Wait traits requirements

The basic_waitable_timer template uses wait traits to allow programs to customize wait and async_wait behavior. [Note: Possible uses of wait traits include:
— To enable timers based on non-realtime clocks.
— Determining how quickly wallclock-based timers respond to system time changes.
— Correcting for errors or rounding timeouts to boundaries.
— Preventing duration overflow. That is, a program may set a timer's expiry e to be Clock::max() (meaning never reached) or Clock::min() (meaning always in the past). As a result, computing the duration until timer expiry as e - Clock::now() may cause overflow. —end note]

For a type Clock meeting the Clock requirements (C++Std [time.clock.req]), a type X meets the WaitTraits requirements if it satisfies the requirements listed below.

In the table below, t denotes a (possibly const) value of type Clock::time_point; and d denotes a (possibly const) value of type Clock::duration.

Table 34. WaitTraits requirements

expression	return type	assertion/note pre/post-condition
`X::to_wait_duration(d)`	`Clock::duration`	Returns a `Clock::duration` value to be used in a `wait` or `async_wait` operation. [Note: The return value is typically representative of the duration `d`. —end note]
`X::to_wait_duration(t)`	`Clock::duration`	Returns a `Clock::duration` value to be used in a `wait` or `async_wait` operation. [Note: The return value is typically representative of the duration from `Clock::now()` until the time point `t`. —end note]