[PATCH] libstdc++: Set active union member in constexpr std::string [PR103295]

[Nathaniel Shead <nathanieloshead@gmail.com>]

Hi,

Below is a patch to fix std::string in constexpr contexts on Clang. This
was originally fixed in the commits attached to PR103295, but a later
commit 98a0d72a seems to have mistakenly undone this.

Tested on x86_64-linux. Verified using clang-14 and clang-15 that the
fix works. I haven't added anything to the test suite, since this issue
is only detected by clang.

This is my first time contributing, so please let me know if I've done
anything wrong or missed something. Thanks!

Nathaniel

-- >8 --

Clang still complains about using std::string in constexpr contexts due
to the changes made in commit 98a0d72a. This patch ensures that we set
the active member of the union as according to [class.union.general] p6.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
        * include/bits/basic_string.h (_M_use_local_data): Set active
          member to _M_local_buf.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 libstdc++-v3/include/bits/basic_string.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 9c2b57f5a1d..2790fd49b05 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -352,8 +352,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
-	  for (_CharT& __c : _M_local_buf)
-	    __c = _CharT();
+	  for (size_type i = 0; i <= _S_local_capacity; ++i)
+	    _M_local_buf[i] = _CharT();
 #endif
 	return _M_local_data();
       }
-- 
2.34.1