[RDF] Forbid calling VariationsFor on snapshots.

Given that snapshots with variations will be enabled via RSnapshotOptions,
the snapshot overload for Vary can be removed, and a static_assert can
be used to signal the failure.

Author: hageboeck

tree/dataframe/inc/ROOT/RDFHelpers.hxx

@@ -222,6 +222,10 @@ namespace Experimental {
 template <typename T>
 RResultMap<T> VariationsFor(RResultPtr<T> resPtr)
 {
+   using SnapshotResult_t = ROOT::RDF::RInterface<ROOT::Detail::RDF::RLoopManager, void>;
+   static_assert(!std::is_same_v<T, SnapshotResult_t>,
+                 "Snapshot with variations only can be enabled via RSnapshotOptions.");
+
    R__ASSERT(resPtr != nullptr && "Calling VariationsFor on an empty RResultPtr");
 
    // populate parts of the computation graph for which we only have "empty shells", e.g. RJittedActions and
@@ -270,9 +274,6 @@ RResultMap<T> VariationsFor(RResultPtr<T> resPtr)
                                         *resPtr.fLoopManager, std::move(nominalAction), std::move(variedAction));
 }
 
-using SnapshotPtr_t = ROOT::RDF::RResultPtr<ROOT::RDF::RInterface<ROOT::Detail::RDF::RLoopManager, void>>;
-SnapshotPtr_t VariationsFor(SnapshotPtr_t resPtr);
-
 /// \brief Add ProgressBar to a ROOT::RDF::RNode
 /// \param[in] df RDataFrame node at which ProgressBar is called.
 ///

tree/dataframe/src/RDFHelpers.cxx

@@ -141,15 +141,7 @@ unsigned int ROOT::RDF::RunGraphs(std::vector<RResultHandle> handles)
    return uniqueLoops.size();
 }
 
-ROOT::RDF::Experimental::SnapshotPtr_t ROOT::RDF::Experimental::VariationsFor(ROOT::RDF::Experimental::SnapshotPtr_t)
-{
-   throw std::logic_error("Varying a Snapshot result is not implemented yet.");
-}
-
-namespace ROOT {
-namespace RDF {
-
-namespace Experimental {
+namespace ROOT::RDF::Experimental {
 
 void ThreadsPerTH3(unsigned int N)
 {
@@ -398,6 +390,5 @@ void AddProgressBar(ROOT::RDataFrame dataframe)
    auto node = ROOT::RDF::AsRNode(dataframe);
    ROOT::RDF::Experimental::AddProgressBar(node);
 }
-} // namespace Experimental
-} // namespace RDF
-} // namespace ROOT
+
+} // namespace ROOT::RDF::Experimental

tree/dataframe/test/dataframe_vary.cxx

@@ -1557,23 +1557,6 @@ TEST_P(RDFVary, VaryTake)
    EXPECT_EQ(sorted(rs["x:1"]), std::vector<int>({1, 2, 3}));
 }
 
-TEST_P(RDFVary, VarySnapshot)
-{
-   const auto fname = "dummy.root";
-   auto h = ROOT::RDataFrame(10)
-               .Define("x", [](ULong64_t e) { return int(e); }, {"rdfentry_"})
-               .Vary(
-                  "x", [](int x) { return ROOT::RVecI{x - 1, x + 1}; }, {"x"}, 2)
-               .Snapshot("t", fname, {"x"});
-   EXPECT_THROW(
-      try { VariationsFor(h); } catch (const std::logic_error &err) {
-         const auto msg = "Varying a Snapshot result is not implemented yet.";
-         EXPECT_STREQ(err.what(), msg);
-         throw;
-      },
-      std::logic_error);
-}
-
 // this is a regression test, we used to read from wrong addresses in this case
 TEST_P(RDFVary, MoreVariedColumnsThanVariations)
 {