[gcc r16-2030] libstdc++: Avoid -Wswitch warning from chrono formatters

[Jonathan Wakely]

https://gcc.gnu.org/g:eeea445e1d0f0aac813d4eccd7b8f55477772dd0

commit r16-2030-geeea445e1d0f0aac813d4eccd7b8f55477772dd0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jul 4 17:03:27 2025 +0100

    libstdc++: Avoid -Wswitch warning from chrono formatters
    
    Add a default case to the switch to suppress warnings about unhandled
    enumeration values. This is a consteval function, so if the default case
    is ever reached it will be an error not silent miscompilation.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/chrono_io.h (__formatter_duration::_S_spec_for):
            Add default case to switch and use __builtin_unreachable.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
index bcf9830fb9e2..72cd569ccd65 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -1841,6 +1841,8 @@ namespace __format
 	      break;
 	    case _None:
 	      break;
+	    default:
+	      __builtin_unreachable();
 	    }
 	  return __res;
 	};