fix: @provides with union type selection set (#147)

```graphql
# A
extend schema @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@key", "@Shareable", "@provides", "@external"])

type Query {
  unouser: User
}

type User @key(fields: "id") {
  id: ID!
  name: String!
  age: String!
}
```

```graphql
# B
extend schema @link(url: "https://specs.apollo.dev/federation/v2.3", import: ["@key", "@provides", "@Shareable", "@external"])

type Query {
  userOrCar: UserOrCar @provides(fields: "... on User { id } ")
}

union UserOrCar = User | Car

type User {
  id: ID! @external
}

type Car {
  uid: ID!
}
```


![graphviz](https://github.com/user-attachments/assets/fe8d022e-7988-4dfc-8b3f-8e79b906c611)

---------

Co-authored-by: Kamil Kisiela <[email protected]>

Author: n1ru4l

.changeset/sour-spiders-brush.md

@@ -0,0 +1,12 @@
+---
+"@theguild/federation-composition": patch
+---
+
+Add support for `@provides` fragment selection sets on union type fields.
+
+```graphql
+type Query {
+  media: [Media] @shareable @provides(fields: "... on Book { title }")
+}
+union Media = Book | Movie
+```

__tests__/subgraph/errors/PROVIDES_ON_NON_OBJECT_FIELD.spec.ts

@@ -1,5 +1,9 @@
 import { expect, test } from "vitest";
-import { graphql, testVersions } from "../../shared/testkit.js";
+import {
+  assertCompositionSuccess,
+  graphql,
+  testVersions,
+} from "../../shared/testkit.js";
 
 testVersions((api, version) => {
   test("PROVIDES_ON_NON_OBJECT_FIELD", () => {
@@ -47,4 +51,144 @@ testVersions((api, version) => {
       }),
     );
   });
+
+  test("PROVIDES_ON_NON_OBJECT_FIELD: is not raised for union field with fragment object type selection sets", () => {
+    const errors = api.composeServices([
+      {
+        name: "users",
+        typeDefs: graphql`
+          extend schema
+            @link(
+              url: "https://specs.apollo.dev/federation/${version}"
+              import: ["@key", "@provides", "@external"]
+            )
+
+          type Query {
+            result: ResultType
+          }
+
+          type EntityA {
+            field1: String @external
+            field2: String @external
+          }
+
+          type EntityB {
+            field1: String @external
+            field2: String @external
+            field3: String @external
+          }
+
+          union UnionType = EntityA | EntityB
+
+          type ResultType {
+            results: [UnionType!]! @provides(fields: "... on EntityA { field1 field2 } ... on EntityB { field1 field2 field3 }")
+          }
+        `,
+      },
+      {
+        name: "other",
+        typeDefs: graphql`
+          extend schema
+            @link(
+              url: "https://specs.apollo.dev/federation/${version}"
+              import: ["@key", "@shareable"]
+            )
+
+          type Query {
+            a: EntityA
+            b: EntityB
+          }
+
+          union UnionType = EntityA | EntityB
+
+          type EntityA @key(fields: "field1") {
+            field1: String @shareable
+            field2: String @shareable
+          }
+
+          type EntityB @key(fields: "field1")  {
+            field1: String @shareable
+            field2: String @shareable
+            field3: String @shareable
+          }
+        `,
+      },
+    ]).errors;
+    expect(errors).toEqual(undefined);
+  });
+
+  test("PROVIDES_ON_NON_OBJECT_FIELD: is not raised for union field with fragment object type selection sets (variant)", () => {
+    const result = api.composeServices([
+      {
+        name: "a",
+        typeDefs: graphql`
+          extend schema
+            @link(
+              url: "https://specs.apollo.dev/federation/v2.3"
+              import: ["@key", "@shareable", "@external"]
+            )
+
+          type Query {
+            media: [Media] @shareable
+          }
+
+          union Media = Book | Movie
+
+          type Book @key(fields: "id") {
+            id: ID!
+          }
+
+          type Movie @key(fields: "id") {
+            id: ID!
+          }
+        `,
+      },
+      {
+        name: "b",
+        typeDefs: graphql`
+          extend schema
+            @link(
+              url: "https://specs.apollo.dev/federation/v2.3"
+              import: ["@key", "@shareable", "@provides", "@external"]
+            )
+
+          type Query {
+            media: [Media] @shareable @provides(fields: "... on Book { title }")
+          }
+
+          union Media = Book | Movie
+
+          type Book @key(fields: "id") {
+            id: ID!
+            title: String @external
+          }
+
+          type Movie @key(fields: "id") {
+            id: ID!
+          }
+        `,
+      },
+      {
+        name: "c",
+        typeDefs: graphql`
+          extend schema
+            @link(
+              url: "https://specs.apollo.dev/federation/v2.3"
+              import: ["@key", "@shareable"]
+            )
+
+          type Book @key(fields: "id") {
+            id: ID!
+            title: String @shareable
+          }
+
+          type Movie @key(fields: "id") {
+            id: ID!
+            title: String @shareable
+          }
+        `,
+      },
+    ]);
+    assertCompositionSuccess(result);
+  });
 });

__tests__/todo.spec.ts

@@ -172,80 +172,4 @@ testImplementations((api) => {
     console.log(result.errors);
     assertCompositionSuccess(result);
   });
-
-  test.todo("provides-on-union", () => {
-    const result = api.composeServices([
-      {
-        name: "a",
-        typeDefs: parse(/* GraphQL */ `
-          extend schema
-            @link(
-              url: "https://specs.apollo.dev/federation/v2.3"
-              import: ["@key", "@shareable", "@external"]
-            )
-
-          type Query {
-            media: [Media] @shareable
-          }
-
-          union Media = Book | Movie
-
-          type Book @key(fields: "id") {
-            id: ID!
-          }
-
-          type Movie @key(fields: "id") {
-            id: ID!
-          }
-        `),
-      },
-      {
-        name: "b",
-        typeDefs: parse(/* GraphQL */ `
-          extend schema
-            @link(
-              url: "https://specs.apollo.dev/federation/v2.3"
-              import: ["@key", "@shareable", "@provides", "@external"]
-            )
-
-          type Query {
-            media: [Media] @shareable @provides(fields: "... on Book { title }")
-          }
-
-          union Media = Book | Movie
-
-          type Book @key(fields: "id") {
-            id: ID!
-            title: String @external
-          }
-
-          type Movie @key(fields: "id") {
-            id: ID!
-          }
-        `),
-      },
-      {
-        name: "c",
-        typeDefs: parse(/* GraphQL */ `
-          extend schema
-            @link(
-              url: "https://specs.apollo.dev/federation/v2.3"
-              import: ["@key", "@shareable"]
-            )
-
-          type Book @key(fields: "id") {
-            id: ID!
-            title: String @shareable
-          }
-
-          type Movie @key(fields: "id") {
-            id: ID!
-            title: String @shareable
-          }
-        `),
-      },
-    ]);
-    console.log(result.errors);
-    assertCompositionSuccess(result);
-  });
 });