Proposed resolution:

This wording is relative to N4741.

1. Edit [time.duration.nonmember] as indicated:

   -1- In the function descriptions that follow, unless stated otherwise, let CD represents the return type of the function.

   […]

   template<class Rep1, class Period1, class Rep2, class Period2>
     constexpr common_type_t<Rep1, Rep2>
       operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
   

   Let CD be common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>.

   -10- Returns: CD(lhs).count() / CD(rhs).count().

[ 2018-06 Rapperswil: Adopted ]

[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[time.duration.nonmember]/1 states

In the function descriptions that follow, CD represents the return type of the function.

From what I could find, many definitions of CD in the paragraphs of [time.duration.nonmember] were lifted to [time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10, which the change rendered ill-formed:

template<class Rep1, class Period1, class Rep2, class Period2>
  constexpr common_type_t<Rep1, Rep2>
    operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Returns: CD(lhs).count() / CD(rhs).count().

In this case, we want CD to mean common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>. That way, the division has the expected semantics of dividing two quantities of the same dimension.