[Clang] Diagnose `extern "C++" int main()`

[MitalAshok]

This currently just seems to be ignored (the name is not mangled).
It is explicitly disallowed by https://eel.is/c++draft/basic.start.main#3.sentence-5

The main function shall not be declared with a linkage-specification.

This includes

extern "C++" { int main(); }

This is also technically ill-formed

extern "C" int main();

but it seems to be used in some places:

llvm-project/libc/startup/linux/do_start.cpp

Line 23 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

llvm-project/libc/startup/gpu/nvptx/start.cpp

Line 15 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

So this should probably just be a warning, not an error.

[shafik]

Using code-blocks are usually better than just using backticks because they allow for easy copy and paste, especially if there is not godbolt link.

[shafik]

Looks like implementation accept this some with diagnostics, gcc gives a diagnostic w/ -pedantic we should probably as well.

[AaronBallman]

That's a very new GCC diagnostic -- 14.1 doesn't issue it. But I agree, Clang should accept and diagnose as an extension same as GCC here. This seems like a good first issue; in CheckMain() (

llvm-project/clang/lib/Sema/SemaDecl.cpp

Line 12212 in 83e6d87

void Sema::CheckMain(FunctionDecl* FD, const DeclSpec& DS) {

) we need a new test for language linkage (FD->getLanguageLinkage() != NoLanguageLinkage) and issue a pedantic diagnostic.

Be sure to test both: extern "C++" int main() {} and extern "C++" { int main() {} }

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Mital Ashok (MitalAshok)

This currently just seems to be ignored (the name is not mangled). It is explicitly disallowed by https://eel.is/c++draft/basic.start.main#3.sentence-5

> The main function shall not be declared with a linkage-specification.

This includes

extern "C++" { int main(); }

This is also technically ill-formed

extern "C" int main();

but it seems to be used in some places:

llvm-project/libc/startup/linux/do_start.cpp

Line 23 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

llvm-project/libc/startup/gpu/nvptx/start.cpp

Line 15 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

So this should probably just be a warning, not an error.

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Mital Ashok (MitalAshok)

This currently just seems to be ignored (the name is not mangled). It is explicitly disallowed by https://eel.is/c++draft/basic.start.main#3.sentence-5

> The main function shall not be declared with a linkage-specification.

This includes

extern "C++" { int main(); }

This is also technically ill-formed

extern "C" int main();

but it seems to be used in some places:

llvm-project/libc/startup/linux/do_start.cpp

Line 23 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

llvm-project/libc/startup/gpu/nvptx/start.cpp

Line 15 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

So this should probably just be a warning, not an error.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mital Ashok (MitalAshok)

This currently just seems to be ignored (the name is not mangled). It is explicitly disallowed by https://eel.is/c++draft/basic.start.main#3.sentence-5

> The main function shall not be declared with a linkage-specification.

This includes

extern "C++" { int main(); }

This is also technically ill-formed

extern "C" int main();

but it seems to be used in some places:

llvm-project/libc/startup/linux/do_start.cpp

Line 23 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

llvm-project/libc/startup/gpu/nvptx/start.cpp

Line 15 in 98e4413

extern "C" int main(int argc, char **argv, char **envp);

So this should probably just be a warning, not an error.

[a-tarasyuk]

can I be assigned to this issue?