This patch series reworks the handling of the empty chron-spec when formatting calendar types. For all types except duration with custom representation, the implementation no-longer delegates to operator<<, and instead configures formatter _M_chron_spec accordingly, and follows with normal format routine.
This patch series should not introduce functional changes, and passes an existing extensive test suite for empty chron-spec, in testsuite/std/time/format/empty_spec.cc. We introduce to new features to the format-spec, that are available only internally: - producing "is not a valid sth" output, by setting _M_debug flag - producing weekday index For the later, we need a new specifier, and '%\0' (embeded null terminator) was used for that purpose. This disallows conflicts with future extension. An alternative, that I would like to inquery, is to reserve '%v' (currently unused) for that purpose. The _M_parse will still reject that format specifier, but we will _M_format would recognize it. And we could add it into _M_parse in future. Use of 'v' seem to be fitting, as close values 'w', 'u' are used to print weekday index, and 'U', 'V', 'W' are used for weeks number of the year. Finally, this patch does not deduplicate the code between the formatters and existing operators<<. Such change may be desired, but as far as I know can be made later without changing the ABI. Most notably, operator<< would benefit from specialized sink that would output to ostream directly. Such _Sink would also be useful for std::print overload accepting ostream.
