Corentin Jabot wrote:

Very rough draft https://isocpp.org/files/papers/D3258R0.pdf

Looks good. I'm however not sure that it makes sense to format
char8_t (char16_t is borderline.)

Or maybe that's only intended for {:?} ?

What about iostream?
This is a story for another paper (One that an enthusiastic
reader is encouraged to write!)

Here's that paper:

Insert at the end of https://eel.is/c++draft/ostream.inserters.character
the following:

template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const char8_t* s);
template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, const char16_t* s);
template<class traits>
  basic_ostream<char, traits>& operator<<(basic_ostream<char, traits>& out, const char32_t* s);

Preconditions: s is not a null pointer.

Effects:
  Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of `out`.
  Creates a character sequence `seq` holding the transcoded representation
  of s [format.string.transcode].
  Determines padding for `seq` as described in [ostream.formatted.reqmts]. Inserts `seq` into
  `out`. Calls `width(0)`.

Returns:
  `out`.

It would be odd to supply inserters without corresponding extractors.

Characters should be handled as well as strings:

template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char8_t);
template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char16_t);
template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& out, char32_t);

Variations of the above for both characters and strings are already defined as deleted in [ostream.general].

Tom.