fix(derive): Allow defaulted value parser for '()' fields

Inspired by #4458

This is a compatible change because it turns a compiler error into a
working case.

clap_derive/src/derives/args.rs

@@ -233,9 +233,9 @@ pub fn gen_augment(
 
                 let implicit_methods = match **ty {
                     Ty::Unit => {
+                        // Leaving out `value_parser` as it will always fail
                         quote_spanned! { ty.span()=>
                             .value_name(#value_name)
-                            #value_parser
                             #action
                         }
                     }

tests/derive/flags.rs

@@ -281,3 +281,54 @@ fn override_implicit_from_flag_positional() {
         Opt::try_parse_from(&["test", "true"]).unwrap()
     );
 }
+
+#[test]
+fn unit_for_negation() {
+    #[derive(Parser, PartialEq, Eq, Debug)]
+    struct Opt {
+        #[arg(long)]
+        arg: bool,
+        #[arg(long, action = ArgAction::SetTrue, overrides_with = "arg")]
+        no_arg: (),
+    }
+
+    assert_eq!(
+        Opt {
+            arg: false,
+            no_arg: ()
+        },
+        Opt::try_parse_from(&["test"]).unwrap()
+    );
+
+    assert_eq!(
+        Opt {
+            arg: true,
+            no_arg: ()
+        },
+        Opt::try_parse_from(&["test", "--arg"]).unwrap()
+    );
+
+    assert_eq!(
+        Opt {
+            arg: false,
+            no_arg: ()
+        },
+        Opt::try_parse_from(&["test", "--no-arg"]).unwrap()
+    );
+
+    assert_eq!(
+        Opt {
+            arg: true,
+            no_arg: ()
+        },
+        Opt::try_parse_from(&["test", "--no-arg", "--arg"]).unwrap()
+    );
+
+    assert_eq!(
+        Opt {
+            arg: false,
+            no_arg: ()
+        },
+        Opt::try_parse_from(&["test", "--arg", "--no-arg"]).unwrap()
+    );
+}