[SYCL] Check if the underlying buffer type is std::string

The implementation should diagnose an error if the underlying type "T"
of a buffer is not device copyable. Currently, we cannot implement a
check for all not device copyable types since sycl::vec is not
trivially copyable on the host and therefore all the types that contain
sycl::vec are also not trivially copyable as well as not device
copyable. The check is implemented for std::string only.

Author: Pavel Samolysov

sycl/include/CL/sycl/buffer.hpp

@@ -44,6 +44,10 @@ template <typename T, int dimensions = 1,
           typename __Enabled = typename detail::enable_if_t<(dimensions > 0) &&
                                                             (dimensions <= 3)>>
 class buffer {
+  // TODO check is_device_copyable<T>::value after converting sycl::vec into a
+  // trivially copyable class.
+  static_assert(!std::is_same<T, std::string>::value,
+      "The underlying data type of a buffer 'T' must be device copyable");
 public:
   using value_type = T;
   using reference = value_type &;

sycl/test/basic_tests/buffer/buffer_for_not_device_copyable.cpp

@@ -0,0 +1,19 @@
+// RUN: %clangxx -fsycl -fsyntax-only -Xclang -verify %s -Xclang -verify-ignore-unexpected=note,warning
+
+#include <string>
+#include <sycl/sycl.hpp>
+
+using namespace sycl;
+
+int main() {
+  static_assert(is_device_copyable_v<int>);
+  std::vector<int> iv(5, 1);
+  buffer b1(iv.data(), range<1>(5));
+
+  static_assert(!is_device_copyable_v<std::string>);
+  std::vector<std::string> sv{"hello", "sycl", "world"};
+  buffer b2(sv.data(), range<1>(3));
+  //expected-error@CL/sycl/buffer.hpp:* {{"The underlying data type of a buffer 'T' must be device copyable"}}
+
+  return 0;
+}