Update to kokkos5 and c++20

[tcclevenger]

Update EKAT to Kokkos 5.0.1 and require C++20.

The most notable change here is the option Kokkos_ENABLE_IMPL_VIEW_LEGACY=ON. Right now we need this since EKAT, Hommexx, and EAMxx use legacy features to subview, in particular. This needs to be addressed as, an impl feature, it will disappear w/o deprecation.

Motivation

This will allow MALI to use updated Trilinos and EAMxx to use modern C++ features.

Testing

CI already uses C++20.

[Copilot]

Pull request overview

Updates EKAT’s build configuration to require C++20 and bumps the Kokkos submodule to 5.0.1, including enabling a Kokkos legacy View implementation switch to preserve current subview behavior.

Changes:

• Set global C++ standard to C++20 and remove per-target C++17 requirement.
• Update extern/kokkos submodule to a newer commit (Kokkos 5.0.1).
• Add Kokkos_ENABLE_IMPL_VIEW_LEGACY=ON CMake option and remove redundant CI C++ standard override.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
CMakeLists.txt	Enforces C++20 and introduces a Kokkos legacy View compatibility option
src/core/CMakeLists.txt	Removes the ekat_core C++17 compile feature requirement
extern/kokkos	Updates the Kokkos submodule revision
cacts.yaml	Removes explicit CMAKE_CXX_STANDARD from CI config since it’s set in CMake

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[Copilot]

set(CMAKE_CXX_STANDARD 20) alone does not strictly require C++20; CMake may still accept older standards if the compiler can’t satisfy it, and may also default to compiler extensions. To truly enforce “require C++20”, also set CMAKE_CXX_STANDARD_REQUIRED to ON and (typically) CMAKE_CXX_EXTENSIONS to OFF near this block.

Suggested change

	set(CMAKE_CXX_STANDARD 20)
	set(CMAKE_CXX_STANDARD 20)
	set(CMAKE_CXX_STANDARD_REQUIRED ON)
	set(CMAKE_CXX_EXTENSIONS OFF)

[Copilot]

Setting CMAKE_CXX_STANDARD at the top-level can leak into parent projects when EKAT is consumed via add_subdirectory, effectively changing the consumer’s global C++ standard. Prefer expressing the requirement on EKAT targets (e.g., via target_compile_features(... PUBLIC cxx_std_20) on public libraries) or only setting the global standard when EKAT is the top-level project (guarded with if (CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)).

Suggested change

	# EKAT requires C++20
	set(CMAKE_CXX_STANDARD 20)
	# EKAT requires C++20 when built as a top-level project
	if (CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
	set(CMAKE_CXX_STANDARD 20)
	endif()

[Copilot]

This hunk removes the explicit per-target language standard requirement for ekat_core. If the intent is to require C++20, it’s more robust to keep the requirement attached to the target (now as cxx_std_20) so the constraint propagates correctly to consumers of ekat_core and doesn’t rely on a global CMAKE_CXX_STANDARD setting.

Suggested change

	target_compile_features(ekat_core PUBLIC cxx_std_20)

[Copilot]

Correct spelling of “unnanounced” to “unannounced”.

Suggested change

	# implementation will go away in the future, unnanounced (it's an "impl" option => no deprecation).
	# implementation will go away in the future, unannounced (it's an "impl" option => no deprecation).

[Copilot]

As written, this reads like an EKAT-level switch, but it only has an effect if EKAT is configuring/building Kokkos from source in the same CMake build (e.g., via add_subdirectory); it won’t change anything when Kokkos is prebuilt and provided via find_package. Consider clarifying that in the option description (or adding a configure-time note) so users aren’t misled into thinking toggling this will reconfigure an externally-provided Kokkos.

Suggested change

	option (Kokkos_ENABLE_IMPL_VIEW_LEGACY "Currently, EKAT is only compatible with Kokkos' legacy view implementation." ON)
	option (Kokkos_ENABLE_IMPL_VIEW_LEGACY "Currently, EKAT is only compatible with Kokkos' legacy view implementation. This option only takes effect when EKAT is configuring/building Kokkos from source in the same CMake build (e.g., via add_subdirectory) and does not reconfigure an externally provided Kokkos found via find_package." ON)