[committed] libstdc++: Fix -Wsign-compare in std::latch::count_down

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

Also add assertions for the precondition on the parameter's value.

libstdc++-v3/ChangeLog:

        * include/std/latch (latch::count_down): Add assertions for
        preconditions. Cast parameter to avoid -Wsign-compare on some
        targets.
---
 libstdc++-v3/include/std/latch | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch
index 146e1860979..1d254aa2581 100644
--- a/libstdc++-v3/include/std/latch
+++ b/libstdc++-v3/include/std/latch
@@ -63,9 +63,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _GLIBCXX_ALWAYS_INLINE void
     count_down(ptrdiff_t __update = 1)
     {
+      __glibcxx_assert(__update >= 0);
       auto const __old = __atomic_impl::fetch_sub(&_M_a,
                                    __update, memory_order::release);
-      if (__old == __update)
+      __glibcxx_assert(__update >= 0);
+      if (__old == static_cast<__detail::__platform_wait_t>(__update))
        __atomic_impl::notify_all(&_M_a);
     }
 
@@ -88,6 +90,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   private:
+    // This alignas is not redundant, it increases the alignment for
+    // long long on x86.
     alignas(__alignof__(__detail::__platform_wait_t)) 
__detail::__platform_wait_t _M_a;
   };
 _GLIBCXX_END_NAMESPACE_VERSION
-- 
2.46.2