`_HAS_CONDITIONAL_EXPLICIT=0` won't work with VS 2022 17.4

[StephanTLavavej]

• Checked for duplicates

Describe the bug

ROOT sets _HAS_CONDITIONAL_EXPLICIT=0 for MSVC's STL:

root/interpreter/cling/lib/Interpreter/CIFactory.cpp

Lines 811 to 816 in 2fc5e7a

	#if (_MSC_VER >= 1926)
	// FIXME: Silly workaround for cling not being able to parse the STL
	// headers anymore after the update of Visual Studio v16.7.0
	// To be checked/removed after the upgrade of LLVM & Clang
	PPOpts.addMacroDef("_HAS_CONDITIONAL_EXPLICIT=0");
	#endif

MSVC's STL (which I work on) has removed this codepath by merging microsoft/STL#2796, which will ship in VS 2022 17.4 Preview 1. As a result, ROOT will fail to build with:

[trimmed...]\utility(151,9): error G08EB1F86: expected member name or ';' after declaration specifiers [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\utility(151,9): error GC66A3811: expected ')' [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\utility(190,24): error G08EB1F86: expected member name or ';' after declaration specifiers [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\utility(190,24): error GC66A3811: expected ')' [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\tuple(268,9): error G08EB1F86: expected member name or ';' after declaration specifiers [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\tuple(268,9): error GC66A3811: expected ')' [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\tuple(302,5): error G28325FEB: non-static data member cannot be constexpr; did you intend to make it const? [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\tuple(302,24): error G68D9A705: member '_Tuple_conditional_explicit_v' declared as a template [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
[trimmed...]\tuple(302,79): error GC66A3811: expected ';' at end of declaration list [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]
CUSTOMBUILD : error : Error loading the default rootcling header files. [F:\gitP\root-project\root\build_amd64\core\G__Core.vcxproj]

Expected behavior

ROOT should build successfully, even when MSVC's STL uses Future Technology (i.e. C++20 explicit(bool) in C++14/17 modes).

Upstream Clang has supported this scenario (with a suppressible warning) since Clang 10.0.0 shipped on 2020-03-24. Example on Compiler Explorer: https://godbolt.org/z/a75ooK6cz

The comment in CIFactory.cpp indicates that this was a known issue (although I was unable to find anything filed in GitHub or Jira) - is ROOT planning to upgrade its Clang/LLVM dependency, which would resolve this?

To Reproduce

Build ROOT with the latest MSVC STL from https://github.com/microsoft/STL, or wait for VS 2022 17.4 Preview 1 to ship (we can't provide an ETA yet, but it'll be some time).

Setup

1. ROOT version: master
2. Operating system: Windows 11
3. How you obtained ROOT: Built it ourselves (or tried to)

Additional context

Supporting explicit(bool) will also improve build throughput, as std::pair, std::tuple, and std::optional's constructor sets were nearly twice the size with the _HAS_CONDITIONAL_EXPLICIT=0 workaround.

[bellenot]

@StephanTLavavej thanks a lot for reporting this! We are on the process of upgrading LLVM & Clang, so this should be solved soon (hopefully).

[bellenot]

@StephanTLavavej FYI, the upgrade of LLVM 13 is done, and this is now fixed in ROOT master (i.e. the workaround has been removed). Thanks again for your feedback!