[MSVC] Root is failed with error G694476FC: static_assert failed "Unexpected size"

[NEIL-smtg]

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Description

We've been using MSVC to build ROOT, and we've recently encountered the following error:

error G694476FC: static_assert failed "Unexpected size"

Our developer has suggested the compiler that was used needs to implement the Defect Report https://cplusplus.github.io/CWG/issues/2518.html (which has been implemented by MSVC and Clang).

Reproducer

Repro step:

1. Open "x64 Native Tools Command Prompt for VS 2022"
2. git clone https://github.com/root-project/root.git
3. mkdir root\build_amd64
4. cd /d root\build_amd64
5. set VSCMD_SKIP_SENDTELEMETRY=1 & "C:\Program Files\Microsoft Visual Studio\2022\Enterprise\Common7\Tools\VsDevCmd.bat" -host_arch=amd64 -arch=amd64
6. python.exe -m pip install pytest 2>&1
7. cmake -G "Visual Studio 17 2022" -A x64 -DCMAKE_SYSTEM_VERSION=10.0.22621.0 -Dtesting=ON -Droottest=ON -Droofit=off .. 2>&1
8. msbuild /m /p:Platform=x64 /p:Configuration=Release ALL_BUILD.vcxproj /t:Rebuild 2>&1

Expected result:

Build succesfully.

ROOT version

We are using the latest commit.

Installation method

git

Operating system

Windows server 2022 data center

Additional context

Detailed log:
Build.log

[bellenot]

So I cannot reproduce the error with the recipe you give. But looking at you log file, I see you set more environment variables. That is:

set _CL_= /Bcapture_repro C:\a\_work\_temp\rwc_project_logs\ROOT\preprocessed_repro_build & set _LINK_= /onfailrepro:C:\a\_work\_temp\rwc_project_logs\ROOT\link_repro_build

Without those variables the build works just fine. This is maybe related to #9445
I'm now trying with the same variables to see if it fails the same

[bellenot]

So I tried with:

cmake -G "Visual Studio 17 2022" -A x64 -Dtesting=ON -Droottest=ON -Droofit=off ..\..\..\git\master
set VSCMD_SKIP_SENDTELEMETRY=1 & "C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\Tools\VsDevCmd.bat" -host_arch=amd64 -arch=amd64 & set _CL_= /Bcapture_repro C:\project_logs\ROOT\preprocessed_repro_build & set _LINK_= /onfailrepro:C:\project_logs\ROOT\link_repro_build
msbuild /m /p:Platform=x64 /p:Configuration=Release ALL_BUILD.vcxproj /t:Rebuild

And it worked...

[StephanTLavavej]

Neil omitted an important bit of info - this involves a very recent change to MSVC's STL, microsoft/STL#4591 . To reproduce it, you'll either have to build and consume our open-source STL, or wait for VS 2022 17.11 Preview 2 to ship (which will contain this commit). We changed the STL to say static_assert(false) instead of static_assert(_Always_false<SomeType>).

[bellenot]

@StephanTLavavej OK, thanks for the info. But then what does that mean for ROOT? It will not build anymore with the upcoming version 17.11 of Visual Studio (again!!) ? What would be required to fix the issue on our side?

[StephanTLavavej]

Correct. To fix this, implement CWG-2518 in Cling (if that's the tool that's emitting this error), allowing static_assert(false) to work in templates. MSVC and Clang have already implemented this Core DR.

[pcanal]

Clang have already implemented this Core DR.

Do you happen to know which version this appeared in? (Or better yet which Merge Request added it). Cling uses Clang underneath; currently version 16; Moving the Clang version used is a long process but backporting a narrow Merge Request might be possible.

[StephanTLavavej]

It shipped in Clang 17, which is the version that microsoft/STL currently requires. https://clang.llvm.org/cxx_dr_status.html documents this.

I believe that llvm/llvm-project@00e2098 implemented it, but there's a slight chance there were followup commits that I'm unaware of.

[bellenot]

OK, thanks a lot for the information! And since it's in Clang/Cling, let's add @vgvassilev in the loop...

[vgvassilev]

We probably should backport this commit. @hahnjo what do you think?

[hahnjo]

Probably, but I'm a bit confused about the error message: Why would Cling output MSVC style error messages?

[StephanTLavavej]

I don't actually know what the G694476FC error code is (the only codes I recognize are MSVC Cnnnn, IntelliSense Ennnn, and Clang named warnings/errors), but the "Unexpected size" is coming from the STL. We have static_assert(false, "Unexpected size") in a template, and after the DR resolution, we expect that this should be activated only when that if constexpr branch is actually taken.

[hahnjo]

I believe that llvm/llvm-project@00e2098 implemented it, but there's a slight chance there were followup commits that I'm unaware of.

Seems reasonable and applies cleanly to our LLVM 16 - I opened #15437 with the backport. I will wait for confirmation that this is all we need before syncing to our LLVM monorepo fork and eventually merging.