platform: add the device_selector constructor

this needs to be in the tail due to device_selector
interactions.

include/CL/sycl/platform.hpp

@@ -88,10 +88,7 @@ class platform
 
        Returns errors via the SYCL exception class.
   */
-  explicit platform(const device_selector &dev_selector) {
-    detail::unimplemented();
-  }
-
+  explicit platform(const device_selector &dev_selector);
 
 #ifdef TRISYCL_OPENCL
   /** Returns the cl_platform_id of the underlying OpenCL platform

include/CL/sycl/platform/detail/platform_tail.hpp

@@ -11,6 +11,24 @@
 namespace cl {
 namespace sycl {
 
+inline
+platform::platform(const device_selector &device_selector) {
+  platform host_platform = {};
+#ifdef TRISYCL_OPENCL
+  if (host_platform.implementation->get_devices(device_selector).empty()) {
+    for (const auto &d : boost::compute::system::platforms()) {
+      auto clplatform = cl::sycl::platform { d };
+      auto devices = clplatform.implementation->get_devices(device_selector);
+      if (!devices.empty()) {
+        *this = std::move(clplatform);
+        return;
+      }
+    }
+  }
+#endif
+  *this = std::move(host_platform);
+}
+
 inline vector_class<device>
 platform::get_devices(info::device_type device_type) const {
   return implementation->get_devices(device_type_selector { device_type });