release/18.x: change the visibility of libc++ header to public in libcxx module

[RichardLuo0]

This PR addresses a problem that headers may not be able to be found if #include is used with std modules.

Consider the following file:

#include <boost/json.hpp>
import std;

int main(int, const char **) { }

boost will include something from libc++, but we are using -nostdinc++, so the compiler can not find any default std header.
Therefore the locally built header needs to be public.

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: RichardLuo (RichardLuo0)

Changes

This PR addresses a problem that headers may not be able to be found if #include is used with std modules.

Consider the following file:

#include <boost/json.hpp>
import std;

int main(int, const char **) { }

boost will include something from libc++, but we are using -nostdinc++, so the compiler can not find any default std header.
Therefore the locally built header needs to be public.

---

Full diff: https://github.com/llvm/llvm-project/pull/91182.diff

1 Files Affected:

• (modified) libcxx/modules/CMakeLists.txt.in (+2-2)

diff --git a/libcxx/modules/CMakeLists.txt.in b/libcxx/modules/CMakeLists.txt.in
index e332d70cc16333..1f8f159e710cb9 100644
--- a/libcxx/modules/CMakeLists.txt.in
+++ b/libcxx/modules/CMakeLists.txt.in
@@ -41,7 +41,7 @@ target_sources(std
     std.cppm
 )
 
-target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+target_include_directories(std SYSTEM PUBLIC @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
 if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
   target_compile_options(std PUBLIC -fno-exceptions)
@@ -67,7 +67,7 @@ target_sources(std.compat
     std.compat.cppm
 )
 
-target_include_directories(std.compat SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+target_include_directories(std.compat SYSTEM PUBLIC @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
 
 if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
   target_compile_options(std.compat PUBLIC -fno-exceptions)

[RichardLuo0]

The checks error seems to have nothing to do with the changes

[tstellar]

@mordante Do you think we should backport this?

[tstellar]

Is there a similar change that is proposed for the main branch?

[RichardLuo0]

Is there a similar change that is proposed for the main branch?

Yes #91240, I forget to mention that here.

[ldionne]

Closing this PR since we are not making any more LLVM 18 releases. This will be included in LLVM 19.