Proposed resolution:

This wording is relative to N4892.

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

   template<class charT, class traits, class Allocator>
     void swap(basic_syncbuf<charT, traits, Allocator>& a,
               basic_syncbuf<charT, traits, Allocator>& b) noexcept;
   

   -1- Effects: Equivalent to a.swap(b).

[ 2022-02-10; Jens comments ]

The desired fix was applied editorially while applying

P2450 C++ Standard Library Issues to be moved in Virtual Plenary, Oct. 2021

[ 2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP. ]

[ 2021-10-14; Reflector poll ]

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

LWG 3498 fixes the inconsistent noexcept-specifiers for member functions of basic_syncbuf, but the proposed resolution in LWG 3498 seems to miss the non-member swap, which also has inconsistent noexcept-specifier: [syncstream.syncbuf.special] says it's noexcept, while [syncstream.syn] says it's not.

Since the non-member swap and the member swap have equivalent effect, and LWG 3498 removes noexcept from the latter, I think it's pretty clear that the former should not be noexcept.