[gcc r12-9895] libstdc++: Fix constexpr functions to conform to older standards

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:374f41517ab2fd78dafe3a0a1874bb9738966e74

commit r12-9895-g374f41517ab2fd78dafe3a0a1874bb9738966e74
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Aug 9 11:11:31 2023 +0100

    libstdc++: Fix constexpr functions to conform to older standards
    
    Some constexpr functions were inadvertently relying on relaxed constexpr
    rules from later standards.
    
    libstdc++-v3/ChangeLog:
    
            * include/experimental/bits/fs_path.h (path::string): Use
            _GLIBCXX17_CONSTEXPR not _GLIBCXX_CONSTEXPR for 'if constexpr'.
            * include/std/charconv (__to_chars_8): Initialize variable for
            C++17 constexpr rules.
    
    (cherry picked from commit b3a2b307b9deea719fb725a86df43b82176fe459)

Diff:
---
 libstdc++-v3/include/experimental/bits/fs_path.h | 2 +-
 libstdc++-v3/include/std/charconv                | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/experimental/bits/fs_path.h b/libstdc++-v3/include/experimental/bits/fs_path.h
index ba6acb2158d..c13195a6ba9 100644
--- a/libstdc++-v3/include/experimental/bits/fs_path.h
+++ b/libstdc++-v3/include/experimental/bits/fs_path.h
@@ -1049,7 +1049,7 @@ namespace __detail
     inline std::basic_string<_CharT, _Traits, _Allocator>
     path::string(const _Allocator& __a) const
     {
-      if _GLIBCXX_CONSTEXPR (is_same<_CharT, value_type>::value)
+      if _GLIBCXX17_CONSTEXPR (is_same<_CharT, value_type>::value)
 	return { _M_pathname.begin(), _M_pathname.end(), __a };
 
       using _WString = basic_string<_CharT, _Traits, _Allocator>;
diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index 533320ea085..b4d37435fa0 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -236,7 +236,7 @@ namespace __detail
       static_assert(is_unsigned<_Tp>::value, "implementation bug");
 
       to_chars_result __res;
-      unsigned __len;
+      unsigned __len = 0;
 
       if _GLIBCXX17_CONSTEXPR (__gnu_cxx::__int_traits<_Tp>::__digits <= 16)
 	{