Keep a selection of only __typename as a struct, not an empty enum

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);
+}

graphql_client/tests/typename_only_selection/query.graphql

@@ -0,0 +1,8 @@
+mutation TypenameOnlyOnUnionMember {
+  doThing {
+    __typename
+    ... on DoThingSuccess {
+      __typename
+    }
+  }
+}

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!
+}

graphql_client/tests/typename_only_selection/typename_only_on_union_member_response.json

@@ -0,0 +1,5 @@
+{
+  "doThing": {
+    "__typename": "DoThingSuccess"
+  }
+}

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")]