libstdc++: Fix -Wunused warning

As noted in PR 66146 comment 35, there is a new warning in the new
std::call_once implementation.

libstdc++-v3/ChangeLog:

	* src/c++11/mutex.cc (std::once_flag::_M_finish): Add
	maybe_unused attribute to variable used in assertion.

diff --git a/libstdc++-v3/src/c++11/mutex.cc b/libstdc++-v3/src/c++11/mutex.cc
index 4d42bed8ecc9017b483d11265a093b930e7f034d..93a5eae391d57f8ab73ead80ea41092428865f71 100644 (file)
--- a/libstdc++-v3/src/c++11/mutex.cc
+++ b/libstdc++-v3/src/c++11/mutex.cc
@@ -74,7 +74,8 @@ std::once_flag::_M_finish(bool returning) noexcept
     }
   else
     {
-      int prev = __atomic_exchange_n(&_M_once, newval, __ATOMIC_RELEASE);
+      int prev [[maybe_unused]]
+       = __atomic_exchange_n(&_M_once, newval, __ATOMIC_RELEASE);
       __glibcxx_assert(prev & _Bits::_Active);
       // Wake any other threads waiting for this execution to finish.
       constexpr int futex_wake = 129; // FUTEX_WAKE_PRIVATE