Change value of TBranchObject::kWarn. Reset it upon reading.

The value conflicted with TBranch::kBranchObject.

tree/tree/inc/TBranchObject.h

@@ -26,7 +26,18 @@
 class TBranchObject : public TBranch {
 
 protected:
-   enum EStatusBits { kWarn = BIT(12) };
+   enum EStatusBits {
+      kWarn = BIT(14)
+   };
+
+   // In version of ROOT older then v6.12, kWarn was set to BIT(12)
+   // which overlaps with TBranch::kBranchObject.  Since it stored
+   // in ROOT files as part of the TBranchObject and that we want
+   // to reset in TBranchObject::Streamer, we need to keep track
+   // of the old value.
+   enum EStatusBitsOldValues {
+      kOldWarn = BIT(12)
+   };
 
    TString     fClassName;        ///< Class name of referenced object
    TObject     *fOldObject;       ///< !Pointer to old object

tree/tree/src/TBranchObject.cxx

@@ -548,6 +548,9 @@ void TBranchObject::Streamer(TBuffer& R__b)
 {
    if (R__b.IsReading()) {
       R__b.ReadClassBuffer(TBranchObject::Class(), this);
+      // We should rewarn in this process.
+      ResetBit(kWarn);
+      ResetBit(kOldWarn);
    } else {
       TDirectory* dirsav = fDirectory;
       fDirectory = 0;  // to avoid recursive calls