[MannXo]

fmt -- -9 notes.txt took -9 as the obsolete -WIDTH option instead of a file name. clap's index_of does not count --, so the value looked exactly like fmt -9 notes.txt. Depending on the position it was dropped and used as the width, rejected with "invalid option", or (fmt -- -9 alone) the file list came out empty and stdin was read.

Operands after -- now go into a separate hidden positional with last(true), so they never reach the -WIDTH detection and are appended to the file list as they are. fmt -9 file and the "only when it is the first option" error are unchanged, and so is --help.

$ fmt -- -9 notes.txt
hidden text
the quick brown fox jumps
$ fmt notes.txt -- -9
the quick brown fox jumps
hidden text

test_fmt_negative_number_file_after_double_dash fails on main.

Closes #15751

[cakebaker]

Thanks for your PR!