[tree] fix TTree::BuildIndex with a formula containing variable array index

roottest/root/tree/index/CMakeLists.txt

@@ -15,3 +15,6 @@ ROOTTEST_ADD_TEST(index64
                   MACRO runindex64.C
                   OUTREF index64.ref)
 
+ROOTTEST_ADD_TEST(varsizearr
+                  MACRO runvarsizearr.C
+                  OUTREF varsizearr.ref)

roottest/root/tree/index/runvarsizearr.C

@@ -0,0 +1,31 @@
+#include "TTree.h"
+#include "TTreeIndex.h"
+#include <vector>
+#include <iostream>
+
+// https://github.com/root-project/root/pull/19619
+int runvarsizearr()
+{
+   const Long64_t nEvents = 2;
+   const size_t nElements = 3;
+   std::vector<int> otherNumbers(nElements);
+   Long64_t eventNumber;
+   TTree t("tree", "tree");
+   t.Branch("eventNumber", &eventNumber);
+   t.Branch("otherNumbers", &otherNumbers);
+   for (Long64_t i = 0; i < nEvents; i++) {
+      eventNumber = i;
+      for (size_t j = 0; j < nElements; ++j)
+         otherNumbers[j] = -2. * eventNumber + 3500 * j;
+      t.Fill();
+   }
+   std::cout << t.GetEntries() << std::endl;
+   // use Scan to show what otherNumbers[2] contains
+   t.Scan("eventNumber:otherNumbers", "", "", nEvents);
+
+   TTreeIndex firstIndex(&t, "otherNumbers[2]", "eventNumber");
+   firstIndex.Print("2"); // before the fix: major was always a garbage value and events were wrongly sorted
+   TTreeIndex secondIndex(&t, "otherNumbers", "eventNumber");
+   secondIndex.Print("2"); // major is otherNumbers[0] since index not specified (fine before and after fix)
+   return 0;
+}

roottest/root/tree/index/varsizearr.ref

@@ -0,0 +1,30 @@
+
+Processing runvarsizearr.C...
+2
+***********************************************
+*    Row   * Instance * eventNumb * otherNumb *
+***********************************************
+*        0 *        0 *         0 *         0 *
+*        0 *        1 *         0 *      3500 *
+*        0 *        2 *         0 *      7000 *
+*        1 *        0 *         1 *        -2 *
+*        1 *        1 *         1 *      3498 *
+*        1 *        2 *         1 *      6998 *
+***********************************************
+
+**********************************************
+*    Index of Tree: tree/tree
+**********************************************
+  serial :  otherNumbers[2] :      eventNumber
+**********************************************
+       0 :             6998 :                1
+       1 :             7000 :                0
+
+**********************************************
+*    Index of Tree: tree/tree
+**********************************************
+  serial :     otherNumbers :      eventNumber
+**********************************************
+       0 :               -2 :                1
+       1 :                0 :                0
+(int) 0

tree/treeplayer/src/TTreeIndex.cxx

@@ -11,6 +11,8 @@
 
 /** \class TTreeIndex
 A Tree Index with majorname and minorname.
+
+If minorname is "0" (default arg in TTree::BuildIndex), just majorname will be used.
 */
 
 #include "TTreeIndex.h"
@@ -180,6 +182,13 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
          fMajorFormula->UpdateFormulaLeaves();
          fMinorFormula->UpdateFormulaLeaves();
       }
+      if ((fMajorFormula->GetNdata() + fMinorFormula->GetNdata()) <= 0) {
+         // Calling GetNdata is essential before calling EvalInstance, otherwise a wrong
+         // result is silently returned by EvalInstance below if formula is value from a variable-sized array
+         // We raise an error to prevent the if clause being optimized-out if we do not use the return
+         Error("TTreeIndex", "In tree entry %lld, Ndata in formula is zero for both '%s' and '%s'", i,
+               fMajorName.Data(), fMinorName.Data());
+      }
       auto GetAndRangeCheck = [this](bool isMajor, Long64_t entry) {
          LongDouble_t ret = (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<LongDouble_t>();
          // Check whether the value (vs significant bits) of ldRet can represent