[io] Add type mismatch check for TObject-derived cases

Adds a check for type mismatch in TFileMerger for the particular case of having
a TObject and trying to merge it against other objects of different types.
Importantly, this includes also the check when the other object is of a
non-TObject-derived type, which would previously lead to a segfault.

Author: vepadulano

io/io/src/TFileMerger.cxx

@@ -661,6 +661,12 @@ Bool_t TFileMerger::MergeOne(TDirectory *target, TList *sourcelist, Int_t type,
                if (!hobj) {
                   TKey *key2 = (TKey*)ndir->GetListOfKeys()->FindObject(keyname);
                   if (key2) {
+                     if (strcmp(key2->GetClassName(), keyclassname) != 0) {
+                        Error("MergeRecursive", "Object type mismatch for key '%s' in file '%s': expected '%s' but found '%s'.",
+                              keyname, nextsource->GetName(), keyclassname, key2->GetClassName());
+                        nextsource = (TFile *)sourcelist->After(nextsource);
+                        return kFALSE;
+                     }
                      hobj = key2->ReadObj();
                      if (!hobj) {
                         switch (fErrBehavior) {

io/io/test/TFileMergerTests.cxx

@@ -18,6 +18,8 @@
 
 #include "gtest/gtest.h"
 
+#include <ROOT/TestSupport.hxx>
+
 static void CreateATuple(TMemFile &file, const char *name, double value)
 {
    auto mytree = new TTree(name, "A tree");
@@ -396,3 +398,44 @@ TEST(TFileMerger, MergeSelectiveTutorial)
       EXPECT_NE(file.Get<TNtuple>("ntuple"), nullptr);
    }
 }
+
+
+TEST(TFileMerger, TypeMismatchErrorTObjectWithNonTObject)
+{
+   struct PathRAII {
+      std::string fPath;
+      PathRAII(const char *path) : fPath(path) {}
+      ~PathRAII() { std::remove(fPath.c_str()); }
+   };
+
+   PathRAII input1("ErrorIfTypeMismatch_input1.root");
+   const auto objname{"myobj"};
+   PathRAII input2("ErrorIfTypeMismatch_input2.root");
+   PathRAII outputfile("ErrorIfTypeMismatch_output.root");
+
+   // Create two files with objects of different types, one must be a TObject-derived type and other a non-TObject-derived type
+   {
+      auto f = std::make_unique<TFile>(input1.fPath.c_str(), "RECREATE");
+      auto t = std::make_unique<TTree>(objname, objname);
+      f->Write();
+   }
+
+   {
+      auto f = std::make_unique<TFile>(input2.fPath.c_str(), "RECREATE");
+      std::vector<int> v{1,2,3};
+      f->WriteObject(&v, objname);
+   }
+
+   // Ensure that TFileMerger detects the type mismatch and errors out.
+   {
+      ROOT::TestSupport::CheckDiagsRAII diagRAII;
+      diagRAII.requiredDiag(kError, "TFileMerger::MergeRecursive", "expected 'TTree' but found 'vector<int>'", /*matchFullMessage*/false);
+      diagRAII.requiredDiag(kError, "TFileMerger::Merge", "error during merge of your ROOT files");
+
+      TFileMerger fm;
+      fm.OutputFile(outputfile.fPath.c_str());
+      fm.AddFile(input1.fPath.c_str(), /*cpProgress*/ false);
+      fm.AddFile(input2.fPath.c_str(), /*cpProgress*/ false);
+      fm.PartialMerge();
+   }
+}