Only LLVM6 and higher contain the necessary intrinsics.

include/hip/hcc_detail/hip_fp16.h

@@ -29,7 +29,7 @@ THE SOFTWARE.
     #include <utility>
 #endif
 
-#if defined(__clang__) && (__clang_major__ > 3)
+#if defined(__clang__) && (__clang_major__ > 5)
     typedef _Float16 _Float16_2 __attribute__((ext_vector_type(2)));
 
     struct __half_raw {