Make sure the op does not change the sign of the operand

Summary: .

Test Plan: .

Reviewed By: soren@fb.com

FB internal diff: D660143

Blame Revision:

folly/small_vector.h

@@ -414,7 +414,8 @@ namespace detail {
   }
   template <class T>
   T* pointerFlagClear(T* p) {
-    return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(p) & ~1);
+    return reinterpret_cast<T*>(
+      reinterpret_cast<uintptr_t>(p) & ~uintptr_t(1));
   }
   inline void* shiftPointer(void* p, size_t sizeBytes) {
     return static_cast<char*>(p) + sizeBytes;