Error out for const scalar return type in reduction

[masterleinad]

Fixes #2090. This pull request also makes sure that we are calling the intended overload for parallel_reduce, i.e., the return value is only copied if the return type is a View or a reducer.

[masterleinad]

The linked example now gives

In file included from /tmp/kokkos-alt/core/src/Kokkos_Parallel.hpp:180:0,
                 from /tmp/kokkos-alt/core/src/Kokkos_OpenMP.hpp:62,
                 from /tmp/kokkos-alt/core/src/decl/Kokkos_Declare_OPENMP.hpp:49,
                 from /tmp/kokkos-alt/example/build_cmake_in_tree/build/kokkos/KokkosCore_Config_DeclareBackend.hpp:47,
                 from /tmp/kokkos-alt/core/src/Kokkos_Core.hpp:56,
                 from /tmp/kokkos-alt/example/build_cmake_in_tree/cmake_example.cpp:1:
/tmp/kokkos-alt/core/src/Kokkos_Parallel_Reduce.hpp: In instantiation of ‘std::enable_if_t<(!(Kokkos::is_view<Arg>::value || Kokkos::is_reducer<ReturnType>::value))> Kokkos::parallel_reduce(const size_t&, const FunctorType&, ReturnType&) [with FunctorType = main()::<lambda(int, int&)>; ReturnType = const int; std::enable_if_t<(!(Kokkos::is_view<Arg>::value || Kokkos::is_reducer<ReturnType>::value))> = void; size_t = long unsigned int]’:
/tmp/kokkos-alt/example/build_cmake_in_tree/cmake_example.cpp:8:68:   required from here
/tmp/kokkos-alt/core/src/Kokkos_Parallel_Reduce.hpp:1778:3: error: static assertion failed: A const return type is only allowed for a View or reducer return type!
   static_assert(
   ^~~~~~~~~~~~~
/tmp/kokkos-alt/core/src/Kokkos_Parallel_Reduce.hpp:1588:22: error: ‘static void Kokkos::Impl::ParallelReduceAdaptor<PolicyType, FunctorType, ReturnType>::execute(const string&, const PolicyType&, const FunctorType&, ReturnType&) [with PolicyType = Kokkos::RangePolicy<Kokkos::OpenMP>; FunctorType = main()::<lambda(int, int&)>; ReturnType = const int; std::__cxx11::string = std::__cxx11::basic_string<char>]’, declared using local type ‘const main()::<lambda(int, int&)>’, is used but never defined [-fpermissive]
   static inline void execute(const std::string& label, const PolicyType& policy,
                      ^~~~~~~
/tmp/kokkos-alt/core/src/Kokkos_Parallel_Reduce.hpp:1588:22: warning: ‘static void Kokkos::Impl::ParallelReduceAdaptor<PolicyType, FunctorType, ReturnType>::execute(const string&, const PolicyType&, const FunctorType&, ReturnType&) [with PolicyType = Kokkos::RangePolicy<Kokkos::OpenMP>; FunctorType = main()::<lambda(int, int&)>; ReturnType = const int]’ used but never defined
CMakeFiles/example.dir/build.make:62: recipe for target 'CMakeFiles/example.dir/cmake_example.cpp.o' failed
make[2]: *** [CMakeFiles/example.dir/cmake_example.cpp.o] Error 1
CMakeFiles/Makefile2:464: recipe for target 'CMakeFiles/example.dir/all' failed

for me.

[dalg24]

Did you check in our Wiki how we actually refer to the reducer argument? I feel like "return type" might be confusing

[masterleinad]

I changed it to "reduction result type".

[masterleinad]

I needed to also allow pointers to be passed in as const.