Move Kokkos::reduction_identity into its own header file

[dalg24]

Move reduction_identity from <Kokkos_NumericTraits.hpp> to a new (public?) header <Kokkos_ReductionIdentity.hpp>

Admittedly this may break code downstream :/

[JBludau]

looks good.

[masterleinad]

Let's see if someone complains. In the end, reduction_identity should only be relevant if people call parallel_reduce but, of course, they might just have included this header for specializing reduction_identity. Anyway, if we merge early, we can hope that users complain early and it would be easy to revert.

Not sure I was clear enough

[dalg24]

Let's see if someone complains. In the end, reduction_identity should only be relevant if people call parallel_reduce but, of course, they might just have included this header for specializing reduction_identity. Anyway, if we merge early, we can hope that users complain early and it would be easy to revert.

I was not clear enough. We know that it will break code downstream (for instance KokkosKernels and ArborX). If we decide to merge it, we are not doing it to see if people complain loud enough and revert, we expect them to update their code.
The main question is do we agree that it is the right thing to do.

edit actually I looked closer and it wouldn't actually break KokkosKernels. The source files that use Kokkos::reduction_identity include <Kokkos_Core.hpp>

[masterleinad]

If this breaks too many projects, we could, of course, include ReductionIdentity.hpp in Kokkos_NumericTraits.hpp again.

[dalg24]

If this breaks too many projects, we could, of course, include ReductionIdentity.hpp in Kokkos_NumericTraits.hpp again.

I would only consider it if guarded with KOKKOS_ENABLE_DEPRECATED_CODE_4

[dalg24]

In Trilinos
https://github.com/trilinos/Trilinos/blob/597f531b9be215f69c06b58c97bbd6a2bd0ccddf/packages/stokhos/src/sacado/kokkos/pce/Sacado_UQ_PCE.hpp#L1315-L1336
https://github.com/trilinos/Trilinos/blob/597f531b9be215f69c06b58c97bbd6a2bd0ccddf/packages/stokhos/src/sacado/kokkos/vector/Sacado_MP_Vector.hpp#L2063-L2084
would need to be updated. That's all I could find.

[Rombur]

I think it's a good change but we should deprecate the current code since we know it will break user code.

[dalg24]

I think it's a good change but we should deprecate the current code since we know it will break user code.

OK as a compromise what if I include the new <Kokkos_ReductionIdentity.hpp> header from <Kokkos_NumericTraits.hpp> (96a9c76) when Kokkos_ENABLE_DEPRECATED_CODE_4 is ON