[ntuple] add test for type trace report

Author: jblomer

tree/ntuple/test/ntuple_show.cxx

@@ -167,7 +167,7 @@ TEST(RNTupleShow, Vectors)
    std::ostringstream os;
    ntuple2->Show(0, os);
    // clang-format off
-   std::string fString{ 
+   std::string fString{
 R"({
   "intVec": [4, 5, 6],
   "floatVecVec": [[0.1, 0.2], [1.1, 1.2]],
@@ -312,7 +312,7 @@ TEST(RNTupleShow, Objects)
    std::ostringstream os;
    ntuple2->Show(0, os);
    // clang-format off
-   std::string fString{ 
+   std::string fString{
 R"({
   "CustomStruct": {
     "a": 4.1,
@@ -663,3 +663,36 @@ TEST(RNTupleShow, Friends)
       writer->Fill();
    }
 }
+
+TEST(RNTupleShow, TypeTraceReport)
+{
+   FileRaii fileGuard("test_ntuple_show_type_trace_report.ntuple");
+   {
+      auto model = RNTupleModel::Create();
+      model->MakeField<std::variant<double, std::vector<std::pair<float, bool>>>>("f");
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+   }
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+
+   // Get the field for the `bool` member of the inner `pair`
+   const auto field = reader->GetModel().GetDefaultEntry().begin()->GetField().GetConstSubfields()[1]
+      ->GetConstSubfields()[0]->GetConstSubfields()[1];
+
+   const auto report = ROOT::Internal::GetTypeTraceReport(*field, reader->GetDescriptor());
+
+   const std::string expected{
+R"(In-memory field/type hierarchy:
+f [std::variant<double,std::vector<std::pair<float,bool>>>] (id: 0)
+  _1 [std::vector<std::pair<float,bool>>] (id: 2)
+    _0 [std::pair<float,bool>] (id: 3)
+      _1 [bool] (id: 5)
+On-disk field/type hierarchy:
+f [std::variant<double,std::vector<std::pair<float,bool>>>] (id: 0)
+  _1 [std::vector<std::pair<float,bool>>] (id: 2)
+    _0 [std::pair<float,bool>] (id: 3)
+      _1 [bool] (id: 5)
+)"};
+
+   EXPECT_EQ(expected, report);
+}