[ntuple] add evolution test for atomic<class>

Author: jblomer

tree/ntuple/test/ntuple_evolution_type.cxx

@@ -236,17 +236,21 @@ TEST(RNTupleEvolution, Enum)
 
 TEST(RNTupleEvolution, CheckAtomic)
 {
-   // TODO(jblomer): enable test with CustomAtomicNotLockFree once linking of libatomic is sorted out.
+   EXPECT_FALSE(std::atomic<CustomAtomicNotLockFree>{}.is_lock_free());
 
    FileRaii fileGuard("test_ntuple_evolution_check_atomic.root");
    {
       auto model = ROOT::RNTupleModel::Create();
       auto atomicInt = model->MakeField<std::atomic<std::int32_t>>("atomicInt");
       auto regularInt = model->MakeField<std::int32_t>("regularInt");
+      auto atomicClass = model->MakeField<std::atomic<CustomAtomicNotLockFree>>("atomicClass");
+      auto regularClass = model->MakeField<CustomAtomicNotLockFree>("regularClass");
       auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
 
       *atomicInt = 7;
       *regularInt = 13;
+      std::fill(std::begin(regularClass->a), std::end(regularClass->a), 137);
+      *atomicClass = *regularClass;
       writer->Fill();
    }
 
@@ -256,6 +260,9 @@ TEST(RNTupleEvolution, CheckAtomic)
    auto v2 = reader->GetView<std::atomic<std::int64_t>>("regularInt");
    auto v3 = reader->GetView<std::int64_t>("atomicInt");
 
+   auto v4 = reader->GetView<CustomAtomicNotLockFree>("atomicClass");
+   auto v5 = reader->GetView<std::atomic<CustomAtomicNotLockFree>>("regularClass");
+
    try {
       reader->GetView<std::atomic<std::byte>>("atomicInt");
       FAIL() << "automatic evolution into an invalid atomic inner type should fail";
@@ -273,6 +280,12 @@ TEST(RNTupleEvolution, CheckAtomic)
    EXPECT_EQ(7, v1(0));
    EXPECT_EQ(13, v2(0));
    EXPECT_EQ(7, v3(0));
+
+   EXPECT_EQ(137, v4(0).a[0]);
+   EXPECT_EQ(137, v4(0).a[99]);
+   CustomAtomicNotLockFree tmp = v5(0);
+   EXPECT_EQ(137, tmp.a[0]);
+   EXPECT_EQ(137, tmp.a[99]);
 }
 
 TEST(RNTupleEvolution, ArrayAsRVec)