compile_commands.json sets wrong compilation.standard

[Tradias]

Issue: C++17 is not enabled when compilation.commands_dir is set, even if compile_commands.json contain the corresponding compile flag.

Consider the following CMake project, configured with -DCMAKE_EXPORT_COMPILE_COMMANDS=on

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)
project(test)
add_executable(test main.cpp)
target_compile_features(test PRIVATE cxx_std_17)
find_program(STANDARDESE_TOOL standardese)
add_custom_target(
  generate-documentation
  COMMAND
    ${STANDARDESE_TOOL}
    "--output.prefix=${CMAKE_CURRENT_BINARY_DIR}/documentation/"
    "--compilation.commands_dir=${CMAKE_BINARY_DIR}"
    "--compilation.standard=c++17" # doesn't help
    "--compilation.gnu_extensions=1" # doesn't help either
   "${CMAKE_SOURCE_DIR}/inc.h")

where main.cpp is

#include "inc.h"

int main(int argc, char** argv) { return (int)My{}; }

and inc.h is

#pragma once

#include <cstddef>

/// Let's use c++17
using My = std::byte;

Building the target generate-documentation

[build] [1/1 100% :: 5.127] cmd.exe /C "cd /D C:\test\build--debug && C:\Users\dennis\bin\standardese.exe --output.prefix=C:/test/build--debug/documentation/ --compilation.commands_dir=C:/test/build--debug --compilation.gnu_extensions=1 --compilation.standard=c++17 --verbose=1 C:/test/inc.h"
[build] parsing C++ files...
[build] [libclang] [error] C:/test/inc.h:6: no type named 'byte' in namespace 'std'
[build] parsing documentation comments...
[build] [standardese comment] [warning] C:/test/inc.h:5: comment does not have a remote entity specified
[build] generating documentation...
[build] writing files in format 'md'...
[build] Build finished with exit code 0

no documentation for using My is being generated.

I tried with clang-cl version 11.1 and 12, where compile_commands.json is:

[
{
  "directory": "C:/test/build--debug",
  "command": "C:\\PROGRA~1\\LLVM\\bin\\CLANG_~1.EXE -g -Xclang -gcodeview -O0 -D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -std=gnu++17 -o CMakeFiles\\test.dir\\main.cpp.obj -c C:\\test\\main.cpp",
  "file": "C:/test/main.cpp"
}
]

and with cl from MSVC 19.29.30040.0 where compile_commands.json is:

[
{
  "directory": "C:/test/build--debug",
  "command": "C:\\PROGRA~2\\MICROS~2\\2019\\COMMUN~1\\VC\\Tools\\MSVC\\1429~1.300\\bin\\Hostx64\\x64\\cl.exe  /nologo /TP /DWIN32 /D_WINDOWS /GR /EHsc /Zi /Ob0 /Od /RTC1 -MDd -std:c++17 /FoCMakeFiles\\test.dir\\main.cpp.obj /FdTARGET_COMPILE_PDB /FS -c C:\\test\\main.cpp",
  "file": "C:/test/main.cpp"
}
]

Note that invoking:

standardese --compilation.standard=c++17 inc.h

works as expected.

I am using standardese v0.5.2 installed with miniconda as explained here.

[saraedum]

So apparently an entry is found in the compilation database for your file, otherwise, this code would not ignore the explicit --compilation.standard flag.

Parsing the entries in the database is handled by cppast here and here. Unless libclang does some mangling to the arguments, I would not be suprised if parsing your second compile_commands.json fails since there is no -std. However, the first one should work from a look at the code.

Does libclang understand gnu++17? I guess it does but could you try to change that to c++17 in your compile commands?

[Tradias]

Changing it to -std=c++17 in compile_commands.json for the first one (clang-cl) yields the same output.

I thought that --compilation args provided through the command line would take precedence over the compile commands. After all standardese --help says:

--compilation.commands_dir arg the directory where a
compile_commands.json is located, its
options have lower priority than the
other ones

[saraedum]

Thanks for pointing this out. This does not seem to be the case, see https://github.com/standardese/standardese/blob/master/tool/generator.cpp#L36. I'll create a separate issue to change the documentation.