Fix building with NVCC as the CXX compiler while the CUDA backend is not enabled

[dalg24]

Fix #5093

@ndellingwood would you please add a nightly build with -DCMAKE_CXX_COMPILER=<prefix>/bin/nvcc_wrapper -DKokkos_ENABLE_CUDA=OFF?

[JBludau]

Is the guard in the TestNumericTraits.hpp really related to the issue or something that came up along the way? Do we know why it needs to be guarded?

sry, I fucked up with the brackets

[dalg24]

Is the guard in the TestNumericTraits.hpp really related to the issue or something that came up along the way? Do we know why it needs to be guarded?

First change is rather obvious, without it you get an error because the OpenMPTarget execution space is not defined.

Second one we hit a known bug

kokkos/core/unit_test/TestNumericTraits.hpp

Lines 600 to 601 in 4e6c254

	// Workaround compiler issue error: expression must have a constant value
	// See kokkos/kokkos#4574

CUDA_VERSION is not defined so I opted for "unconditional" removal when using NVCC with the CUDA backend disabled

[masterleinad]

Looks OK to me.

[JBludau]

Looks good to me

[ndellingwood]

@ndellingwood would you please add a nightly build with -DCMAKE_CXX_COMPILER=<prefix>/bin/nvcc_wrapper -DKokkos_ENABLE_CUDA=OFF?

Will do

[ndellingwood]

@dalg24 I'm testing a local build with the desired config (compile with nvcc_wrapper, cuda backend disabled) to prep for the new nightly build.
Successful compilation (tested with cuda/10.1+gcc/7.3.0), I encountered warning messages related to desul:

/ascldap/users/ndellin/kokkos/core/src/../../tpls/desul/include/desul/atomics/Compare_Exchange_ScopeCaller.hpp(39): warning: calling a constexpr __host__ function("operator==") from a __host__ __device__ function("atomic_compare_exchange") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.
          detected during:
            instantiation of "T desul::atomic_compare_exchange(T *, T, T, desul::MemoryOrderRelaxed, desul::MemoryScopeCaller) [with T=Kokkos::Impl::HostThreadTeamData::pair_int_t]" 
/ascldap/users/ndellin/kokkos/core/src/Kokkos_Atomics_Desul_Wrapper.hpp(227): here
            instantiation of "T Kokkos::atomic_compare_exchange(T *, desul::Impl::dont_deduce_this_parameter_t<const T>, desul::Impl::dont_deduce_this_parameter_t<const T>) [with T=Kokkos::Impl::HostThreadTeamData::pair_int_t]" 
/ascldap/users/ndellin/kokkos/core/src/impl/Kokkos_HostThreadTeam.cpp(225): here

/ascldap/users/ndellin/kokkos/core/src/../../tpls/desul/include/desul/atomics/Compare_Exchange_ScopeCaller.hpp(39): warning: calling a constexpr __host__ function from a __host__ __device__ function is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.
          detected during:
            instantiation of "T desul::atomic_compare_exchange(T *, T, T, desul::MemoryOrderRelaxed, desul::MemoryScopeCaller) [with T=Kokkos::Impl::HostThreadTeamData::pair_int_t]" 
/ascldap/users/ndellin/kokkos/core/src/Kokkos_Atomics_Desul_Wrapper.hpp(227): here
            instantiation of "T Kokkos::atomic_compare_exchange(T *, desul::Impl::dont_deduce_this_parameter_t<const T>, desul::Impl::dont_deduce_this_parameter_t<const T>) [with T=Kokkos::Impl::HostThreadTeamData::pair_int_t]" 
/ascldap/users/ndellin/kokkos/core/src/impl/Kokkos_HostThreadTeam.cpp(225): here

Should I open an issue (and if so does this go to the kokkos repo or an external desul repo)?

[dalg24]

Plz add to the developer meeting agenda today. It is technically a desul issue, which detect CUDA rather than taking it as a configuration option.