[SYCL] Turn read-only accessor iterator into const_iterator

sycl/include/sycl/accessor.hpp

@@ -1209,7 +1209,12 @@ class __SYCL_SPECIAL_CLASS __SYCL_TYPE(accessor) accessor :
   using reference = DataT &;
   using const_reference = const DataT &;
 
-  using iterator = typename detail::accessor_iterator<DataT, Dimensions>;
+  // When accessor is read-only, we should not be able to modify it through the
+  // iterator and therefore, effectively returning const_iterator here
+  using iterator = typename detail::accessor_iterator<
+      typename std::conditional<AccessMode == access_mode::read, const DataT,
+                                DataT>::type,
+      Dimensions>;
   using const_iterator =
       typename detail::accessor_iterator<const DataT, Dimensions>;
   using difference_type =

sycl/test/basic_tests/accessor/iterator-member-types.cpp

@@ -0,0 +1,65 @@
+// RUN: %clangxx -fsycl -c %s
+//
+// Purpose of this test is to check that [accessor|host_accessor]::iterator and
+// ::const_iterator are aliased to the correct type.
+// FIXME: extend this test to also check ::reverse_iterator and
+// ::const_reverse_iterator
+#include <sycl/sycl.hpp>
+
+#include <type_traits>
+
+template <typename DataT, int Dimensions, sycl::access_mode AccessMode,
+          sycl::target AccessTarget = sycl::target::device>
+void check_accessor() {
+  using IteratorDataT =
+      typename std::conditional<AccessMode == sycl::access_mode::read,
+                                const DataT, DataT>::type;
+  static_assert(
+      std::is_same_v<sycl::detail::accessor_iterator<IteratorDataT, Dimensions>,
+                     typename sycl::accessor<DataT, Dimensions, AccessMode,
+                                             AccessTarget>::iterator>);
+
+  // const_iterator should always be specialized as const DataT
+  static_assert(std::is_same_v<
+                sycl::detail::accessor_iterator<const DataT, Dimensions>,
+                typename sycl::accessor<DataT, Dimensions, AccessMode,
+                                             AccessTarget>::const_iterator>);
+}
+
+template <typename DataT, int Dimensions, sycl::access_mode AccessMode>
+void check_host_accessor() {
+  using IteratorDataT =
+      typename std::conditional<AccessMode == sycl::access_mode::read,
+                                const DataT, DataT>::type;
+  static_assert(
+      std::is_same_v<sycl::detail::accessor_iterator<IteratorDataT, Dimensions>,
+                     typename sycl::host_accessor<DataT, Dimensions,
+                                                  AccessMode>::iterator>);
+
+  // const_iterator should always be specialized as const DataT
+  static_assert(
+      std::is_same_v<sycl::detail::accessor_iterator<const DataT, Dimensions>,
+                     typename sycl::host_accessor<DataT, Dimensions,
+                                                  AccessMode>::const_iterator>);
+}
+
+struct user_defined_t {
+  char c;
+  float f;
+  double d;
+  sycl::vec<int, 3> v3;
+};
+
+int main() {
+
+  check_accessor<int, 1, sycl::access_mode::read>();
+  check_accessor<float, 2, sycl::access_mode::write>();
+  check_accessor<user_defined_t, 3, sycl::access_mode::read_write>();
+
+  check_host_accessor<user_defined_t, 1, sycl::access_mode::read>();
+  check_host_accessor<int, 2, sycl::access_mode::write>();
+  check_host_accessor<float, 3, sycl::access_mode::read_write>();
+
+  return 0;
+
+}