[ntuple] don't try to construct an Emulated TClass as a RClassField

When constructing a RClassField we need some information that we don't
have when the TClass is Emulated (or less), therefore we now check
that we have at least an Interpreted TClass in RFieldBase::Create().
If not, we follow the same logic as when we have no TClass at all
(i.e. either create an emulated field or fail, depending on the user
options).

tree/ntuple/v7/src/RFieldBase.cxx

@@ -594,7 +594,10 @@ ROOT::Experimental::RFieldBase::Create(const std::string &fieldName, const std::
 
       if (!result) {
          auto cl = TClass::GetClass(canonicalType.c_str());
-         if (cl != nullptr) {
+         // NOTE: if the class is not at least "Interpreted" we don't have enough information to
+         // properly construct the RClassField (e.g. we are missing the list of bases), so in that
+         // situation we rely on field emulation instead.
+         if (cl != nullptr && cl->GetState() >= TClass::kInterpreted) {
             createContextGuard.AddClassToStack(canonicalType);
             if (cl->GetCollectionProxy()) {
                result = std::make_unique<RProxiedCollectionField>(fieldName, canonicalType);

tree/ntuple/v7/src/RFieldMeta.cxx

@@ -73,6 +73,12 @@ ROOT::Experimental::RClassField::RClassField(std::string_view fieldName, std::st
    if (fClass == nullptr) {
       throw RException(R__FAIL("RField: no I/O support for type " + std::string(className)));
    }
+   if (fClass->GetState() < TClass::kInterpreted) {
+      throw RException(R__FAIL(std::string("RField: we don't have enough information about class '") +
+                               std::string(className) +
+                               " to construct a RClassField based on it (maybe it's an emulated class, or it was just"
+                               " forward declared)."));
+   }
    // Avoid accidentally supporting std types through TClass.
    if (fClass->Property() & kIsDefinedInStd) {
       throw RException(R__FAIL(std::string(className) + " is not supported"));
@@ -101,7 +107,9 @@ ROOT::Experimental::RClassField::RClassField(std::string_view fieldName, std::st
       fTraits |= kTraitTriviallyDestructible;
 
    int i = 0;
-   for (auto baseClass : ROOT::Detail::TRangeStaticCast<TBaseClass>(*fClass->GetListOfBases())) {
+   const auto *bases = fClass->GetListOfBases();
+   assert(bases);
+   for (auto baseClass : ROOT::Detail::TRangeStaticCast<TBaseClass>(*bases)) {
       if (baseClass->GetDelta() < 0) {
          throw RException(R__FAIL(std::string("virtual inheritance is not supported: ") + std::string(className) +
                                   " virtually inherits from " + baseClass->GetName()));

tree/ntuple/v7/test/ntuple_emulated.cxx

@@ -88,10 +88,19 @@ struct Outer_Simple {
       ASSERT_EQ(inner->GetSubFields()[0]->GetFieldName(), "fInt1");
    }
 
-   // Now test loading entries with a reader
+   // Now test loading entries with a reader.
+   // NOTE: using a TFile-based reader exercises the code path where the user-defined type
+   // gets loaded as an Emulated TClass, which we must make sure we handle properly.
    RNTupleDescriptor::RCreateModelOptions cmOpts;
    cmOpts.fEmulateUnknownTypes = true;
-   reader = RNTupleReader::Open(cmOpts, "ntpl", fileGuard.GetPath());
+
+   ROOT::TestSupport::CheckDiagsRAII diagRAII;
+   diagRAII.optionalDiag(kWarning, "TClass::Init", "no dictionary for class",
+                         /*matchFullMessage=*/false);
+   std::unique_ptr<TFile> file(TFile::Open(fileGuard.GetPath().c_str()));
+   std::unique_ptr<ROOT::RNTuple> ntpl(file->Get<ROOT::RNTuple>("ntpl"));
+   reader = RNTupleReader::Open(cmOpts, *ntpl);
+
    reader->LoadEntry(0);
 }