Adding Kokkos::to_array (#6375)

* Added Kokkos::to_Array for Kokkos::Array (analogous to std::to_array for
std::array)

* Changed KOKKOS_INLINE_FUNCTION into KOKKOS_FUNCTION (for inline template
functions)

* Added back to_Array tests

* Fixed surperfluous KOKKOS_INLINE_FUNCTION

* Fix for "set but not used" error for variables in a static_assert under
nvcc

* Guard against Kokkos::to_Array<long>({0, 1, 3}), as that is not
supported under gcc8

* Renamed to_Array to to_array and to_Array_impl to to_array_impl

core/src/Kokkos_Array.hpp

@@ -356,6 +356,32 @@ struct KOKKOS_DEPRECATED
 template <typename T, typename... Us>
 Array(T, Us...)->Array<T, 1 + sizeof...(Us)>;
 
+namespace Impl {
+
+template <typename T, size_t N, size_t... I>
+KOKKOS_FUNCTION constexpr Array<std::remove_cv_t<T>, N> to_array_impl(
+    T (&a)[N], std::index_sequence<I...>) {
+  return {{a[I]...}};
+}
+
+template <typename T, size_t N, size_t... I>
+KOKKOS_FUNCTION constexpr Array<std::remove_cv_t<T>, N> to_array_impl(
+    T(&&a)[N], std::index_sequence<I...>) {
+  return {{std::move(a[I])...}};
+}
+
+}  // namespace Impl
+
+template <typename T, size_t N>
+KOKKOS_FUNCTION constexpr auto to_array(T (&a)[N]) {
+  return Impl::to_array_impl(a, std::make_index_sequence<N>{});
+}
+
+template <typename T, size_t N>
+KOKKOS_FUNCTION constexpr auto to_array(T(&&a)[N]) {
+  return Impl::to_array_impl(std::move(a), std::make_index_sequence<N>{});
+}
+
 }  // namespace Kokkos
 
 //<editor-fold desc="Support for structured binding">

core/unit_test/TestArray.cpp

@@ -18,6 +18,11 @@
 
 namespace {
 
+// nvcc errors on variables only used in static_asserts
+// Passing those variables to this function should eliminate the warning
+template <typename... Ts>
+KOKKOS_FUNCTION constexpr void maybe_unused(Ts&&...) {}
+
 KOKKOS_FUNCTION constexpr bool test_array() {
   constexpr Kokkos::Array<int, 3> a{{1, 2}};
 
@@ -160,4 +165,29 @@ constexpr bool test_array_specialization_kokkos_swap() {
 
 static_assert(test_array_specialization_kokkos_swap());
 
+constexpr bool test_to_array() {
+  // copies a string literal
+  [[maybe_unused]] auto a1 = Kokkos::to_array("foo");
+  static_assert(a1.size() == 4);
+  maybe_unused(a1);
+
+  // deduces both element type and length
+  [[maybe_unused]] auto a2 = Kokkos::to_array({0, 2, 1, 3});
+  static_assert(std::is_same_v<decltype(a2), Kokkos::Array<int, 4>>);
+  maybe_unused(a2);
+
+// gcc8 doesn't support the implicit conversion
+#if !defined(KOKKOS_COMPILER_GNU) || (KOKKOS_COMPILER_GNU >= 910)
+  // deduces length with element type specified
+  // implicit conversion happens
+  [[maybe_unused]] auto a3 = Kokkos::to_array<long>({0, 1, 3});
+  static_assert(std::is_same_v<decltype(a3), Kokkos::Array<long, 3>>);
+  maybe_unused(a3);
+#endif
+
+  return true;
+}
+
+static_assert(test_to_array());
+
 }  // namespace