bug#80443: Warn upon chmod + file

Dan Jacobson <[email protected]> writes:

> Any reason
> $ chmod + file
> should not trigger a warning?
>
> https://unix.stackexchange.com/questions/804566/shouldnt-chmod-file-be-an-error
> https://debbugs.gnu.org/80436

As Stephen Harris posted, POSIX requires it to behave as it currently does:

    +
    
    If perm is not specified, the '+' operation shall not change the
    file mode bits.

    If who is not specified, the file mode bits represented by perm for
    the owner, group, and other permissions, except for those with
    corresponding bits in the file mode creation mask of the invoking
    process, shall be set.
    

    Otherwise, the file mode bits represented by the specified who and
    perm values shall be set.

Also the grammar just below that on the POSIX page [1] allows an "op"
without a "permlist".

Collin

[1] https://pubs.opengroup.org/onlinepubs/9799919799/utilities/chmod.html