[committed] libstdc++: Fix nodiscard warnings in perf test for memory pools

[Jonathan Wakely]

The use of unnamed std::lock_guard temporaries was intentional here, as
they were used like barriers (but std::barrier isn't available until
C++20). But that gives nodiscard warnings, because unnamed temporary
locks are usually unintentional. Use named variables in new block scopes
instead.

libstdc++-v3/ChangeLog:

        * testsuite/performance/20_util/memory_resource/pools.cc: Fix
        -Wunused-value warnings about unnamed std::lock_guard objects.
---

Tested x86_64-linux. Pushed to trunk.

 .../performance/20_util/memory_resource/pools.cc  | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git 
a/libstdc++-v3/testsuite/performance/20_util/memory_resource/pools.cc 
b/libstdc++-v3/testsuite/performance/20_util/memory_resource/pools.cc
index 8a2a5f1facf..5d57d1a5a0c 100644
--- a/libstdc++-v3/testsuite/performance/20_util/memory_resource/pools.cc
+++ b/libstdc++-v3/testsuite/performance/20_util/memory_resource/pools.cc
@@ -167,7 +167,10 @@ void test_lists_resource_per_thread()
   auto run_test = [&mx] (std::pmr::memory_resource* memres,
                         __gnu_test::time_counter* timers)
   {
-    std::lock_guard<std::mutex>{mx};  // block until the mutex can be locked
+    {
+      // block until the mutex can be locked
+      std::lock_guard<std::mutex> wait_for_gate_to_be_unlocked{mx};
+    }
     populate_lists(memres, timers);
   };
 
@@ -239,7 +242,10 @@ void test_lists_shared_resource()
   auto run_test = [&mx] (std::pmr::memory_resource* memres,
                         __gnu_test::time_counter* timers)
   {
-    std::lock_guard<std::mutex>{mx};  // block until the mutex can be locked
+    {
+      // block until the mutex can be locked
+      std::lock_guard<std::mutex> wait_for_gate_to_be_unlocked{mx};
+    }
     populate_lists(memres, timers);
   };
 
@@ -307,7 +313,10 @@ void test_cross_thread_dealloc()
   [&, num_threads] (std::pmr::memory_resource* memres, int i, bool with_exit)
   {
     std::size_t counter = 0;
-    std::lock_guard<std::mutex>{mx};
+    {
+      // block until the mutex can be locked
+      std::lock_guard<std::mutex> wait_for_gate_to_be_unlocked{mx};
+    }
     // Fill this thread's buffer with allocations:
     for (X& x : allocs[i])
     {
-- 
2.47.0