From 79225d3a2d520456d458ed10ecef3d3a8bd2e06f Mon Sep 17 00:00:00 2001
From: Ed Page <eopage@gmail.com>
Date: Mon, 7 Nov 2022 06:07:48 -0600
Subject: [PATCH] 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 |  2 +-
 tests/derive/flags.rs           | 51 +++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/clap_derive/src/derives/args.rs b/clap_derive/src/derives/args.rs
index 15f676be..ad2a6463 100644
--- a/clap_derive/src/derives/args.rs
+++ b/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
                         }
                     }
diff --git a/tests/derive/flags.rs b/tests/derive/flags.rs
index 63fb216a..12eb7d51 100644
--- a/tests/derive/flags.rs
+++ b/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()
+    );
+}