fix(derive): name the count type in standing presence checks

conformance/tests/update_from.rs

@@ -642,3 +642,42 @@ fn a_standing_group_member_conflicts_with_a_sibling_flag() {
         "standing --yaml still conflicts with --strict",
     );
 }
+
+/// A counted `u8` whose presence is "not the default". The generated
+/// `!= Default::default()` must name `u8`, or `serde_json`'s `PartialEq<Value> for u8`
+/// makes the comparison ambiguous — hk's CLI hit that on every build that pulls
+/// `serde_json` into the same crate.
+#[derive(Cli, Debug, PartialEq)]
+#[usage(bin = "counted", completion)]
+struct Counted {
+    #[usage(short, long, global, count, overrides("--quiet"))]
+    verbose: u8,
+    #[usage(short, long, global, overrides("--verbose"))]
+    quiet: bool,
+    #[usage(subcommand)]
+    command: CountedCmd,
+}
+
+#[derive(Subcommands, Debug, PartialEq)]
+enum CountedCmd {
+    Run,
+}
+
+#[test]
+fn a_standing_count_compiles_beside_serde_json() {
+    // Keep `serde_json` live in this module so its `PartialEq` impls stay in scope.
+    let _ = serde_json::json!({"n": 1u8});
+
+    let a = argv(["-vv", "run"]);
+    let mut counted = Counted::parse_from(&a).expect("two -v");
+    assert_eq!(counted.verbose, 2);
+
+    // A second line that says nothing about `-v` must keep the standing count —
+    // presence is "not the default", and that comparison has to name `u8`.
+    let a = argv(["run"]);
+    counted
+        .try_update_from(&a)
+        .expect("the standing count answers for itself");
+    assert_eq!(counted.verbose, 2);
+    assert!(!counted.quiet);
+}

derive/src/codegen.rs

@@ -3233,7 +3233,13 @@ fn standing_presence(field: &Field) -> Option<TokenStream> {
         }),
         Kind::Flag { .. } | Kind::Arg { .. } => Some(match field.shape {
             Shape::Bool => quote!(__usage_s.#ident),
-            Shape::Count => quote!(__usage_s.#ident != ::std::default::Default::default()),
+            // Name the field's type: `Default::default()` alone is ambiguous when an
+            // adopter also brings `serde_json::Value`'s `PartialEq<u8>` into scope
+            // (hk's `verbose: u8` count under a workspace that pulls serde_json).
+            Shape::Count => {
+                let ty = &field.ty;
+                quote!(__usage_s.#ident != <#ty as ::std::default::Default>::default())
+            }
             Shape::Optional => quote!(__usage_s.#ident.is_some()),
             // Nowhere to put "absent", so the field always holds something. The same
             // reading of the type that makes it required in the first place.
@@ -4918,7 +4924,8 @@ fn merge_present(field: &Field) -> TokenStream {
     match field.shape {
         Shape::Bool => quote!(partial.#given || partial.#ident),
         Shape::Count => {
-            quote!(partial.#given || partial.#ident != ::std::default::Default::default())
+            let ty = &field.ty;
+            quote!(partial.#given || partial.#ident != <#ty as ::std::default::Default>::default())
         }
         Shape::Optional => quote!(partial.#given || partial.#ident.is_some()),
         Shape::Required | Shape::Many => quote!(partial.#given || !partial.#ident.is_empty()),