[gcc r14-7007] libstdc++: Remove std::__unicode::__null_sentinel

[gcc r14-7007] libstdc++: Remove std::__unicode::__null_sentinel

[Jonathan Wakely]

https://gcc.gnu.org/g:eb846114ed7c49e426fccb826a3f81b7abbc84be

commit r14-7007-geb846114ed7c49e426fccb826a3f81b7abbc84be
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jan 8 11:46:56 2024 +0000

    libstdc++: Remove std::__unicode::__null_sentinel
    
    The name __null_sentinel is defined as a macro by newlib, so we can't
    use it as an identifier. That variable is not actually used by
    libstdc++, it was added because P2728R6 proposes std::uc::null_sentinel.
    Since we don't need it and it breaks bootstrap for newlib targets, just
    remove it. A null sentinel can still be used by constructing a
    _Null_sentinel_t object as needed, rather than having a named object of
    that type predefined.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/unicode.h (__null_sentinel): Remove.
            * testsuite/17_intro/names.cc: Add __null_sentinel.

Diff:
---
 libstdc++-v3/include/bits/unicode.h      | 2 --
 libstdc++-v3/testsuite/17_intro/names.cc | 1 +
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/unicode.h b/libstdc++-v3/include/bits/unicode.h
index 66f8399fdfb..e49498a0531 100644
--- a/libstdc++-v3/include/bits/unicode.h
+++ b/libstdc++-v3/include/bits/unicode.h
@@ -83,8 +83,6 @@ namespace __unicode
       { return *__it == iter_value_t<_It>{}; }
   };
 
-  inline constexpr _Null_sentinel_t __null_sentinel;
-
   template<typename _FromFmt, typename _ToFmt,
 	   input_iterator _Iter, sentinel_for<_Iter> _Sent = _Iter,
 	   typename _ErrorHandler = _Repl>
diff --git a/libstdc++-v3/testsuite/17_intro/names.cc b/libstdc++-v3/testsuite/17_intro/names.cc
index 5e77e9f2ab0..53c5aff219d 100644
--- a/libstdc++-v3/testsuite/17_intro/names.cc
+++ b/libstdc++-v3/testsuite/17_intro/names.cc
@@ -140,6 +140,7 @@
 
 // These clash with newlib so don't use them.
 # define __lockable		cannot be used as an identifier
+# define __null_sentinel	cannot be used as an identifier
 # define __packed		cannot be used as an identifier
 # define __unused		cannot be used as an identifier
 # define __used			cannot be used as an identifier