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[RFC PATCH 3/4] libstdc++ atomic_futex: Use std::chrono::steady_clock as reference clock

The user-visible effect of this change is that std::future::wait_for now
uses std::chrono::steady_clock to determine the timeout. This makes it
immune to changes made to the system clock. It also means that anyone using
their own clock types with std::future::wait_until will have the timeout
converted to std::chrono::steady_clock rather than

Now that use of both std::chrono::steady_clock and
std::chrono::system_clock are correctly supported for the wait
timeout, I believe that std::chrono::steady_clock is a better choice
for the reference clock that is used by std::future::wait_for, and all
other clocks are converted to, since it is guaranteed to advance
steadily. The previous behaviour of converting to
std::chrono::system_clock risks timeouts changing dramatically when
the system clock is changed.

 libstdc++-v3/include/bits/atomic_futex.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index c2b3df03592..7ba86dfeec7 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
   template <unsigned _Waiter_bit = 0x80000000>
   class __atomic_futex_unsigned : __atomic_futex_unsigned_base
-    typedef chrono::system_clock __clock_t;
+    typedef chrono::steady_clock __clock_t;
     // This must be lock-free and at offset 0.
     atomic<unsigned> _M_data;
     _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo,
-	const chrono::time_point<__clock_t, _Dur>& __atime)
+	const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	  const chrono::time_point<_Clock, _Duration>& __atime)
-	// DR 887 - Sync unknown clock to known clock.
 	const typename _Clock::time_point __c_entry = _Clock::now();
 	const __clock_t::time_point __s_entry = __clock_t::now();
 	const auto __delta = __atime - __c_entry;
     template<typename _Duration>
     _M_load_when_equal_until(unsigned __val, memory_order __mo,
-	const chrono::time_point<__clock_t, _Duration>& __atime)
+	const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) == __val)

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