I just came across [time.duration.io]p4:

> If Period​::​type is micro, but the character U+00B5 cannot be represented in the encoding used for charT, the unit suffix "us" is used instead of "μs".

How is the determination as to whether the character can be represented to be done?  It seems this would involve consulting the locale.  Or is this effectively implementation defined behavior?

Perhaps this is worth an LWG issue to at least clarify the behavior?