[SYCL] Assignment operators for 0-dim accessors (#8069)

This patch implements and provides tests for assignment operators for
the non-read-only, 0-dimensional variants of the buffer accessor, local
accessor, and host accessor classes.

Implemented according to [Table 72 of the SYCL 2020
specification](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_interface_for_buffer_host_accessors)
and the following PRs:
KhronosGroup/SYCL-Docs#278,
KhronosGroup/SYCL-Docs#334

Author: Andrew Lamzed-Short

sycl/include/sycl/accessor.hpp

@@ -2012,6 +2012,22 @@ class __SYCL_EBO __SYCL_SPECIAL_CLASS __SYCL_TYPE(accessor) accessor :
     return *(getQualifiedPtr() + LinearIndex);
   }
 
+  template <int Dims = Dimensions,
+            typename = detail::enable_if_t<AccessMode != access_mode::atomic &&
+                                           !IsAccessReadOnly && Dims == 0>>
+  const accessor &operator=(const value_type &Other) const {
+    *getQualifiedPtr() = Other;
+    return *this;
+  }
+
+  template <int Dims = Dimensions,
+            typename = detail::enable_if_t<AccessMode != access_mode::atomic &&
+                                           !IsAccessReadOnly && Dims == 0>>
+  const accessor &operator=(value_type &&Other) const {
+    *getQualifiedPtr() = std::move(Other);
+    return *this;
+  }
+
   template <int Dims = Dimensions,
             typename = detail::enable_if_t<(Dims > 0) && (IsAccessAnyWrite ||
                                                           IsAccessReadOnly)>>
@@ -2780,6 +2796,20 @@ class __SYCL_EBO __SYCL_SPECIAL_CLASS __SYCL_TYPE(local_accessor) local_accessor
     return Property();
 #endif
   }
+
+  template <int Dims = Dimensions, typename = detail::enable_if_t<
+                                       !std::is_const_v<DataT> && Dims == 0>>
+  const local_accessor &operator=(const value_type &Other) const {
+    *local_acc::getQualifiedPtr() = Other;
+    return *this;
+  }
+
+  template <int Dims = Dimensions, typename = detail::enable_if_t<
+                                       !std::is_const_v<DataT> && Dims == 0>>
+  const local_accessor &operator=(value_type &&Other) const {
+    *local_acc::getQualifiedPtr() = std::move(Other);
+    return *this;
+  }
 };
 
 /// Image accessors.
@@ -2957,6 +2987,7 @@ class __SYCL_EBO host_accessor
                              access::placeholder::false_t>;
 
   constexpr static int AdjustedDim = Dimensions == 0 ? 1 : Dimensions;
+  constexpr static bool IsAccessReadOnly = AccessMode == access::mode::read;
 
   template <typename T, int Dims>
   struct IsSameAsBuffer
@@ -3111,6 +3142,23 @@ class __SYCL_EBO host_accessor
       const detail::code_location CodeLoc = detail::code_location::current())
       : host_accessor(BufferRef, CommandGroupHandler, AccessRange, AccessOffset,
                       PropertyList, CodeLoc) {}
+
+  template <int Dims = Dimensions,
+            typename = detail::enable_if_t<AccessMode != access_mode::atomic &&
+                                           !IsAccessReadOnly && Dims == 0>>
+  const host_accessor &
+  operator=(const typename AccessorT::value_type &Other) const {
+    *AccessorT::getQualifiedPtr() = Other;
+    return *this;
+  }
+
+  template <int Dims = Dimensions,
+            typename = detail::enable_if_t<AccessMode != access_mode::atomic &&
+                                           !IsAccessReadOnly && Dims == 0>>
+  const host_accessor &operator=(typename AccessorT::value_type &&Other) const {
+    *AccessorT::getQualifiedPtr() = std::move(Other);
+    return *this;
+  }
 };
 
 template <typename DataT, int Dimensions, typename AllocatorT>

sycl/test/basic_tests/accessor/host-acc-assign-op.cpp

@@ -0,0 +1,15 @@
+// RUN: %clangxx -fsycl -fsycl-targets=%sycl_triple %s -o %t.out
+// RUN: %t.out
+
+#include <sycl/sycl.hpp>
+
+int main() {
+  int Expected = 64;
+  int Data = 32;
+  sycl::buffer<int, 1> DataBuffer(&Data, sycl::range<1>(1));
+  sycl::host_accessor<int, 0> HostAcc{DataBuffer};
+
+  HostAcc = Expected;
+
+  assert(HostAcc == Expected);
+}