[libc++] Remove use of internal glibc macros to determine if c8rtomb() and mbrtoc8() are present

[tahonermann]

Summary

When support for declaring the c8rtomb() and mbrtoc8() functions within the std namespace was added in commit 7e7013c5d4b1b3996c8dba668c5a94bb33b2999b, internal glibc macros were used to determine if C2X extensions are enabled. Specifically, a check for whether __GLIBC_USE is defined and whether __GLIBC_USE(ISOC2X) is non-0 was added. __GLIBC_USE is an internal detail of the glibc implementation that may be changed or removed in the future potentially leading to inconsistency or compilation failures. This change removes the use of the internal glibc macro to avoid such problems. Unfortunately, without another mechanism to determine if C2X extensions are enabled, this removal will result in inconsistent declarations of the c8rtomb() and mbrtoc8() functions; when C++ char8_t support is not enabled, but C2X extensions are, these functions will be declared in the global namespace but not in the std namespace. This situation will improve when C23 support is finalized and the check can be re-implemented using __STDC_VERSION__.

[ldionne]

TBH this seems reasonable to me. If this seems reasonable to you, I'll go with your judgement here.

libcxx/include/__config

1223

Nitpick, but when you upload your patches, please consider using arc diff so that context is available in the diffs!

[tahonermann]

Thanks @ldionne, I'm heading out of town for the weekend, so I'll wait to push this until Monday.

I meant to get this in before the Clang 16 branch, but life and stuff. I'll file an issue to cherry-pick the change for the release branch once committed. Please let me know if you have any thoughts on that.

libcxx/include/__config

1223

Oops, I forgot the -U99999 option when generating the diff.

[ldionne]

In D142588#4084087, @tahonermann wrote:

Thanks @ldionne, I'm heading out of town for the weekend, so I'll wait to push this until Monday.

I meant to get this in before the Clang 16 branch, but life and stuff. I'll file an issue to cherry-pick the change for the release branch once committed. Please let me know if you have any thoughts on that.

That sounds fine. The release manager should reach out to me and I'll let them know it's good to go.

[tahonermann]

Rebase and test update to ignore any declarations present in the global namespace when char8_t support is not enabled.

[tahonermann]

CI failures are due to test time outs in unrelated tests.

Assuming no new concerns about the added changes to update a test, I'll commit this later today.

[tahonermann]

Sigh, I forgot to tag the commit with a link to this differential review. Closing manually.

[tahonermann]

https://github.com/llvm/llvm-project/issues/60517 has been filed to backport this change for the LLVM 16 release.

libcxx/include/__config

 #    define _LIBCPP_PACKED_BYTE_FOR_AIX_END /* empty */
 #  endif
 
 #  if __has_attribute(__packed__)
 #    define _LIBCPP_PACKED __attribute__((__packed__))
 #  else
 #    define _LIBCPP_PACKED
 #  endif
 
 // c8rtomb() and mbrtoc8() were added in C++20 and C23. Support for these
 // functions is gradually being added to existing C libraries. The conditions
 // below check for known C library versions and conditions under which these
 // functions are declared by the C library.
 #  define _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
 // GNU libc 2.36 and newer declare c8rtomb() and mbrtoc8() in C++ modes if
-// __cpp_char8_t is defined or if C2X extensions are enabled. Unfortunately,
-// determining the latter depends on internal GNU libc details. If the
-// __cpp_char8_t feature test macro is not defined, then a char8_t typedef
-// will be declared as well.
-#  if defined(_LIBCPP_GLIBC_PREREQ) && defined(__GLIBC_USE)
-#    if _LIBCPP_GLIBC_PREREQ(2, 36) && (defined(__cpp_char8_t) || __GLIBC_USE(ISOC2X))
+// __cpp_char8_t is defined or if C2X extensions are enabled. Determining
+// the latter depends on internal GNU libc details that are not appropriate
+// to depend on here, so any declarations present when __cpp_char8_t is not
+// defined are ignored.
+#  if defined(_LIBCPP_GLIBC_PREREQ)
+#    if _LIBCPP_GLIBC_PREREQ(2, 36) && defined(__cpp_char8_t)
 #      undef _LIBCPP_HAS_NO_C8RTOMB_MBRTOC8
 #    endif
 #  endif
 
 // There are a handful of public standard library types that are intended to
 // support CTAD but don't need any explicit deduction guides to do so. This
 // macro is used to mark them as such, which suppresses the
 // '-Wctad-maybe-unsupported' compiler warning when CTAD is used in user code

libcxx/test/std/strings/c.strings/no_c8rtomb_mbrtoc8.verify.cpp

 //   error: 'error' diagnostics seen but not expected:
 //     File /usr/include/uchar.h Line 38: cannot combine with previous 'type-name' declaration specifier
 //     File /usr/include/uchar.h Line 39: cannot combine with previous 'type-name' declaration specifier
 //   2 errors generated.
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <uchar.h>
 
+#include "test_macros.h"
+
+// When C++ char8_t support is not enabled, definitions of these functions that
+// match the C2X declarations may still be present in the global namespace with
+// a char8_t typedef substituted for the C++ char8_t type. If so, these are not
+// the declarations we are looking for, so don't test for them.
+#if !defined(TEST_HAS_NO_CHAR8_T)
 using U = decltype(::c8rtomb);
 using V = decltype(::mbrtoc8);
-#if defined(_LIBCPP_HAS_NO_C8RTOMB_MBRTOC8)
+#  if defined(_LIBCPP_HAS_NO_C8RTOMB_MBRTOC8)
 // expected-error@-3 {{no member named 'c8rtomb' in the global namespace}}
 // expected-error@-3 {{no member named 'mbrtoc8' in the global namespace}}
-#else
+#  else
 // expected-no-diagnostics
-#endif
+#  endif
+#else
+// expected-no-diagnostics
+#endif