fmt: treat -N operands after -- as file names

src/uu/fmt/src/fmt.rs

@@ -71,6 +71,7 @@ mod options {
     pub const QUICK: &str = "quick";
     pub const TAB_WIDTH: &str = "tab-width";
     pub const FILES_OR_WIDTH: &str = "files";
+    pub const FILES_AFTER_DASHES: &str = "files-after-dashes";
 }
 
 pub type FileOrStdReader = BufReader<Box<dyn Read + 'static>>;
@@ -283,6 +284,13 @@ fn extract_files(matches: &ArgMatches) -> UResult<Vec<OsString>> {
                 Some(Ok(x.clone()))
             }
         })
+        .chain(
+            matches
+                .get_many::<OsString>(options::FILES_AFTER_DASHES)
+                .into_iter()
+                .flatten()
+                .map(|x| Ok(x.clone())),
+        )
         .collect();
 
     if files.as_ref().is_ok_and(Vec::is_empty) {
@@ -501,6 +509,13 @@ pub fn uu_app() -> Command {
                 .value_parser(clap::value_parser!(OsString))
                 .allow_negative_numbers(true),
         )
+        .arg(
+            Arg::new(options::FILES_AFTER_DASHES)
+                .action(ArgAction::Append)
+                .value_parser(clap::value_parser!(OsString))
+                .last(true)
+                .hide(true),
+        )
 }
 
 #[cfg(test)]

tests/by-util/test_fmt.rs

@@ -152,6 +152,33 @@ fn test_fmt_positional_width_not_first() {
         .stderr_contains("fmt: invalid option -- 1; -WIDTH is recognized only when it is the first\noption; use -w N instead");
 }
 
+#[test]
+fn test_fmt_negative_number_file_after_double_dash() {
+    use uutests::util::TestScenario;
+    use uutests::util_name;
+
+    let scene = TestScenario::new(util_name!());
+    scene.fixtures.write("-9", "hidden text\n");
+    scene.fixtures.write("f", "the quick brown fox jumps\n");
+    for args in [&["--", "-9", "f"][..], &["-u", "--", "-9", "f"]] {
+        scene
+            .ucmd()
+            .args(args)
+            .succeeds()
+            .stdout_only("hidden text\nthe quick brown fox jumps\n");
+    }
+    scene
+        .ucmd()
+        .args(&["f", "--", "-9"])
+        .succeeds()
+        .stdout_only("the quick brown fox jumps\nhidden text\n");
+    scene
+        .ucmd()
+        .args(&["--", "-9"])
+        .succeeds()
+        .stdout_only("hidden text\n");
+}
+
 #[test]
 fn test_fmt_width_not_valid_number() {
     new_ucmd!()