add partial spec and base for std::hash support

Signed-off-by: Tikhomirova, Kseniya <kseniya.tikhomirova@intel.com>

Author: KseniyaTikhomirova

libsycl/include/sycl/__impl/detail/obj_base.hpp

@@ -18,17 +18,20 @@
 #include <sycl/__impl/detail/config.hpp>
 
 #include <cassert>
+#include <optional>
 #include <type_traits>
 #include <utility>
 
 _LIBSYCL_BEGIN_NAMESPACE_SYCL
 
 namespace detail {
 
-template <class Impl, class SyclObject> class ObjBase {
+template <typename Impl, typename SyclObject> class ObjBase;
+template <typename Impl, typename SyclObject>
+class ObjBase<Impl &, SyclObject> {
 public:
   using ImplType = Impl;
-  using Base = ObjBase<Impl, SyclObject>;
+  using Base = ObjBase<Impl &, SyclObject>;
 
 protected:
   ImplType &impl;
@@ -57,6 +60,19 @@ Obj createSyclObjFromImpl(
   return Obj::Base::createSyclProxy(ImplObj);
 }
 
+// std::hash support (4.5.2. Common reference semantics)
+template <typename T> struct HashBase {
+  size_t operator()(const T &Obj) const {
+#ifdef __SYCL_DEVICE_ONLY__
+    (void)Obj;
+    return 0;
+#else
+    auto &Impl = sycl::detail::getSyclObjImpl(Obj);
+    return std::hash<std::decay_t<decltype(Impl)>>{}(Impl);
+#endif
+  }
+};
+
 } // namespace detail
 
 _LIBSYCL_END_NAMESPACE_SYCL

libsycl/include/sycl/__impl/platform.hpp

@@ -31,7 +31,7 @@ class platform_impl;
 
 // 4.6.2. Platform class
 class _LIBSYCL_EXPORT platform
-    : public detail::ObjBase<detail::platform_impl, platform> {
+    : public detail::ObjBase<detail::platform_impl &, platform> {
 public:
   /// Constructs a platform object that is a copy of the platform which contains
   /// the device returned by default_selector_v.
@@ -104,9 +104,13 @@ class _LIBSYCL_EXPORT platform
   typename detail::is_platform_info_desc<Param>::return_type
   get_info_impl() const;
 
-  friend detail::ObjBase<detail::platform_impl, platform>;
+  friend detail::ObjBase<detail::platform_impl &, platform>;
 }; // class platform
 
 _LIBSYCL_END_NAMESPACE_SYCL
 
+template <>
+struct std::hash<sycl::platform>
+    : public sycl::detail::HashBase<sycl::platform> {};
+
 #endif // _LIBSYCL___IMPL_PLATFORM_HPP