Add support for Kokkos::fence in the fwd mode

Although this function doesn't need to be differentiated and
is correctly used by Clad automatically, this custom pushforward
prevents Clad from throwing a warning during that.

include/clad/Differentiator/KokkosBuiltins.h

@@ -67,6 +67,11 @@ inline void resize_pushforward(const I& arg, View& v, const size_t n0,
   ::Kokkos::resize(arg, d_v, n0, n1, n2, n3, n4, n5, n6, n7);
 }
 
+/// Fence
+template <typename S> void fence_pushforward(const S& s, const S& /*d_s*/) {
+  ::Kokkos::fence(s);
+}
+
 /// Parallel for
 template <class... PolicyParams, class FunctorType> // range policy
 void parallel_for_pushforward(

unittests/Kokkos/ParallelFor.cpp

@@ -102,6 +102,22 @@ template <typename View> struct Foo {
   void operator()(const int i) const { res(i) = x * i; }
 };
 
+double parallel_for_functor_simplest_case_fence(double x) {
+  Kokkos::View<double[5], Kokkos::HostSpace> res("res");
+
+  // Kokkos::fence("named fence"); // Does not work on some versions of Kokkos.
+
+  Foo<Kokkos::View<double[5], Kokkos::HostSpace>> f(res, x);
+
+  f(0); // FIXME: this is a workaround to put Foo::operator() into the
+        // differentiation plan. This needs to be solved in clad.
+
+  Kokkos::parallel_for(5, f);
+  Kokkos::fence();
+
+  return res(3);
+}
+
 double parallel_for_functor_simplest_case_intpol(double x) {
   Kokkos::View<double[5], Kokkos::HostSpace> res("res");
 
@@ -191,6 +207,10 @@ double parallel_for_functor_simplest_case_mdpol_space_and_anon(double x) {
 TEST(ParallelFor, FunctorSimplestCases) {
   const double eps = 1e-8;
 
+  auto df0 = clad::differentiate(parallel_for_functor_simplest_case_fence, 0);
+  for (double x = 3; x <= 5; x += 1)
+    EXPECT_NEAR(df0.execute(x), 3, eps);
+
   auto df1 = clad::differentiate(parallel_for_functor_simplest_case_intpol, 0);
   for (double x = 3; x <= 5; x += 1)
     EXPECT_NEAR(df1.execute(x), 3, eps);