CMake: should warn or indicate that DEAL_II_WITH_KOKKOS and DEAL_II_WITH_BOOST is always on.

[yurivict]

Version: 9.5.0
FreeBSD 13.2

[RichardYCJ]

According to the release page, kokkos is a required dependency after version 9.5.0

[drwells]

Yes, Kokkos is required as of 9.5.0 (and we have a bundled version in case people do not have it installed).

It sounds like the error message printed when someone provides -DDEAL_II_WITH_KOKKOS=OFF is wrong - we should fix that.

[yurivict]

Does DEAL_II_WITH_KOKKOS=OFF disable anything at all? Is DEAL_II_WITH_KOKKOS needed?

[tamiko]

@drwells @yurivict Correct, there is simply no error message.
Neither for DEAL_II_WITH_KOKKOS, nor for DEAL_II_WITH_BOOST.
This is due to

dealii/cmake/configure/configure_20_boost.cmake

Lines 30 to 32 in 01b805b

	set(DEAL_II_WITH_BOOST ON # Always true. We need it :-]
	CACHE BOOL "Build deal.II with support for boost." FORCE
	)

which implies that we never trigger:

dealii/cmake/configure/configure_20_boost.cmake

Lines 179 to 191 in 01b805b

	#
	# DEAL_II_WITH_BOOST is always required.
	#
	if(NOT DEAL_II_WITH_BOOST)
	if(DEAL_II_FEATURE_AUTODETECTION)
	feature_error_message("BOOST")
	else()
	message(FATAL_ERROR "\n"
	"Unmet configuration requirements: "
	"DEAL_II_WITH_BOOST required, but set to OFF!\n\n"
	)
	endif()
	endif()

So this is "wrong" for about 11 years. Same for Kokkos as the logic was simply copied.
We could simply remove the second code block and issue a warning when we switch the flag to ON.