From 546e4ad5df6c5fc33ba8fd2b9d6354f059c8aac5 Mon Sep 17 00:00:00 2001
From: Anton Shevchenko <ashevchenko@twitter.com>
Date: Tue, 4 Aug 2026 17:28:55 -0700
Subject: [PATCH] Keep a selection of only __typename as a struct, not an empty
 enum

`ExpandedSelection::render` turns a fieldless struct into the enum of its
variants. With no variants either -- a union member narrowed to with nothing
but `__typename` -- that produced `enum X {}`, which cannot deserialize.

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 .../tests/typename_only_selection.rs          | 26 +++++++++++++++++++
 .../typename_only_selection/query.graphql     |  8 ++++++
 .../typename_only_selection/schema.graphql    | 22 ++++++++++++++++
 ...ypename_only_on_union_member_response.json |  5 ++++
 .../src/codegen/selection.rs                  |  5 ++--
 5 files changed, 64 insertions(+), 2 deletions(-)
 create mode 100644 graphql_client/tests/typename_only_selection.rs
 create mode 100644 graphql_client/tests/typename_only_selection/query.graphql
 create mode 100644 graphql_client/tests/typename_only_selection/schema.graphql
 create mode 100644 graphql_client/tests/typename_only_selection/typename_only_on_union_member_response.json

diff --git a/graphql_client/tests/typename_only_selection.rs b/graphql_client/tests/typename_only_selection.rs
new file mode 100644
index 00000000..90c9cc98
--- /dev/null
+++ b/graphql_client/tests/typename_only_selection.rs
@@ -0,0 +1,26 @@
+use graphql_client::*;
+
+const RESPONSE: &str =
+    include_str!("typename_only_selection/typename_only_on_union_member_response.json");
+
+#[derive(GraphQLQuery)]
+#[graphql(
+    query_path = "tests/typename_only_selection/query.graphql",
+    schema_path = "tests/typename_only_selection/schema.graphql",
+    response_derives = "Debug, PartialEq, Eq"
+)]
+pub struct TypenameOnlyOnUnionMember;
+
+#[test]
+fn typename_only_on_union_member_deserialization() {
+    let response_data: typename_only_on_union_member::ResponseData =
+        serde_json::from_str(RESPONSE).unwrap();
+
+    let expected = typename_only_on_union_member::ResponseData {
+        do_thing: typename_only_on_union_member::TypenameOnlyOnUnionMemberDoThing::DoThingSuccess(
+            typename_only_on_union_member::TypenameOnlyOnUnionMemberDoThingOnDoThingSuccess {},
+        ),
+    };
+
+    assert_eq!(response_data, expected);
+}
diff --git a/graphql_client/tests/typename_only_selection/query.graphql b/graphql_client/tests/typename_only_selection/query.graphql
new file mode 100644
index 00000000..0b19e321
--- /dev/null
+++ b/graphql_client/tests/typename_only_selection/query.graphql
@@ -0,0 +1,8 @@
+mutation TypenameOnlyOnUnionMember {
+  doThing {
+    __typename
+    ... on DoThingSuccess {
+      __typename
+    }
+  }
+}
diff --git a/graphql_client/tests/typename_only_selection/schema.graphql b/graphql_client/tests/typename_only_selection/schema.graphql
new file mode 100644
index 00000000..67fc3e78
--- /dev/null
+++ b/graphql_client/tests/typename_only_selection/schema.graphql
@@ -0,0 +1,22 @@
+schema {
+  query: Query
+  mutation: Mutation
+}
+
+type Query {
+  things: [String!]!
+}
+
+type Mutation {
+  doThing: DoThingResult!
+}
+
+union DoThingResult = DoThingSuccess | DoThingFailure
+
+type DoThingSuccess {
+  thingId: ID!
+}
+
+type DoThingFailure {
+  reason: String!
+}
diff --git a/graphql_client/tests/typename_only_selection/typename_only_on_union_member_response.json b/graphql_client/tests/typename_only_selection/typename_only_on_union_member_response.json
new file mode 100644
index 00000000..fa474f9d
--- /dev/null
+++ b/graphql_client/tests/typename_only_selection/typename_only_on_union_member_response.json
@@ -0,0 +1,5 @@
+{
+  "doThing": {
+    "__typename": "DoThingSuccess"
+  }
+}
diff --git a/graphql_client_codegen/src/codegen/selection.rs b/graphql_client_codegen/src/codegen/selection.rs
index 1823e6d0..db4dce8b 100644
--- a/graphql_client_codegen/src/codegen/selection.rs
+++ b/graphql_client_codegen/src/codegen/selection.rs
@@ -629,8 +629,9 @@ impl<'a> ExpandedSelection<'a> {
                 .collect();
 
             // If we only have an `on` field, turn the struct into the enum
-            // of the variants.
-            if fields.peek().is_none() {
+            // of the variants. With no variants either, this would produce an
+            // enum with no variants, which can never deserialize.
+            if fields.peek().is_none() && !on_variants.is_empty() {
                 let item = quote! {
                     #response_derives
                     #[serde(tag = "__typename")]