diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -3715,7 +3715,7 @@
       std::string_view Proto;
       {
         ScopedOverride<const char *> SaveFirst(First, &*ProtoSourceName.begin()),
-            SaveLast(Last, &*ProtoSourceName.end());
+            SaveLast(Last, &*ProtoSourceName.rbegin() + 1);
         Proto = parseBareSourceName();
       }
       if (Proto.empty())