[gcc r12-8908] libstdc++: Set active union member in constexpr std::string [PR103295]

[Jonathan Wakely]

https://gcc.gnu.org/g:a088d93c210f9b662d706e2fcf63a59d05fe27c1

commit r12-8908-ga088d93c210f9b662d706e2fcf63a59d05fe27c1
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Fri Nov 11 22:23:31 2022 +1100

    libstdc++: Set active union member in constexpr std::string [PR103295]
    
    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>
    (cherry picked from commit 52672be7d328df50f9a05ce3ab44ebcae50fee1b)

Diff:
---
 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 9777deaf2e9..f1a0e6984a1 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();
       }