Proposed resolution:

This wording is relative to N5054.

1. Modify [time.duration.io] as indicated:

   template<class charT, class traits, class Rep, class Period>
     basic_ostream<charT, traits>&
       operator<<(basic_ostream<charT, traits>& os, const duration<Rep, Period>& d);
   

   -1- Effects: Inserts the duration `d` onto the stream `os` as if it were implemented as follows:

   basic_ostringstream<charT, traits> s;
   s.flags(os.flags());
   s.imbue(os.getloc());
   s.precision(os.precision());
   s << d.count() << units-suffix;
   return os << s.str();
   

   where units-suffix depends onis a character sequence of `charT` determined by the type `Period::type` as follows. In the following list, each quoted character sequence denotes the corresponding STATICALLY-WIDEN<charT> character sequence: […]

   [Drafting note:

   (1) The existing list is unchanged, including:

   Otherwise, if `Period::type` is `micro`, it is implementation-defined whether units-suffix is `"μs"` (`"\u00b5\u0073"`) or `"us"`.

   (2) Since `%q` in [time.format] refers to the duration's unit suffix as specified in [time.duration.io], this change also specifies the appropriate `charT` representation for `%q`. ]

[time.duration.io] specifies operator<< for `duration` as if by:

basic_ostringstream<charT, traits> s;
s.flags(os.flags());
s.imbue(os.getloc());
s.precision(os.precision());
s << d.count() << units-suffix;
return os << s.str();

For `Period::type` equal to `micro`, units-suffix is implementation-defined to be either `"μs"` (`"\u00b5\u0073"`) or `"us"`.

The type and encoding of units-suffix are not specified in terms of `charT`. In particular, if `"μs"` denotes an ordinary string literal, then for a wide stream the expression above inserts a `const char*` into a basic_ostream<wchar_t>. [ostream.inserters.character] specifies this operation by applying `out.widen()` to each individual `char`.

This does not perform transcoding. For example, when the ordinary literal encoding is UTF-8, the micro sign is represented by multiple char code units. Widening those code units individually does not produce the corresponding micro sign in the wide literal encoding.

For example:

std::wostringstream os;
os << std::chrono::microseconds{1};

should be able to produce:

1μs

when the implementation chooses the `μs` representation, but the current specification does not provide semantics that produce that result.

The same problem affects `%q` in [time.format], whose replacement is specified as "the duration's unit suffix as specified in [time.duration.io]".

This is similar to LWG 4522, which clarified that formatting to `wchar_t` requires transcoding rather than widening narrow code units.

The unit suffix should therefore be specified directly as a sequence of `charT`, using the literal encoding associated with `charT`.