[SYCL] Adjust accessor::value_type in accordance with the spec (#7096)

`value_type` should be defined as `const DataT` for read-only accessors.
This also makes `accessor::iterator` of a read-only accessor non-writeable
as it should be in accordance with the spec.

sycl/include/sycl/accessor.hpp

@@ -1205,13 +1205,17 @@ class __SYCL_SPECIAL_CLASS __SYCL_TYPE(accessor) accessor :
   friend class sycl::ext::intel::esimd::detail::AccessorPrivateProxy;
 
 public:
-  using value_type = DataT;
+  // 4.7.6.9.1. Interface for buffer command accessors
+  // value_type is defined as const DataT for read_only accessors, DataT
+  // otherwise
+  using value_type = typename std::conditional<AccessMode == access_mode::read,
+                                               const DataT, DataT>::type;
   using reference = DataT &;
   using const_reference = const DataT &;
 
-  using iterator = typename detail::accessor_iterator<DataT, Dimensions>;
+  using iterator = typename detail::accessor_iterator<value_type, Dimensions>;
   using const_iterator =
-      typename detail::accessor_iterator<const DataT, Dimensions>;
+      typename detail::accessor_iterator<const value_type, Dimensions>;
   using difference_type =
       typename std::iterator_traits<iterator>::difference_type;
 

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

@@ -0,0 +1,57 @@
+// 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 AccessorT =
+      typename sycl::accessor<DataT, Dimensions, AccessMode, AccessTarget>;
+  static_assert(std::is_same_v<sycl::detail::accessor_iterator<
+                                   typename AccessorT::value_type, Dimensions>,
+                               typename AccessorT::iterator>);
+
+  static_assert(
+      std::is_same_v<sycl::detail::accessor_iterator<
+                         const typename AccessorT::value_type, Dimensions>,
+                     typename AccessorT::const_iterator>);
+}
+
+template <typename DataT, int Dimensions, sycl::access_mode AccessMode>
+void check_host_accessor() {
+  using AccessorT = typename sycl::host_accessor<DataT, Dimensions, AccessMode>;
+  static_assert(std::is_same_v<sycl::detail::accessor_iterator<
+                                   typename AccessorT::value_type, Dimensions>,
+                               typename AccessorT::iterator>);
+
+  static_assert(
+      std::is_same_v<sycl::detail::accessor_iterator<
+                         const typename AccessorT::value_type, Dimensions>,
+                     typename AccessorT::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;
+}