auto v = MatMul(scope, t, u);
    TF_ASSERT_OK(scope.status());
    CHECK_NOTNULL(v.node());

    auto x = Const(scope, {{5.0, 6.0}, {7.0, 8.0}});
    auto y = Const(scope, {{1.0, 0.0}, {0.0, 1.0}});
    auto z = MatMul(scope, x, y);
    TF_ASSERT_OK(scope.status());
    CHECK_NOTNULL(z.node());

    if (expected) {
      // Construct backward graph.

  Session* new_session;
  Status status = NewSession(session_options, &new_session);
  TF_CHECK_OK(status) << status;
  impl_.reset(new Impl(new_session, scope.graph_as_shared_ptr()));
  CHECK_NOTNULL(impl()->session_.get());
}

// Define destructor here so we can forward declare `Impl` in client_session.h.
// If we define a dtor in the header file or use the default dtor,
// unique_ptr<Impl> needs the complete type.