From c1f11f53cb1e6285ddcb8bad249f303fb7ece7e9 Mon Sep 17 00:00:00 2001
From: Jakob Blomer <jblomer@cern.ch>
Date: Thu, 5 Nov 2026 22:53:13 +0100
Subject: [PATCH] [ntuple] fix recreation of streamer field with type alias

(cherry picked from commit 38e71de3c92e60fa81e0248fe8ff995db5af8eff)
---
 tree/ntuple/src/RNTupleDescriptor.cxx   |  3 ++-
 tree/ntuple/test/StreamerField.hxx      | 10 ++++++++++
 tree/ntuple/test/StreamerFieldLinkDef.h |  3 +++
 tree/ntuple/test/rfield_streamer.cxx    | 18 ++++++++++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/tree/ntuple/src/RNTupleDescriptor.cxx b/tree/ntuple/src/RNTupleDescriptor.cxx
index 7f12307c94790..30aeff66362c5 100644
--- a/tree/ntuple/src/RNTupleDescriptor.cxx
+++ b/tree/ntuple/src/RNTupleDescriptor.cxx
@@ -69,7 +69,8 @@ std::unique_ptr<ROOT::RFieldBase>
 ROOT::RFieldDescriptor::CreateField(const RNTupleDescriptor &ntplDesc, const ROOT::RCreateFieldOptions &options) const
 {
    if (GetStructure() == ROOT::ENTupleStructure::kStreamer) {
-      auto streamerField = std::make_unique<ROOT::RStreamerField>(GetFieldName(), GetTypeName());
+      const auto &typeName = GetTypeAlias().empty() ? GetTypeName() : GetTypeAlias();
+      auto streamerField = std::make_unique<ROOT::RStreamerField>(GetFieldName(), typeName);
       streamerField->SetOnDiskId(fFieldId);
       return streamerField;
    }
diff --git a/tree/ntuple/test/StreamerField.hxx b/tree/ntuple/test/StreamerField.hxx
index 8ff32e52f9dd1..3b5f7d15c80e3 100644
--- a/tree/ntuple/test/StreamerField.hxx
+++ b/tree/ntuple/test/StreamerField.hxx
@@ -96,4 +96,14 @@ struct TemperatureKelvin {
    float fValue;
 };
 
+template <typename T>
+struct MEtoEDMStreamer {
+   T fMember;
+};
+
+template <typename T>
+struct MEtoEDMClass {
+   T fMember;
+};
+
 #endif // ROOT_RNTuple_Test_StreamerField
diff --git a/tree/ntuple/test/StreamerFieldLinkDef.h b/tree/ntuple/test/StreamerFieldLinkDef.h
index 707d59654ba20..e8449954ff170 100644
--- a/tree/ntuple/test/StreamerFieldLinkDef.h
+++ b/tree/ntuple/test/StreamerFieldLinkDef.h
@@ -26,4 +26,7 @@
 #pragma link C++ options = rntupleStreamerMode(true) class TemperatureCelsius + ;
 #pragma link C++ options = rntupleStreamerMode(true) class TemperatureKelvin + ;
 
+#pragma link C++ options = rntupleStreamerMode(true) class MEtoEDMStreamer<long long>+;
+#pragma link C++ class MEtoEDMClass<long long>+;
+
 #endif
diff --git a/tree/ntuple/test/rfield_streamer.cxx b/tree/ntuple/test/rfield_streamer.cxx
index 6f35f3f6ba8a4..ad57ed4bd211e 100644
--- a/tree/ntuple/test/rfield_streamer.cxx
+++ b/tree/ntuple/test/rfield_streamer.cxx
@@ -377,3 +377,21 @@ TEST(RField, StreamerClassMismatch)
                          false /* matchFullMessage */);
    reader->LoadEntry(0);
 }
+
+TEST(RField, StreamerLong64TemplateArg)
+{
+   FileRaii fileGuard("test_ntuple_rfield_streamer_long64_template_arg.root");
+   {
+      auto model = RNTupleModel::Create();
+      model->AddField(RFieldBase::Create("s", "MEtoEDMStreamer<Long64_t>").Unwrap());
+      model->AddField(RFieldBase::Create("c", "MEtoEDMClass<Long64_t>").Unwrap());
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+      writer->Fill();
+   }
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   const auto &s = dynamic_cast<const ROOT::RStreamerField &>(reader->GetModel().GetConstField("s"));
+   EXPECT_STREQ("MEtoEDMStreamer<Long64_t>", s.GetClass()->GetName());
+   const auto &c = dynamic_cast<const ROOT::RClassField &>(reader->GetModel().GetConstField("c"));
+   EXPECT_STREQ("MEtoEDMClass<Long64_t>", c.GetClass()->GetName());
+}