[visitor-plugin-common] fix: declare masked fragment types for unselected union members

.changeset/fix-masked-union-fragment-members.md

@@ -0,0 +1,5 @@
+---
+'@graphql-codegen/visitor-plugin-common': patch
+---
+
+Emit a fragment member type for every union or interface member when fragment spreads are masked or combined, including members that select no fields. Nested spreads were referring to those names, and TypeScript reported them as missing.

packages/plugins/other/visitor-plugin-common/src/selection-set-to-object.ts

@@ -1259,7 +1259,18 @@ export class SelectionSetToObject<
           : this.buildFragmentTypeName(fragmentName, fragmentSuffix, typeName);
 
       if (possibleFields.length === 0) {
-        return [];
+        const namesEveryMember =
+          hasMultipleTypes &&
+          (this._config.inlineFragmentTypes === 'mask' ||
+            this._config.inlineFragmentTypes === 'combine');
+        if (!namesEveryMember) {
+          return [];
+        }
+        return {
+          name: declarationName,
+          content: this.getEmptyObjectType(),
+          isUnionType: false,
+        };
       }
 
       const flatFields = possibleFields.map(selectionObject => {

packages/plugins/typescript/operations/tests/ts-documents.spec.ts

@@ -5326,6 +5326,69 @@ function test(q: GetEntityBrandDataQuery): void {
       `);
     });
 
+    it('#10920 - keeps masked fragment member types that a nested spread references', async () => {
+      const testSchema = buildSchema(/* GraphQL */ `
+        type Human {
+          id: String!
+          name: String!
+        }
+        type Robot {
+          id: String!
+          registeredAt: String!
+        }
+        union User = Human | Robot
+        type Task {
+          id: String!
+          assignee: User!
+        }
+        type Query {
+          task(id: String!): Task!
+        }
+      `);
+
+      const fragment = parse(/* GraphQL */ `
+        fragment HumanName_User on User {
+          ... on Human {
+            name
+          }
+        }
+
+        fragment TaskUser_User on User {
+          ... on Human {
+            id
+          }
+          ... on Robot {
+            id
+          }
+          ...HumanName_User
+        }
+
+        fragment TaskDetail_Task on Task {
+          id
+          assignee {
+            ...TaskUser_User
+          }
+        }
+
+        query GetTask($id: String!) {
+          task(id: $id) {
+            ...TaskDetail_Task
+          }
+        }
+      `);
+
+      const { content } = await plugin(
+        testSchema,
+        [{ location: '', document: fragment }],
+        { inlineFragmentTypes: 'mask' },
+        { outputFile: 'graphql.ts' },
+      );
+
+      expect(content).toContain('type HumanName_User_Human_Fragment =');
+      expect(content).toContain('type HumanName_User_Robot_Fragment =');
+      await validate(content);
+    });
+
     it('#6874 - generates types when parent type differs from spread fragment member types', async () => {
       const testSchema = buildSchema(/* GraphQL */ `
         interface Animal {