[RF][HF] Re-use any type of RooAbsArgs in ParamHistFunc::createParamSet

When connecting preprocessing functions to nuisance parameters via
`Measurement::AddPreprocessFunction`, the nuisance parameter is replaced
by a RooAbsReal that is not a RooRealVar, and the implementation of
`ParamHistFunc::createParamSet` should be able to deal with that.

So far, `createParamSet` assumed that any pre-existing argument in the
workspace with the nuisance parameter name is of type RooRealVar, which
is not valid in that case. Therefore, using preprocessing functions in
place of these parameters resulted in a segfault.

To fix this, the code is generalized to any RooAbsArg type.

Closes #12225.

Author: guitargeek

roofit/histfactory/src/ParamHistFunc.cxx

@@ -317,10 +317,8 @@ RooArgList ParamHistFunc::createParamSet(RooWorkspace& w, const std::string& Pre
       gamma.setConstant( false );
 
       w.import( gamma, RooFit::RecycleConflictNodes() );
-      RooRealVar* gamma_wspace = (RooRealVar*) w.var( VarName );
-
-      paramSet.add( *gamma_wspace );
 
+      paramSet.add(*w.arg(VarName));
     }
   }
 
@@ -351,10 +349,8 @@ RooArgList ParamHistFunc::createParamSet(RooWorkspace& w, const std::string& Pre
         gamma.setConstant( false );
 
         w.import( gamma, RooFit::RecycleConflictNodes() );
-        RooRealVar* gamma_wspace = (RooRealVar*) w.var( VarName );
-
-        paramSet.add( *gamma_wspace );
 
+        paramSet.add(*w.arg(VarName));
       }
     }
   }
@@ -388,10 +384,8 @@ RooArgList ParamHistFunc::createParamSet(RooWorkspace& w, const std::string& Pre
           gamma.setConstant( false );
 
           w.import( gamma, RooFit::RecycleConflictNodes() );
-          RooRealVar* gamma_wspace = (RooRealVar*) w.var( VarName );
-
-          paramSet.add( *gamma_wspace );
 
+          paramSet.add(*w.arg(VarName));
         }
       }
     }
@@ -431,10 +425,12 @@ RooArgList ParamHistFunc::createParamSet(RooWorkspace& w, const std::string& Pre
   RooArgList params = ParamHistFunc::createParamSet( w, Prefix, vars );
 
   for (auto comp : params) {
-    auto var = static_cast<RooRealVar*>(comp);
-
-    var->setMin( gamma_min );
-    var->setMax( gamma_max );
+    // If the gamma is subject to a preprocess function, it is a RooAbsReal and
+    // we don't need to set the range.
+    if(auto var = dynamic_cast<RooRealVar*>(comp)) {
+      var->setMin( gamma_min );
+      var->setMax( gamma_max );
+    }
   }
 
   return params;