Merge "Avoid undefined compiler behavior"
diff --git a/libc/malloc_debug/tests/malloc_debug_system_tests.cpp b/libc/malloc_debug/tests/malloc_debug_system_tests.cpp
index 68b3a1e..9d33189 100644
--- a/libc/malloc_debug/tests/malloc_debug_system_tests.cpp
+++ b/libc/malloc_debug/tests/malloc_debug_system_tests.cpp
@@ -511,7 +511,7 @@
 
   for (size_t i = 0; i < kMaxThreads; i++) {
     std::thread malloc_thread([&thread_mask, &run, &allocs, i] {
-      thread_mask.fetch_or(1 << i);
+      thread_mask.fetch_or(1U << i);
       while (!run)
         ;
       for (auto ptr : allocs[i]) {