diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 95a04937ec9eac1419ebedb4916acc74b43b0e52..cda7d2cc2a2591f71fcdc988edeebe1aa860cccd 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -3090,6 +3090,7 @@ void ASTReader::ReadPragmaDiagnosticMappings(DiagnosticsEngine &Diag) {
           break; // no more diag/map pairs for this location.
         }
         diag::Mapping Map = (diag::Mapping)F.PragmaDiagMappings[Idx++];
+        // The user bit gets set by WritePragmaDiagnosticMappings.
         Diag.setDiagnosticMapping(DiagID, Map, Loc);
       }
     }