[gcc r13-7626] libstdc++: Add missing constexpr specifiers in <format>

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

https://gcc.gnu.org/g:52010f9449a7bf4f9d662e2708df301e85768a15

commit r13-7626-g52010f9449a7bf4f9d662e2708df301e85768a15
Author: Deev Patel <pateldeev@nevada.unr.edu>
Date:   Mon Jul 24 21:10:52 2023 +0100

    libstdc++: Add missing constexpr specifiers in <format>
    
    A couple of virtual functions in the libstdc++ format header are marked
    constexpr in the base class, but not in the derived class. This was
    causing build failures when trying to compile latest gcc libstdc++ with
    clang 16 using c++20. Adding the constexpr specifier resolves the issue.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (_Formatting_scanner::_M_on_chars): Add
            missing constexpr specifier.
            (_Formatting_scanner::_M_format_arg): Likewise.
    
    (cherry picked from commit 96482ffe60d9bdec802fcad705c69641b2a3e040)

Diff:
---
 libstdc++-v3/include/std/format | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 9d5981e4882..9a3896b8339 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -3553,14 +3553,14 @@ namespace __format
 
       using iterator = typename _Scanner<_CharT>::iterator;
 
-      void
+      constexpr void
       _M_on_chars(iterator __last) override
       {
 	basic_string_view<_CharT> __str(this->begin(), __last);
 	_M_fc.advance_to(__format::__write(_M_fc.out(), __str));
       }
 
-      void
+      constexpr void
       _M_format_arg(size_t __id) override
       {
 	using _Context = basic_format_context<_Out, _CharT>;