/std:c++latest not preserved when transferring compile command

[Sam McCall]

InterpolatingCompilationDatabase will translate clang --driver-mode=cl /std:c++latest foo.cpp into clang --driver-mode=cl foo.hpp.

This is because we expect the value after "/std:" to name a language version, but in MSVC-compat mode it's a hardcoded table in the driver instead.

[Fabian Keßler]

I figured out a similar issue with msvc 16.11.3 for the source file it is called correctly, but when clangd is parsing the header file, it omits it.

[paulcleggSIG]

This is still an issue as of clangd 14.0.5 and msvc 17.3.4. We use clang-cl, our CompilationDatabase / compiler_commands.json specify -std:c++latest and the clangd invocation (via vscode-clangd) output omits the argument unless we redundantly configure our .clangd config file like:

CompileFlags:
Add: [
"-std:c++latest"
]
CompilationDatabase: "C:/path/to/cmake/out/folder/with/compiler_commands"

For recent language standards, msvc does name a language version in a way that seems 1:1 mappable to my eye.

/std:c++14
/std:c++17
/std:c++20
/std:c++latest

[paulcleggSIG]

c++latest is the outlier, as it is a moving target.

However, cmake (3.22.2), has a mapping for 23 (and soon 26) to the best msvc match: c++latest. Those mapping choices are certainly debatable but are overridable in a defined way via CMAKE_CXX23_STANDARD_COMPILE_OPTION, so if clangd would accept the -std argument verbatim from compiler_commands.json when it's present, I think that would be more intuitive than the current behavior.

[Egor Mikhaylov]

This still happens in clangd 16.0.0. Quite annoying, since Clangd otherwise requires zero configuration other than compiler_commands.json.