N4885 [time.clock.cast.utc]/2:

template<class Duration>
  auto operator()(const time_point<SourceClock, Duration>& t) const
    -> decltype(SourceClock::to_utc(t));

Mandates: SourceClock::to_utc(t) returns a utc_time<Duration>, where Duration is a valid chrono::duration specialization.

Confusingly, the Duration that this operator() is templated on, is not the same as the Duration in the return type! What this means to say is:

Mandates: SourceClock::to_utc(t) returns a utc_time<Duration2>, where Duration2 is a valid chrono::duration specialization.

Attempting to static_assert that this operator() returns utc_time<Duration> matching the template parameter exactly, will fail to compile (observe that [time.clock.tai.overview] specifies that tai_clock::to_utc() returns utc_time<common_type_t<Duration, seconds>>). Which just happened to me, hence the issue. While I was confused by the wording, the intent seems clear upon a re-read, and I believe that this is an editorial issue, not an LWG issue. Thanks to @MattStephanson for diagnosing this and explaining it to me.

This occurs 4 times:

• [time.clock.cast.sys]/2
• [time.clock.cast.sys]/5
• [time.clock.cast.utc]/2
• [time.clock.cast.utc]/5