Proposed resolution:

This wording is relative to N4928.

1. Modify [syncstream.osyncstream.overview] as indicated:

   namespace std {
     template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
     class basic_osyncstream : public basic_ostream<charT, traits> {
     public:
       […]
       using syncbuf_type = basic_syncbuf<charT, traits, Allocator>;
       […]
       
       // assignment
       basic_osyncstream& operator=(basic_osyncstream&&) noexcept;
   
       […]
     
     private:
       syncbuf_type sb; // exposition only
     };
   }
   

[ 2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP. ]

[ 2023-02-06; Reflector poll ]

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

The synopsis of std::basic_osyncstream ([syncstream.osyncstream.overview]) indicates that it's member functions behave as if it hold a std::basic_syncbuf as its subobject, and according to [functions.within.classes], std::basic_osyncstream's move assignment operator should call std::basic_syncbuf's move assignment operator.

However, currently std::basic_osyncstream's move assignment operator is noexcept, while std::basic_syncbuf's is not. So when an exception is thrown from move assignment between std::basic_syncbuf objects, std::terminate should be called.

It's clarified in LWG 3498 that an exception can escape from std::basic_syncbuf's move assignment operator. Is there any reason that an exception shouldn't escape from std::basic_osyncstream's move assignment operator?