_PADDING
 #if defined(ALTERNATE_LAYOUT) && defined(SUBCLASS_PADDING)
 template <class _CharT, size_t = sizeof(_CharT)> struct __padding {
   unsigned char __xx[sizeof(_CharT) - 1];
@@ -41,6 +46,13 @@ template <class _CharT, size_t = sizeof(_CharT)> struct __padding {
 
 template <class _CharT> struct __padding<_CharT, 1> {};
 #endif
+#else // !UB_PADDING
+template <size_t _PaddingSize> struct __padding {
+   char __padding_[_PaddingSize];
+ };
+
+ template <> struct __padding<0> {};
+#endif
 
 template <class _CharT, class _Traits, class _Allocator> class basic_string {
 public:
@@ -77,7 +89,12 @@ template <class _CharT, class _Traits, class _Allocator> class basic_string {
     };
 #else // !SUBCLASS_PADDING
 
+#ifdef UB_PADDING
     unsigned char __padding[sizeof(value_type) - 1];
+#else
+    [[no_unique_address]] __padding<sizeof(value_type) - 1> __padding_;
+#endif
+
 #ifdef BITMASKS
     unsigned char __size_;
 #else // !BITMASKS