sort: don't rewrite a -t<chars> file operand after --

src/uu/sort/src/sort.rs

@@ -2155,7 +2155,16 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     // https://github.com/uutils/coreutils/issues/2424#issuecomment-863825242,
     // and the same rewrite in `cut`), so rewrite every attached `-t<chars>`
     // argument to its long form, which preserves the separator verbatim.
+    // Arguments after `--` are file operands and must be left untouched.
+    let mut seen_double_dash = false;
     let args = args.into_iter().map(|x| {
+        if seen_double_dash {
+            return x;
+        }
+        if x == "--" {
+            seen_double_dash = true;
+            return x;
+        }
         // Non-UTF-8 separators are rejected later anyway, so lossy conversion
         // here only affects arguments that cannot become a valid separator.
         let as_str = x.to_string_lossy();

tests/by-util/test_sort.rs

@@ -2097,6 +2097,15 @@ fn test_separator_attached_equals() {
         .stdout_only("b=a=d\na=b=c\n");
 }
 
+#[test]
+fn test_separator_like_file_operand_after_double_dash() {
+    // After `--`, a file named `-t=` is an operand and must not be rewritten
+    // to `--field-separator==`.
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("-t=", "b\na\n");
+    ucmd.args(&["--", "-t="]).succeeds().stdout_only("a\nb\n");
+}
+
 #[test]
 fn test_separator_attached_equals_double() {
     // `-t==` selects the two-character separator `==`, which GNU rejects.