[io] read versioned class from file even if in-memory-class is unversioned

io/io/src/TBufferFile.cxx

@@ -3618,9 +3618,10 @@ Int_t TBufferFile::ReadClassBuffer(const TClass *cl, void *pointer, const TClass
             // (tracking) was not enabled.  So let's create the StreamerInfo if it is the
             // one for the current version, otherwise let's complain ...
             // We could also get here when reading a file prior to the introduction of StreamerInfo.
-            // We could also get here if there old class version was '1' and the new class version is higher than 1
+            // We could also get here if the old class version was '1' and the new class version is higher than 1
+            // We could also get here if the stored-in-file class version was '3' and the current-in-memory class version is not defined
             // AND the checksum is the same.
-            if (v2file || version == cl->GetClassVersion() || version == 1 ) {
+            if (v2file || version == cl->GetClassVersion() || version == 1 || (version > 0 && cl->GetClassVersion() == -1) ) {
                R__LOCKGUARD(gInterpreterMutex);
 
                // We need to check if another thread did not get here first

roottest/root/io/evolution/versions/execROOT5306.cxx

@@ -0,0 +1,34 @@
+#include "TInterpreter.h"
+#include "TFile.h"
+
+#ifndef SUBCLASS_HH
+#define SUBCLASS_HH
+class MySubClass
+{
+public:
+   int id;
+};
+#endif
+#ifdef __ROOTCLING__
+#pragma link C++ class MySubClass+;
+#endif
+
+// https://its.cern.ch/jira/browse/ROOT-5306
+int execROOT5306()
+{
+   // Original file was generated as:
+   // .L MySubClass.cxx+
+   // TFile f("/tmp/mysub.root", "RECREATE");
+   // MySubClass msc;
+   // msc.id = 33;
+   // f.WriteObjectAny(&msc, "MySubClass", "msc");
+   // f.Close();
+   // with MySubClass.cxx containing: class MySubClass { public: int id; ClassDef(MySubClass, 3) };
+   TFile f("mysub.root", "READ");
+   auto msc = f.Get<MySubClass>("msc");
+   if (msc->id != 33) {
+      printf("Error: id is %d vs 33\n", msc->id);
+      return 3;
+   }
+   return 0;
+}

roottest/root/io/evolution/versions/execROOT5306.ref

@@ -0,0 +1,3 @@
+
+Processing execROOT5306.cxx+...
+(int) 0