[libcxx-commits] [libcxx] [libc++][chrono][threading] Implement LWG 3504: `condition_variable::wait_for` is overspecified (PR #222443)

[Vishank Singh via libcxx-commits]

================
@@ -141,29 +147,7 @@ class _LIBCPP_EXPORTED_FROM_ABI condition_variable {
 
   template <class _Rep, class _Period>
   _LIBCPP_HIDE_FROM_ABI cv_status wait_for(unique_lock<mutex>& __lk, const chrono::duration<_Rep, _Period>& __d) {
-    using namespace chrono;
-    if (__d <= __d.zero())
-      return cv_status::timeout;
-    using __ns_rep                   = nanoseconds::rep;
-    steady_clock::time_point __c_now = steady_clock::now();
-
-#  if _LIBCPP_HAS_COND_CLOCKWAIT
-    using __clock_tp_ns     = time_point<steady_clock, nanoseconds>;
-    __ns_rep __now_count_ns = std::__safe_nanosecond_cast(__c_now.time_since_epoch()).count();
-#  else
-    using __clock_tp_ns     = time_point<system_clock, nanoseconds>;
-    __ns_rep __now_count_ns = std::__safe_nanosecond_cast(system_clock::now().time_since_epoch()).count();
-#  endif
-
-    __ns_rep __d_ns_count = std::__safe_nanosecond_cast(__d).count();
-
-    if (__now_count_ns > numeric_limits<__ns_rep>::max() - __d_ns_count) {
-      __do_timed_wait(__lk, __clock_tp_ns::max());
-    } else {
-      __do_timed_wait(__lk, __clock_tp_ns(nanoseconds(__now_count_ns + __d_ns_count)));
-    }
-
-    return steady_clock::now() - __c_now < __d ? cv_status::no_timeout : cv_status::timeout;
+    return wait_until(__lk, std::__rel_to_abs(__d));
----------------
VishankSingh wrote:

No, this is only LWG 3504. The previous non-predicate `condition_variable::wait_for` had a custom implementation that manually converted to nanoseconds and measured elapsed time. 
We can replace that entire path with the standard-mandated form `return wait_until(lock, *rel-to-abs*(rel_time));`

The other overloads were already simple one-liners (`now() + rel_time`), so they only needed a small change.

https://github.com/llvm/llvm-project/pull/222443