Minor bug in in N4775 8.3.8(6) example

[lewissbaker]

https://github.com/GorNishanov/coroutines-ts/blob/2e7b3dacef35b9fbc895bca0a4b32392d76a6819/Coroutines/expressions.tex#L137

I think this example needs to use std::chrono::duration_cast to cast the input duration to the system_clock duration:

 template <class Rep, class Period>
 auto operator co_await(std::chrono::duration<Rep, Period> d) {
   struct awaiter {
     std::chrono::system_clock::duration duration;
     ...
     awaiter(std::chrono::system_clock::duration d) : duration(d){}
     bool await_ready() const { return duration.count() <= 0; }
     void await_resume() {}
     void await_suspend(std::experimental::coroutine_handle<> h){...}
   };
-  return awaiter{d};
+  return awaiter{std::chrono::duration_cast<std::chrono::system_clock::duration>(d)};
 }

[GorNishanov]

I have not checked the chrono wording, but, I did check with compilers and they seem to like it: https://godbolt.org/z/_Bj1Ke

[lewissbaker]

It seems to fail with an error about ambiguous conversion if you pass 1ns instead of 1s. Adding explicit duration_cast fixes this. https://godbolt.org/z/tLy_SR

[GorNishanov]

Cool. Thank you very much. I can probably fix it editorially.

[GorNishanov]

Though... It seems like duration correctly disables implicit conversion if there is a chance for an overflow. Hence, it works for 1s, 1ms, but, does not want to work for 1ns.

In this case, coercion might not be the right tool, since, it may overflow and wait for a long time, as opposed to for a little.

[lewissbaker]

I would have thought that it should have been detecting overflow if going the other way. system_clock::duration is std::chrono::microseconds so converting nanoseconds to microseconds should just be dividing by 1000 which can never overflow (assuming the same rep).

According to the cppreference the implicit conversion is disabled if there is a potential loss of precision. So that must be why it's failing for ns -> us conversion.

[lewissbaker]

So I guess it's fine. Really depends whether you want to enable the implicit truncation behaviour or not.