Implicit include directories are incomplete, resulting in compile errors (CMake + Clang-Tidy)

[Viatorus]

In order to make clang-tidy (and cppcheck and other static analyzer happy) they need all includes of an compiler.

But here:

emscripten/cmake/Modules/Platform/Emscripten.cmake

Lines 371 to 376 in bb16735

	if (NOT DEFINED CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES)
	set(CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include")
	endif()
	if (NOT DEFINED CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES)
	set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include")
	endif()

Only include/ is set. But there are more includes if you run: echo | em++ -xc++ -E -v -

"${EMSCRIPTEN_SYSROOT}/include/compat
"${EMSCRIPTEN_SYSROOT}/include/c++/v1
??"/upstream/lib/clang/20/include
... even more

Maybe cmake_parse_implicit_include_info from CMakeParseImplicitIncludeInfo should be used (it works).

Without the complete list, a simple program is not compileable.

Version of emscripten/emsdk:
3.1.74/4.0.0

Failing command line in full:
.em++ -std=c++20 -isystem $EMSCRIPTEN/cache/sysroot/include/ test.cpp

./em++ -std=c++20 -isystem /home/neubertt/projects/emsdk/upstream/emscripten/cache/sysroot/include/ test.cpp

#include <string>

 In file included from test.cpp:1:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/string:589:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__algorithm/max.h:14:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__algorithm/max_element.h:15:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__iterator/iterator_traits.h:14:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__concepts/constructible.h:13:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__concepts/destructible.h:13:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__type_traits/is_nothrow_destructible.h:14:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__type_traits/is_destructible.h:16:
 In file included from $EMSCRIPTEN/cache/sysroot/include/c++/v1/__type_traits/remove_all_extents.h:13:
 $EMSCRIPTEN/cache/sysroot/include/c++/v1/cstddef:45:5: error: <cstddef> tried including <stddef.h> but didn't find libc++'s <stddef.h> header.           This usually means that your header search paths are not configured properly.           The header search paths should contain the C++ Standard Library headers before           any C Standard Library, and you are probably using compiler flags that make that           not be the case.
    45 | #   error <cstddef> tried including <stddef.h> but didn't find libc++'s <stddef.h> header. \
       |     ^

[sbc100]

Can you give an example of how you are using clang-tidy / cppcheck such that it fails? Since you are referring to cmake I imagine they must be getting their information from some kind of cmake metatdata file? Could show the exact command that fail?

[sbc100]

Ah, I see the problem. In Emscripten.cmake we bypass a lot of stuff by default to speed things up. This includes the auto-detection of the include dirs. Can you try adding -DEMSCRIPTEN_FORCE_COMPILERS=OFF to your cmake to see if that fixes the issue?

Also, I do have a fix in flight but I'm trying to figure out how that CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES is used by other tools. I can't currently see how it is. Those paths do not appear in compile_commands.json .. so how are the linters extacting them?

[Viatorus]

Hello, thank you for taking a look at it.

Most projects have this in their CMake file:

# Generate compile_commands.json to make it easier to work with clang based tools
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
# Include implicit directories in the compile commands file
set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})

Take a look: https://github.com/search?q=set%28CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES+%24%7BCMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES%7D%29&type=code

This doesn't yet work for Emscripten.

One workaround would be to do:

if (NOT EMSCRIPTEN)
  set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
endif ()

But this would mean to adjust many project files.

[sbc100]

Include implicit directories in the compile commands file

set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})

So this is needed to make thing like clang-tidy work? Seems like a big of hack, but if that is what folks do we can certainly make sure we are compatible with that.

[sbc100]

This doesn't yet work for Emscripten.

When you say it "doesn't yet work" for emscripten what exactly do you mean? We do set CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES, right? Are you jut saying that that this list is incomplete are is somehow missing for you?

[Viatorus]

Seems like a big of hack

Since clang-tidy (and other tools which use compile_commands.json) don't know/test the compiler internals, they rely on a complete include directory list, that's why CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES must be used.

"doesn't yet work"

"yet" means - if could be fixed ;)

Are you jut saying that that this list is incomplete are is somehow missing for you?

Yes. Because the list is incomplete, even Emscripten itself cannot compile a simple program anymore (see the code example above). I think the include problem comes from the fact, that instead this include order is used:

emsdk/upstream/emscripten/cache/sysroot/include/compat
emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
emsdk/upstream/lib/clang/20/include
emsdk/upstream/emscripten/cache/sysroot/include

This one is used:

emsdk/upstream/emscripten/cache/sysroot/include // <-- Comes from CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES
emsdk/upstream/emscripten/cache/sysroot/include/compat
emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
emsdk/upstream/lib/clang/20/include
emsdk/upstream/emscripten/cache/sysroot/include

[sbc100]

Ok great. I have an initial fix in #23535. As a followup I hope to completely remove EMSCRIPTEN_FORCE_COMPILERS which is the cause of this issue.