[SYCL] Disable span reductions before C++17

Necessary because span.hpp is only available >= C++17.

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/include/CL/sycl/reduction.hpp

@@ -100,6 +100,7 @@ reduction(T *Var, const T &Identity, BinaryOperation Combiner,
   return {Var, Identity, Combiner, InitializeToIdentity};
 }
 
+#if __cplusplus >= 201703L
 /// Constructs a reduction object using the reduction variable referenced by
 /// the given sycl::span \p Span, reduction operation \p Combiner, and
 /// optional reduction properties.
@@ -149,6 +150,7 @@ reduction(span<T, Extent> Span, const T &Identity, BinaryOperation Combiner,
       PropList.has_property<property::reduction::initialize_to_identity>();
   return {Span, Identity, Combiner, InitializeToIdentity};
 }
+#endif
 
 } // namespace sycl
 } // __SYCL_INLINE_NAMESPACE(cl)

sycl/include/sycl/ext/oneapi/reduction.hpp

@@ -413,6 +413,7 @@ template <typename T, class BinaryOperation> class reducer_element {
 /// Specialization of 'reducer' class for array reductions accepting a span
 /// in cases where the identity value is not known.
 // TODO: Support alternative algorithms for different values of Extent
+#if __cplusplus >= 201703L
 template <typename T, size_t Extent, class BinaryOperation>
 class reducer<span<T, Extent>, BinaryOperation,
               enable_if_t<!IsKnownIdentityOp<T, BinaryOperation>::value>> {
@@ -571,6 +572,7 @@ class reducer<span<T, Extent>, BinaryOperation,
 private:
   marray<T, Extent> MValue;
 };
+#endif
 
 /// Base non-template class which is a base class for all reduction
 /// implementation classes. It is needed to detect the reduction classes.
@@ -1026,6 +1028,7 @@ class reduction_impl : private reduction_impl_base {
 /// Specialization of reduction_impl for spans
 // TODO: Consider hoisting some functionality into reduction_impl_base
 // TODO: Refactor to remove IsUSM and IsPlaceholder arguments?
+#if __cplusplus >= 201703L
 template <typename T, size_t Extent, class BinaryOperation, int Dims,
           bool IsUSM, access::placeholder IsPlaceholder>
 class reduction_impl<span<T, Extent>, BinaryOperation, Dims, IsUSM,
@@ -1273,6 +1276,7 @@ class reduction_impl<span<T, Extent>, BinaryOperation, Dims, IsUSM,
 
   bool InitializeToIdentity;
 };
+#endif
 
 /// These are the forward declaration for the classes that help to create
 /// names for additional kernels. It is used only when there are