[gcc r15-7859] libstdc++: Avoid '-Wunused-parameter' for 'out' in member function 'std::codecvt_base::result std::_

[Thomas Schwinge]

https://gcc.gnu.org/g:bf07f9a9832494a7270a7099195f47f2126e8e07

commit r15-7859-gbf07f9a9832494a7270a7099195f47f2126e8e07
Author: Thomas Schwinge <tschwinge@baylibre.com>
Date:   Wed Feb 19 20:39:25 2025 +0100

    libstdc++: Avoid '-Wunused-parameter' for 'out' in member function 'std::codecvt_base::result std::__format::{anonymous}::__encoding::conv(std::string_view, std::string&) const'
    
    In a newlib configuration:
    
        ../../../../../source-gcc/libstdc++-v3/src/c++20/format.cc: In member function ‘std::codecvt_base::result std::__format::{anonymous}::__encoding::conv(std::string_view, std::string&) const’:
        ../../../../../source-gcc/libstdc++-v3/src/c++20/format.cc:100:35: error: unused parameter ‘out’ [-Werror=unused-parameter]
          100 |   conv(string_view input, string& out) const
              |                           ~~~~~~~~^~~
    
            libstdc++-v3/
            * src/c++20/format.cc (conv): Tag 'out' as '[[maybe_unused]]'.

Diff:
---
 libstdc++-v3/src/c++20/format.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++20/format.cc b/libstdc++-v3/src/c++20/format.cc
index 1a24fcab7f76..6967d53259da 100644
--- a/libstdc++-v3/src/c++20/format.cc
+++ b/libstdc++-v3/src/c++20/format.cc
@@ -97,7 +97,7 @@ struct __encoding : locale::facet
 
   // Convert `input` to UTF-8, using `out` to hold the result.
   codecvt_base::result
-  conv(string_view input, string& out) const
+  conv(string_view input, [[maybe_unused]] string& out) const
   {
     if (input.empty()) [[unlikely]]
       return codecvt_base::noconv;