[time.duration.cons]/3:

Constraints: No overflow is induced in the conversion and treat_­as_­floating_­point_­v<rep> is true or both ratio_­divide<Period2, period>​::​den is 1 and treat_­as_­floating_­point_­v<Rep2> is false.

1. There are a bit too many conjunctions in this sentence, making the sentence hard to parse.
2. It's unclear what "No overflow is induced in the conversion" means. Based on LWG 2094, I guess it means that no overflow shall occur when computing ratio_divide, but this is not obvious.
   (My initial reading is that duration<int, micro> us = duration<int, milli>::max(); should be an error due to overflow, but the example shows that ms is convertible to duration<int, micro>, and we can't SFINAE on argument value).