Proposed resolution:

This wording is relative to N4885.

1. Modify [syncstream.syncbuf.assign] as indicated:

   basic_syncbuf& operator=(basic_syncbuf&& rhs) noexcept;
   

   -1- Effects: […]

   -2- Postconditions: […]

   -3- Returns: […]

   -4- Remarks: […]

   void swap(basic_syncbuf& other) noexcept;
   

   -5- Preconditions: […]

   -6- Effects: […]

[ 2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-06-23; Reflector poll ]

Set status to Tentatively Ready after eight votes in favour during reflector poll.

[ 2021-05-22 Tim adds PR ]

The move assignment is specified to call emit() which can throw, and there's nothing in the wording providing for catching/ignoring the exception, so it can't be noexcept. The swap needs to call basic_streambuf::swap, which isn't noexcept, so it shouldn't be noexcept either.

[ 2020-11-21; Reflector prioritization ]

Set priority to 3 during reflector discussions.

The synopsis in [syncstream.syncbuf.overview] shows the move assignment operator and swap member as potentially throwing. The detailed descriptions in [syncstream.syncbuf.assign] are noexcept.

Daniel:

This mismatch is already present in the originally accepted paper P0053R7, so this is nothing that could be resolved editorially.