[RF] Fixing issues in RooSimultaneous with ConditionalObservables

`RooSimultaneous` is a PDF class that performs the simultaneous fitting
of multiple pairs of a PDF and a dataset.

`RooFit::ConditionalObservables(RooAbsArgs...)` is a command argument to
the `fitTo()` method of the `RooAbsPdf` classes, which specifies
variables that should be excluded from the normalization set of a PDF
when fitting.

If I use the command argument for fitting using the class, the class
does not deal with the conditional variable well, and the conditional
variables become floating parameters. This should be fixed.

Closes #19166

Author: qogksdnr

roofit/roofitcore/src/FitHelpers.cxx

@@ -745,7 +745,17 @@ std::unique_ptr<RooAbsReal> createNLL(RooAbsPdf &pdf, RooAbsData &data, const Ro
 
       RooArgSet normSet;
       pdf.getObservables(data.get(), normSet);
-      normSet.remove(projDeps, true, true);
+
+      if (dynamic_cast<RooSimultaneous const*>(&pdf)) {
+         for (auto i : projDeps) {
+            auto res = normSet.find(i->GetName());
+            if (res != nullptr) {
+               res->setAttribute("__conditional__");
+            }
+         }
+      } else {
+         normSet.remove(projDeps);
+      }
 
       pdf.setAttribute("SplitRange", splitRange);
       pdf.setStringAttribute("RangeName", rangeName);

roofit/roofitcore/src/RooSimultaneous.cxx

@@ -1217,6 +1217,10 @@ RooSimultaneous::compileForNormSet(RooArgSet const &normSet, RooFit::Detail::Com
 
       std::unique_ptr<RooArgSet> pdfNormSet{
          std::unique_ptr<RooArgSet>(pdfClone->getVariables())->selectByAttrib("__obs__", true)};
+      std::unique_ptr<RooArgSet> condVarSet{
+         std::unique_ptr<RooArgSet>(pdfClone->getVariables())->selectByAttrib("__conditional__", true)};
+
+      pdfNormSet->remove(*condVarSet, true, true);
 
       if (rangeName) {
          pdfClone->setNormRange(RooHelpers::getRangeNameForSimComponent(rangeName, splitRange, catName).c_str());

roofit/roofitcore/test/testRooSimultaneous.cxx

@@ -456,3 +456,54 @@ TEST(RooSimultaneous, DuplicateExtendedPdfs)
    EXPECT_FLOAT_EQ(simPdfVal, 0.05);
    EXPECT_DOUBLE_EQ(simPdfVal, simPdfRef.getVal(normSet));
 }
+
+/// GitHub issue #19166.
+/// RooSimultaneous::fitTo() should work well with ConditionalOvservables.
+/// We test this by performing the similar test to a test case in
+/// RooSimultaneousSingleChannelCrossCheck.
+TEST_P(TestStatisticTest, RooSimultaneousSingleChannelCrossCheckWithCondVar)
+{
+   using namespace RooFit;
+
+   // silence log output
+   RooHelpers::LocalChangeMsgLevel changeMsgLvl(RooFit::WARNING);
+
+   RooWorkspace ws;
+   ws.factory("Uniform::uniform(width[1, 3])");
+   ws.factory("expr::sigma(\"@0*@1\", width, width_scale[0.8, 0.5, 1.5])");
+   ws.factory("Gaussian::model(x[-20, 20], mean[3, -20., 20.], sigma)");
+
+   RooRealVar &width = *ws.var("width");
+   RooAbsPdf &widthModel = *ws.pdf("uniform");
+   std::unique_ptr<RooDataSet> protoData{widthModel.generate(width, 100)};
+
+   RooRealVar &x = *ws.var("x");
+   RooAbsPdf &model = *ws.pdf("model");
+
+   std::unique_ptr<RooDataSet> data{model.generate(x, *protoData)};
+
+   RooCategory cat("cat", "cat");
+   cat.defineType("physics");
+
+   RooArgSet params;
+   RooArgSet initialParams;
+   model.getParameters(data->get(), params);
+   params.snapshot(initialParams);
+
+   RooSimultaneous modelSim("modelSim", "modelSim", {{"physics", &model}}, cat);
+
+   RooDataSet combData("combData", "combData", {x, width}, Index(cat), Import({{"physics", data.get()}}));
+
+   using namespace RooFit;
+
+   params.assign(initialParams);
+   std::unique_ptr<RooFitResult> resDirect{
+      model.fitTo(*data, ConditionalObservables(width), Save(), PrintLevel(-1), _evalBackend)};
+
+   params.assign(initialParams);
+   std::unique_ptr<RooFitResult> resSimWrapped{
+      modelSim.fitTo(combData, ConditionalObservables(width), Save(), PrintLevel(-1), _evalBackend)};
+
+   EXPECT_TRUE(resSimWrapped->isIdentical(*resDirect))
+      << "Inconsistency in RooSimultaneous wrapping with ConditionalObservables";
+}