[ntuple] clarify/improve record field reconciliation

Record fields are used for
  1) untyped records
  2) emulated classes and as
  3) base class for the pair and tuple fields.

These three cases use different constructors and have different
reconciliation logic wrt. to the on-disk information.

Make this more clear through code comments. Shortcut reconciliation for
emulated fields.

Author: jblomer

tree/ntuple/inc/ROOT/RField/RFieldRecord.hxx

@@ -63,6 +63,7 @@ class RRecordField : public RFieldBase {
 
    /// If `emulatedFromType` is non-empty, this field was created as a replacement for a ClassField that we lack a
    /// dictionary for and reconstructed from the on-disk information.
+   /// Used by the public constructor and by Internal::CreateEmulatedRecordField().
    RRecordField(std::string_view fieldName, std::vector<std::unique_ptr<RFieldBase>> itemFields,
                 std::string_view emulatedFromType);
 
@@ -82,6 +83,8 @@ protected:
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
    void ReadInClusterImpl(RNTupleLocalIndex localIndex, void *to) final;
 
+   /// Used by RPairField and RTupleField descendants. These descendants have their own logic to attach the subfields
+   /// that ensure that the resulting memory layout matches std::pair or std::tuple, resp.
    RRecordField(std::string_view fieldName, std::string_view typeName);
 
    void AttachItemFields(std::vector<std::unique_ptr<RFieldBase>> itemFields);
@@ -106,6 +109,8 @@ protected:
 public:
    /// Construct a RRecordField based on a vector of child fields. The ownership of the child fields is transferred
    /// to the RRecordField instance.
+   /// The resulting field uses a memory layout for its values as if there was a struct consisting of the passed
+   /// item fields.
    RRecordField(std::string_view fieldName, std::vector<std::unique_ptr<RFieldBase>> itemFields);
    RRecordField(RRecordField &&other) = default;
    RRecordField &operator=(RRecordField &&other) = default;

tree/ntuple/src/RField.cxx

@@ -607,6 +607,13 @@ void ROOT::RRecordField::ReadInClusterImpl(RNTupleLocalIndex localIndex, void *t
 
 void ROOT::RRecordField::ReconcileOnDiskField(const RNTupleDescriptor &desc)
 {
+   if (fTraits & kTraitEmulatedField) {
+      // The field has been explicitly constructed following the on-disk information. No further reconcilation needed.
+      return;
+   }
+   // Note that the RPairField and RTupleField descendants have their own reconcilation logic
+   R__ASSERT(GetTypeName().empty());
+
    const auto &fieldDesc = desc.GetFieldDescriptor(GetOnDiskId());
    EnsureMatchingOnDiskField(fieldDesc, kDiffTypeName | kDiffTypeVersion);