[c++] default to C++17 for most builds

[jameslamb]

Follow-up to #7013

@h-vetinari made a good point there that we probably could safely default to C++17 in most builds here: #7013 (comment)

See the notes on that PR... C++17 has been supported for 6+ years in most compilers by now, so I don't think we lose much portability by doing this. And this should make things easier for upgrading dependencies (for example, linking to newer CUDA Toolkit, Boost, etc.).

Notes for Reviewers

There will still be one use of C++11 left

For as long as the R package supports R 3.x, the core source code (excluding anything to do with GPUs or MPI) still needs to be compilable with C++11:

LightGBM/R-package/configure.win

Lines 10 to 20 in de4c88f

	# As described in "Using C++ code" in "Writing R Extensions",
	# Rtools35 shipped with g++ 4.9, which didn't support C++17.
	#
	# Testing here for C++17 support, to account for that possibility
	# and to continue supporting R 3.6.
	#
	CXX17=`"${R_EXE}" CMD config CXX17`
	CXX17STD=`"${R_EXE}" CMD config CXX17STD`
	CXX="${CXX17} ${CXX17STD}"
	CXXFLAGS=`"${R_EXE}" CMD config CXX17FLAGS`
	CXX_STD="CXX17"

Moving the minimum R version to R 4.x would eliminate that. R 4.0 was released 5.5 years ago (April 2020) so it's probably been long enough... but let's not make that a part of this PR. I'll open a separate RFC issue.

update: R minimum version moved to 4.0 in #7038

[jameslamb]

There are 2 CI jobs failing, both using older build toolchains.

• R 3.6 + Rtools35 + MinGW: https://github.com/microsoft/LightGBM/actions/runs/17383949128/job/49347055936?pr=7016
• Visual Studio 2015: https://ci.appveyor.com/project/guolinke/lightgbm/builds/52664949

Those are both toolchains we've previously discussed dropping. Let's drop those and then make C++17 the default in LightGBM.

• R 3.6 + Rtools35 + MinGW: [RFC] [R-package] Drop R 3.x support? #7023
• Visual Studio 2015: [ci] [C++] drop support for Visual Studio 2015 (fixes #6803) #7022

[StrikerRUS]

Should we also update wording here?

LightGBM/CMakeLists.txt

Lines 85 to 91 in 061f59a

	elseif(MSVC)
	if(MSVC_VERSION LESS 1900)
	message(
	FATAL_ERROR
	"The compiler ${CMAKE_CXX_COMPILER} doesn't support required C++11 features. Please use a newer MSVC."
	)
	endif()

Not sure, but should we use CXX17 here now?

LightGBM/R-package/README.md

Line 77 in 061f59a

CXX11=g++-8

I just checked: we can safely remove -build/c++11 rule from filter for cpplint.

LightGBM/.pre-commit-config.yaml

Line 31 in 061f59a

- --filter=-build/c++11,-build/include_subdir,-build/include_what_you_use,-build/header_guard,-whitespace/indent_namespace,-whitespace/line_length

[jameslamb]

The previous message was probably technically still true, but this rewording is a bit simpler and is consistent with the other enforce-a-minimum-compiler-version errors.

It's enough for users to be told that the version of a compiler they're using is older than what LightGBM supports... saying for MSVC that that's because of "C++11 features" doesn't add much information.

[jameslamb]

I tested all of this on a Mac with R 4.4.2 tonight, confirmed it worked as expected.

[jameslamb]

All great suggestions, thanks @StrikerRUS !

I looked for more such things like this and didn't find any:

git grep -E -i 'c\+\+.*11'

I've addressed the 3 you mentioned, left inline comments explaining those choices.

[StrikerRUS]

LGTM, thank you for working on this!

[jameslamb]

It's good that we did this, and that it'll be in the next release... just noticed that C++11/C++14 support was recently removed in R-devel!!

wch/r-source@486e1ee