libstdc++: Fix -Wunused-variable warning in <format>

libstdc++-v3/ChangeLog:

	* include/std/format (format_parse_context::check_dynamic_spec):
	Add [[maybe_unused]] attribute and comment.

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 3280dadfb90e149eeb78e73a8d1133c8a976442c..52243eb54792470cab99806e495f7a89a5c5d1be 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -4323,6 +4323,11 @@ namespace __format
     constexpr void
     basic_format_parse_context<_CharT>::check_dynamic_spec(size_t __id) noexcept
     {
+      // This call enforces the Mandates: condition that _Ts contains valid
+      // types and each type appears at most once. It could be a static_assert
+      // but this way failures give better diagnostics, due to calling the
+      // non-constexpr __invalid_dynamic_spec function.
+      [[maybe_unused]]
       constexpr bool __ok = __check_dynamic_spec_types<_Ts...>();
 
       if consteval {