Proposed resolution:

This wording is relative to N4910.

1. Modify [time.syn], header <chrono> synopsis, as indicated:

   […]
   // [time.duration.alg], specialized algorithms
   template<class Rep, class Period>
     constexpr common_type_t<duration<Rep, Period>> abs(duration<Rep, Period> d);
   […]
   

2. Modify [time.duration.alg] as indicated:

   [Drafting note: This will cause abs to reduce the period before returning it, much like the other arithmetic operators.

   This is not a breaking change, because code that was using abs with a non-reduced period before did not compile. ]

   template<class Rep, class Period>
     constexpr common_type_t<duration<Rep, Period>> abs(duration<Rep, Period> d);
   

   -1- Constraints: numeric_limits<Rep>::is_signed is true.

   -2- Returns: If d >= d.zero(), return +d, otherwise return -d.

[ 2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD. ]

[ 2022-08-23; Reflector poll: NAD ]

Not ill-formed, implementation should do a conversion. Changing it to return the reduced duration as an improvement would be for LEWG.

Currently [time.duration.alg] specifies abs(duration) as:

Returns: if d >= d.zero(), return d, otherwise return -d.

Because unary minus on durations is defined to return common_type_t<duration>(-rep_), and common_type_t for durations is specified to reduce the period, this is ill-formed with durations such as duration<int, ratio<1000, 1000>>, or any other type where the numerator and denominator of the period are not coprime.