[DF] Add test for TChain with TTrees with different names

tree/dataframe/test/dataframe_simple.cxx

@@ -2,6 +2,7 @@
 #include <gtest/gtest.h>
 #include <ROOT/RDataFrame.hxx>
 #include <ROOT/TSeq.hxx>
+#include <TChain.h>
 #include <TFile.h>
 #include <TGraph.h>
 #include <TInterpreter.h>
@@ -927,6 +928,28 @@ TEST(RDFSimpleTests, ScalarValuesCollectionWeights)
    EXPECT_ANY_THROW(*h);
 }
 
+TEST_P(RDFSimpleTests, ChainWithDifferentTreeNames)
+{
+	const auto fname1 = "test_chainwithdifferenttreenames_1.root";
+	const auto fname2 = "test_chainwithdifferenttreenames_2.root";
+	{
+		ROOT::RDataFrame(10).Define("x", [] { return 1; }).Snapshot<int>("t1", fname1, {"x"});
+		ROOT::RDataFrame(10).Define("x", [] { return 3; }).Snapshot<int>("t2", fname2, {"x"});
+	}
+
+   // add trees to chain
+   TChain c("t1");
+   c.Add(fname1);
+   c.Add((std::string(fname2) + "/t2").c_str());
+
+   // pass chain to RDF and process trees with different names
+   ROOT::RDataFrame df2(c);
+   EXPECT_DOUBLE_EQ(*df2.Mean("x"), 2);
+
+   gSystem->Unlink(fname1);
+   gSystem->Unlink(fname2);
+}
+
 // run single-thread tests
 INSTANTIATE_TEST_CASE_P(Seq, RDFSimpleTests, ::testing::Values(false));