GPU: Workaround for Clang Frontend issue

This is fixed with Clang >= 20 and C++23 (P2280R4)

Author: davidrohr

GPU/GPUTracking/Base/cuda/GPUReconstructionCUDAGenRTC.cxx

@@ -38,7 +38,11 @@ int32_t GPUReconstructionCUDA::genRTC(std::string& filename, uint32_t& nCompile)
 {
   std::string rtcparam = std::string("#define GPUCA_RTC_CODE\n") +
                          std::string(GetProcessingSettings().rtc.optSpecialCode ? "#define GPUCA_RTC_SPECIAL_CODE(...) __VA_ARGS__\n" : "#define GPUCA_RTC_SPECIAL_CODE(...)\n") +
+#ifndef GPUCA_HIP_WORKAROUND_CONSTEXPR // TODO: Fixme, once we have C++ P2280R4 in Clang
                          std::string(GetProcessingSettings().rtc.optConstexpr ? "#define GPUCA_RTC_CONSTEXPR constexpr\n" : "#define GPUCA_RTC_CONSTEXPR\n") +
+#else
+                         std::string("#define GPUCA_RTC_CONSTEXPR\n") +
+#endif
                          GPUParamRTC::generateRTCCode(param(), GetProcessingSettings().rtc.optConstexpr);
   if (filename == "") {
     filename = "/tmp/o2cagpu_rtc_";

GPU/GPUTracking/Base/hip/CMakeLists.txt

@@ -270,3 +270,8 @@ add_dependencies(GPUTrackingHIPExternalProvider O2::GPUTracking) # must not depe
 if(NOT DEFINED GPUCA_HIP_HIPIFY_FROM_CUDA OR "${GPUCA_HIP_HIPIFY_FROM_CUDA}")
   add_dependencies(GPUTrackingHIPExternalProvider ${MODULE}_HIPIFIED)
 endif()
+
+set_source_files_properties("${GPUCA_HIP_SOURCE_DIR}/GPUReconstructionHIPGenRTC.cxx"
+TARGET_DIRECTORY O2::GPUTrackingHIP
+PROPERTIES
+COMPILE_DEFINITIONS "GPUCA_HIP_WORKAROUND_CONSTEXPR")