First I'll say that fmt::print(ostream&) does the right thing and just
calls ostream.write.
https://github.com/fmtlib/fmt/blob/35c0286cd8f1365bffbc417021e8cd23112f6c8f/include/fmt/ostream.h#L55-L67
But in the paper wording, things arent exactly clear.

The paper says
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2093r12.html#wording

> void vprint_nonunicode(ostream& os, string_view fmt, format_args args);
>
> 4 Effects: Writes the result of vformat(os.getloc(), fmt, args) to os.
>
> Throws: As specified in [format.err.report] or system_error if a call
> by the implementation to an operating system or other underlying API
> results in an error that prevents the function from meeting its
> specifications.

Iostreams have their own error handling mechanisms with their 3 error
flags. They can even throw exceptions if one opts-in. All IO functions
follow the same convention regarding these flags, and std::print that
accepts ostream should probably follow
it. https://en.cppreference.com/w/cpp/named_req/UnformattedOutputFunction
The easiest way is to just call ostream.write() like in fmtlib.

I'm writing this because with the current wording I get the impression
that system_error should be thrown if failbit gets set (or even if it
was set before the call to std::print). That exception should be used
only when some C APIs return error like WriteConsoleW() on Windows.