Improve composition hints around types that are strictly @external

.changeset/weak-bottles-crash.md

@@ -0,0 +1,5 @@
+---
+"@apollo/composition": patch
+---
+
+Stop emitting "inconsistent value type" hints against definitions where the type is marked `@external` or all fields are marked `@external`.

composition-js/src/__tests__/hints.test.ts

@@ -1353,4 +1353,86 @@ describe('when a directive causes an implicit federation version upgrade', () =>
     assertCompositionSuccess(result);
     expect(result).toNotRaiseHints();
   });
-})
+});
+
+describe('when a partially-defined type is marked @external or all fields are marked @external', () => {
+  describe('value types', () => {
+    it('with type marked @external', () => {
+      const meSubgraph = gql`
+        type Query {
+          me: Account
+        }
+
+        type Account @key(fields: "id") {
+          id: ID!
+          name: String
+          permissions: Permissions
+        }
+
+        type Permissions {
+          canView: Boolean
+          canEdit: Boolean
+        }
+      `;
+
+      const accountSubgraph = gql`
+        type Query {
+          account: Account
+        }
+
+        type Account @key(fields: "id") {
+          id: ID!
+          permissions: Permissions @external
+          isViewer: Boolean @requires(fields: "permissions { canView }")
+        }
+
+        type Permissions @external {
+          canView: Boolean
+        }
+      `;
+
+      const result = mergeDocuments(meSubgraph, accountSubgraph);
+      expect(result).toNotRaiseHints();
+    });
+
+    it('with all fields marked @external', () => {
+      const meSubgraph = gql`
+        type Query {
+          me: Account
+        }
+
+        type Account @key(fields: "id") {
+          id: ID!
+          name: String
+          permissions: Permissions
+        }
+
+        type Permissions {
+          canView: Boolean
+          canEdit: Boolean
+          canDelete: Boolean
+        }
+      `;
+
+      const accountSubgraph = gql`
+        type Query {
+          account: Account
+        }
+
+        type Account @key(fields: "id") {
+          id: ID!
+          permissions: Permissions @external
+          isViewer: Boolean @requires(fields: "permissions { canView canEdit }")
+        }
+
+        type Permissions {
+          canView: Boolean @external
+          canEdit: Boolean @external
+        }
+      `;
+
+      const result = mergeDocuments(meSubgraph, accountSubgraph);
+      expect(result).toNotRaiseHints();
+    });
+  });
+});

composition-js/src/merging/merge.ts

@@ -965,9 +965,9 @@ class Merger {
         typeDescription = 'interface'
         break;
     }
-    for (const source of sources) {
+    for (const [index, source] of sources.entries()) {
       // As soon as we find a subgraph that has the type but not the field, we hint.
-      if (source && !source.field(field.name)) {
+      if (source && !source.field(field.name) && !this.areAllFieldsExternal(index, source)) {
         this.mismatchReporter.reportMismatchHint({
           code: hintId,
           message: `Field "${field.coordinate}" of ${typeDescription} type "${dest}" is defined in some but not all subgraphs that define "${dest}": `,
@@ -1081,6 +1081,10 @@ class Merger {
     return this.metadata(sourceIdx).isFieldFullyExternal(field);
   }
 
+  private areAllFieldsExternal(sourceIdx: number, type: ObjectType | InterfaceType): boolean {
+    return type.fields().every(f => this.isExternal(sourceIdx, f));
+  }
+
   private validateAndFilterExternal(sources: (FieldDefinition<any> | undefined)[]): (FieldDefinition<any> | undefined)[] {
     const filtered: (FieldDefinition<any> | undefined)[] = [];
     for (const [i, source] of sources.entries()) {