diff --git a/clang/lib/Serialization/ASTReaderInternals.h b/clang/lib/Serialization/ASTReaderInternals.h
--- a/clang/lib/Serialization/ASTReaderInternals.h
+++ b/clang/lib/Serialization/ASTReaderInternals.h
@@ -62,7 +62,7 @@
       // Just use a linear scan unless we have more than a few IDs.
       if (Found.empty() && !Data.empty()) {
         if (Data.size() <= 4) {
-          for (auto I : Found)
+          for (auto I : Data)
             if (I == ID)
               return;
           Data.push_back(ID);