improve behavior if mdspan is enabled and alpaka_CUDA_EXPT_EXTENDED_L…

…AMBDA is disabled

- Instead automatically disabling mdspan if alpaka_CUDA_EXPT_EXTENDED_LAMBDA is off throw an error. That's more expected behavior than an enabled feature is off. Also the old behavior does not worked, because mdspan was still compiled and throws a compiler error.
- Fix bug, that the Clang as CUDA compiler was also affected by the CMake options alpaka_CUDA_EXPT_EXTENDED_LAMBDA, which sets a nvcc flag.

cmake/alpakaCommon.cmake

@@ -951,19 +951,16 @@ endif()
 
 if (NOT alpaka_USE_MDSPAN STREQUAL "OFF")
     if (MSVC AND (alpaka_CXX_STANDARD LESS 20))
-        message(WARNING "std::mdspan on MSVC requires C++20. Please enable C++20 via alpaka_CXX_STANDARD. Use of std::mdspan has been disabled.")
-        set(alpaka_USE_MDSPAN "OFF" CACHE STRING "Use std::mdspan with alpaka" FORCE)
+        message(FATAL_ERROR "std::mdspan on MSVC requires C++20. Please enable C++20 via alpaka_CXX_STANDARD. Use of std::mdspan has been disabled.")
     endif ()
 
     if (alpaka_ACC_GPU_CUDA_ENABLE AND (CMAKE_CUDA_COMPILER_ID STREQUAL "NVIDIA") AND (CMAKE_CXX_COMPILER_ID STREQUAL "Clang"))
         # this issue actually only occurs when the host compiler (not the CXX compiler) is clang, but cmake does not let us query the host compiler id
         # see: https://gitlab.kitware.com/cmake/cmake/-/issues/20901
-        message(WARNING "std::mdspan does not work with nvcc and clang as host compiler. Use of std::mdspan has been disabled.")
-        set(alpaka_USE_MDSPAN "OFF" CACHE STRING "Use std::mdspan with alpaka" FORCE)
+        message(FATAL_ERROR "std::mdspan does not work with nvcc and clang as host compiler. Use of std::mdspan has been disabled.")
     endif ()
 
-    if (alpaka_ACC_GPU_CUDA_ENABLE AND (NOT alpaka_CUDA_EXPT_EXTENDED_LAMBDA STREQUAL ON))
-        message(WARNING "std::mdspan requires nvcc's extended lambdas. Use of std::mdspan has been disabled.")
-        set(alpaka_USE_MDSPAN "OFF" CACHE STRING "Use std::mdspan with alpaka" FORCE)
+    if (alpaka_ACC_GPU_CUDA_ENABLE AND CMAKE_CUDA_COMPILER_ID STREQUAL "NVIDIA" AND (NOT alpaka_CUDA_EXPT_EXTENDED_LAMBDA STREQUAL ON))
+        message(FATAL_ERROR "std::mdspan requires nvcc's extended lambdas.")
     endif()
 endif()

script/job_generator/generate_job_yaml.py

@@ -345,11 +345,16 @@ def job_variables(job: Dict[str, Tuple[str, str]]) -> Dict[str, str]:
         variables["ALPAKA_CI_STDLIB"] = "libstdc++"
         variables["CMAKE_CUDA_ARCHITECTURES"] = job[SM_LEVEL][VERSION]
         variables["ALPAKA_CI_CUDA_VERSION"] = job[ALPAKA_ACC_GPU_CUDA_ENABLE][VERSION]
+        variables["alpaka_CUDA_EXPT_EXTENDED_LAMBDA"] = "OFF"
 
     if job[DEVICE_COMPILER][NAME] == NVCC:
         # general configuration, if nvcc is the CUDA compiler
         variables["ALPAKA_CI_CUDA_COMPILER"] = "nvcc"
 
+        # MdSpan requires alpaka_CUDA_EXPT_EXTENDED_LAMBDA
+        if job[MDSPAN][VERSION] == ON_VER:
+            variables["alpaka_CUDA_EXPT_EXTENDED_LAMBDA"] = "ON"
+
         # configuration, if GCC is the CUDA host compiler
         if job[HOST_COMPILER][NAME] == GCC:
             variables["ALPAKA_CI_CXX"] = "g++"