[SYCL] Fix bit_cast for half type (#4603)

After removing of half type trivial default constructor the bit_cast to sycl::half became impossible. Added helper struct for that case
Signed-off-by: mdimakov maxim.dimakov@intel.com

Author: maximdimakov

sycl/include/CL/sycl/bit_cast.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <cstdint>
 #include <type_traits>
 
 #if __cpp_lib_bit_cast
@@ -20,6 +21,21 @@ namespace sycl {
 // forward decl
 namespace detail {
 inline void memcpy(void *Dst, const void *Src, std::size_t Size);
+
+namespace half_impl {
+class half;
+}
+using half = cl::sycl::detail::half_impl::half;
+
+template <typename T>
+#ifdef __SYCL_DEVICE_ONLY__
+using lowering_half_type =
+    typename std::conditional<std::is_same<T, half>::value, _Float16, T>::type;
+#else
+using lowering_half_type =
+    typename std::conditional<std::is_same<T, half>::value, std::uint16_t,
+                              T>::type;
+#endif
 }
 
 template <typename To, typename From>
@@ -41,7 +57,8 @@ constexpr
 #if __has_builtin(__builtin_bit_cast)
   return __builtin_bit_cast(To, from);
 #else  // __has_builtin(__builtin_bit_cast)
-  static_assert(std::is_trivially_default_constructible<To>::value,
+  static_assert(std::is_trivially_default_constructible<
+                    typename sycl::detail::lowering_half_type<To>>::value,
                 "To must be trivially default constructible");
   To to;
   sycl::detail::memcpy(&to, &from, sizeof(To));

sycl/test/regression/bit_cast.hpp

@@ -0,0 +1,12 @@
+// The purpose of this test is to check that the following code can be
+// successfully compiled
+
+#include <CL/sycl.hpp>
+
+int main() {
+  sycl::half x;
+  int16_t a = sycl::bit_cast<int16_t>(x);
+  sycl::bit_cast<sycl::half>(a);
+
+  return 0;
+}

sycl/test/regression/bit_cast_lin.cpp

@@ -0,0 +1,4 @@
+// RUN: %clangxx -fsycl -fsycl-host-compiler=g++ -fsycl-host-compiler-options='-std=c++17' %s -o %t.out
+// UNSUPPORTED: windows
+
+#include "bit_cast.hpp"

sycl/test/regression/bit_cast_win.cpp

@@ -0,0 +1,4 @@
+// RUN: %clangxx -fsycl -fsycl-host-compiler=cl -fsycl-host-compiler-options='/std:c++17' %s -o %t.out
+// UNSUPPORTED: linux
+
+#include "bit_cast.hpp"