test(parser): Show flag behavior

tests/builder/flags.rs

@@ -222,3 +222,22 @@ fn leading_dash_stripped() {
     let cmd = Command::new("mycat").arg(Arg::new("filename").long("--filename"));
     cmd.debug_assert();
 }
+
+#[test]
+fn optional_value() {
+    let cmd = Command::new("flag").args([arg!(-f --flag "some flag").action(ArgAction::SetTrue)]);
+
+    let m = cmd.clone().try_get_matches_from(vec![""]).unwrap();
+    assert!(!*m.get_one::<bool>("flag").expect("defaulted by clap"));
+
+    let m = cmd.clone().try_get_matches_from(vec!["", "-f"]).unwrap();
+    assert!(*m.get_one::<bool>("flag").expect("defaulted by clap"));
+
+    cmd.clone()
+        .try_get_matches_from(vec!["", "-f", "true"])
+        .unwrap_err();
+
+    cmd.clone()
+        .try_get_matches_from(vec!["", "-f", "false"])
+        .unwrap_err();
+}

tests/derive/flags.rs

@@ -329,3 +329,27 @@ fn unit_for_negation() {
         Opt::try_parse_from(["test", "--arg", "--no-arg"]).unwrap()
     );
 }
+
+#[test]
+#[should_panic = "Argument `alice`'s action SetTrue is incompatible with `num_args(0..=1)`"]
+fn optional_value_flag() {
+    #[derive(Parser, PartialEq, Eq, Debug)]
+    struct Opt {
+        #[arg(short, long, num_args=0..=1)]
+        alice: bool,
+    }
+
+    assert_eq!(Opt { alice: false }, Opt::try_parse_from(["test"]).unwrap());
+    assert_eq!(
+        Opt { alice: true },
+        Opt::try_parse_from(["test", "-a"]).unwrap()
+    );
+    assert_eq!(
+        Opt { alice: true },
+        Opt::try_parse_from(["test", "-a", "true"]).unwrap()
+    );
+    assert_eq!(
+        Opt { alice: false },
+        Opt::try_parse_from(["test", "-a", "false"]).unwrap()
+    );
+}