date: `iso-8601` argument format discrepancy

[my4ng]

The coreutils version currently accepts more argument option-value formats than that of the GNU version:

GNU: -I[FMT], --iso-8601[=FMT] output date/time in ISO 8601 format... coreutils: -I, --iso-8601 [<FMT>] output date/time in ISO 8601 format...

This means the following arguments are accepted by coreutils but not GNU:

• date -I=seconds
• date -I seconds
• date --iso-8601 seconds

GNU will treat =seconds as the value in the first one, and treat seconds as the positional argument [+FORMAT] in the next two.

coreutils will also reject some that are accepted by GNU since it will take the positional argument as the argument value, e.g. date -I 05091009:

error: invalid value '05091009' for '--iso-8601 [<FMT>]'
  [possible values: date, hours, minutes, seconds, ns]

This cannot be simply fixed by setting require_equals for the clap argument, since it would also preclude the correct syntax -Iseconds. There probably are more examples of this discrepancy in coreutils (please comment below).

This is a known limitation in clap, and I have commented in this clap issue and made a fix PR.

[BenWiederhake]

The first part seems to be no big problem: After all, having an extension to GNU is fine, as long as we remain drop-in compatible.

The second part seems to be a bigger problem. Also see #4254 because it seems more and more that clap is an awesome project, but simply solves a different problem than we have. Here's a demonstration that uutils-args will be able to handle this particular case correctly: https://github.com/uutils/uutils-args/pull/113/files#diff-8da4a3571369459ddecc9e114ae27fef1bd0f3c8ae3e81334ef463b425981ab3R137-R165

[my4ng]

Indeed. They are very hesitant to introduce breaking changes because how many things depend on it.

Which particular part in the diff are you referring to?