seq: Do not allow -w and -f to be specified at the same time

src/uu/seq/src/error.rs

@@ -28,6 +28,10 @@ pub enum SeqError {
     /// No arguments were passed to this function, 1 or more is required
     #[error("missing operand")]
     NoArguments,
+
+    /// Both a format and equal width where passed to seq
+    #[error("format string may not be specified when printing equal width strings")]
+    FormatAndEqualWidth,
 }
 
 fn parse_error_type(e: &ParseNumberError) -> &'static str {

src/uu/seq/src/seq.rs

@@ -114,6 +114,10 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         format: matches.get_one::<String>(OPT_FORMAT).map(|s| s.as_str()),
     };
 
+    if options.equal_width && options.format.is_some() {
+        return Err(SeqError::FormatAndEqualWidth.into());
+    }
+
     let first = if numbers.len() > 1 {
         match numbers[0].parse() {
             Ok(num) => num,

tests/by-util/test_seq.rs

@@ -19,6 +19,14 @@ fn test_no_args() {
         .stderr_contains("missing operand");
 }
 
+#[test]
+fn test_format_and_equal_width() {
+    new_ucmd!()
+        .args(&["-w", "-f", "%f", "1"])
+        .fails_with_code(1)
+        .stderr_contains("format string may not be specified");
+}
+
 #[test]
 fn test_hex_rejects_sign_after_identifier() {
     new_ucmd!()