We (seastar) recently hit a build error with gcc 9 because the fix in 0998d2fd59e7a5eb3a3566c57625702bbdc6a05f is missing.

Given that P1286R2 is about c++11 and newer, could the fix be backported to gcc 9?

The original commit message is:

    Implement P1286R2, Contra CWG1778
    
    The C++11 requirement that an explicit exception-specification on a
    defaulted function match the implicit one was found to be problematic for
    std::atomic.  This paper, adopted in February, simply removes that
    requirement: if an explicitly defaulted function has a different
    exception-specification, that now works just like a user-written function:
    either it isn't noexcept when it could be, or it is noexcept and will call
    terminate if an exception is thrown.