[ntuple] fix TClass initializatin in RField<UserClass>

In the templated version of the RClassField, the type name passed to
TClass::GetClass() must not use the RNTuple normalized name but the
demangled name or the meta normalized name. Otherwise, RNTuple may
normalize, e.g., `long long` to `std::int64_t`, which in turn gets
normalized by Meta to `unsigned long`.

Author: jblomer

tree/ntuple/inc/ROOT/RField.hxx

@@ -225,6 +225,8 @@ public:
    /// For polymorphic classes (that declare or inherit at least one virtual method), return the expected dynamic type
    /// of any user object. If the class is not polymorphic, return nullptr.
    const std::type_info *GetPolymorphicTypeInfo() const;
+   /// Return the TClass instance backing this field.
+   const TClass *GetClass() const { return fClass; }
    void AcceptVisitor(ROOT::Detail::RFieldVisitor &visitor) const final;
 };
 
@@ -316,7 +318,7 @@ template <typename T, typename = void>
 class RField final : public RClassField {
 public:
    static std::string TypeName() { return ROOT::Internal::GetRenormalizedTypeName(typeid(T)); }
-   RField(std::string_view name) : RClassField(name, TypeName())
+   RField(std::string_view name) : RClassField(name, Internal::GetDemangledTypeName(typeid(T)))
    {
       static_assert(std::is_class_v<T>, "no I/O support for this basic C++ type");
    }

tree/ntuple/test/CustomStruct.hxx

@@ -103,6 +103,12 @@ public:
    T fMember;
 };
 
+class EdmContainer {
+public:
+   // Used to test that the streamer info for fWrapper will use long long
+   EdmWrapper<long long> fWrapper;
+};
+
 template <typename T>
 struct EdmHashTrait {
    using value_type = T;

tree/ntuple/test/CustomStructLinkDef.h

@@ -29,6 +29,8 @@
 
 #pragma link C++ class EdmWrapper<CustomStruct> +;
 #pragma link C++ class EdmHash < 1> + ;
+#pragma link C++ class EdmWrapper<long long>+;
+#pragma link C++ class EdmContainer;
 
 #pragma link C++ class DataVector < int, double> + ;
 #pragma link C++ class DataVector < int, float> + ;

tree/ntuple/test/rfield_class.cxx

@@ -371,3 +371,19 @@ TEST(RNTuple, PolymorphicPointer)
       EXPECT_THROW(writer->Fill(), ROOT::RException);
    }
 }
+
+TEST(RNTuple, TClassMetaName)
+{
+   auto f1 = ROOT::RClassField("f", "EdmWrapper<long long>");
+   EXPECT_STREQ("EdmWrapper<Long64_t>", f1.GetClass()->GetName());
+
+   auto f2 = std::make_unique<ROOT::RField<EdmWrapper<long long>>>("f");
+   EXPECT_STREQ("EdmWrapper<Long64_t>", static_cast<ROOT::RClassField *>(f2.get())->GetClass()->GetName());
+
+   auto f3 = RFieldBase::Create("f", "EdmWrapper<long long>").Unwrap();
+   EXPECT_STREQ("EdmWrapper<Long64_t>", static_cast<ROOT::RClassField *>(f3.get())->GetClass()->GetName());
+
+   auto f4 = RFieldBase::Create("f", "EdmContainer").Unwrap();
+   EXPECT_STREQ("EdmWrapper<Long64_t>",
+                static_cast<const ROOT::RClassField *>(f4->GetConstSubfields()[0])->GetClass()->GetName());
+}