std::experimental::latch::count_down (3) - Linux Man Pages
void count_down( ptrdiff_t n = 1); (1) (concurrency TS)
Atomically decrements the internal counter by n without blocking the caller.
If n is greater than the value of the internal counter or is negative, the behavior is undefined.
This operation synchronizes_with all calls that block on this latch and all is_ready calls on this latch that returns true.
n - the value by which the internal counter is decreased