Merge default array arguments for external types

.changeset/sixty-plums-enjoy.md

@@ -0,0 +1,5 @@
+---
+"@apollo/composition": patch
+---
+
+Allow merging external types when using arrays as default arguments.

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

@@ -614,6 +614,47 @@ describe('composition', () => {
     `);
   });
 
+  it('merges default arguments when they are arrays', () => {
+    const subgraphA = {
+      name: 'subgraph-a',
+      typeDefs: gql`
+        type Query {
+          a: A @shareable
+        }
+
+        type A @key(fields: "id") {
+          id: ID
+          get(ids: [ID] = []): [B] @external
+          req: Int @requires(fields: "get { __typename }")
+        }
+
+        type B @key(fields: "id", resolvable: false) {
+          id: ID
+        }
+      `
+    };
+    const subgraphB = {
+      name: 'subgraph-b',
+      typeDefs: gql`
+        type Query {
+          a: A @shareable
+        }
+
+        type A @key(fields: "id") {
+          id: ID
+          get(ids: [ID] = []): [B]
+        }
+
+        type B @key(fields: "id") {
+          id: ID
+        }
+      `
+    };
+
+    const result = composeAsFed2Subgraphs([subgraphA, subgraphB]);
+    assertCompositionSuccess(result);
+  });
+
   describe('merging of type references', () => {
     describe('for field types', () => {
       it('errors on incompatible types', () => {

composition-js/src/merging/merge.ts

@@ -1794,7 +1794,9 @@ class Merger {
         if (!sameType(destArg.type!, arg.type!) && !this.isStrictSubtype(arg.type!, destArg.type!)) {
           invalidArgsTypes.add(name);
         }
-        if (destArg.defaultValue !== arg.defaultValue) {
+        const defaultIsArray = Array.isArray(destArg.defaultValue) && Array.isArray(arg.defaultValue);
+        const defaultArgumentsAreEqual = defaultIsArray ? arrayEquals(destArg.defaultValue, arg.defaultValue) : destArg.defaultValue === arg.defaultValue;
+        if (!defaultArgumentsAreEqual) {
           invalidArgsDefaults.add(name);
         }
       }

composition-js/src/merging/reporter.ts

@@ -1,4 +1,4 @@
-import { addSubgraphToASTNode, assert, ErrorCodeDefinition, joinStrings, MultiMap, NamedSchemaElement, printSubgraphNames, SubgraphASTNode } from '@apollo/federation-internals';
+import { addSubgraphToASTNode, ErrorCodeDefinition, joinStrings, MultiMap, NamedSchemaElement, printSubgraphNames, SubgraphASTNode } from '@apollo/federation-internals';
 import { ASTNode, GraphQLError } from 'graphql';
 import { CompositionHint, HintCodeDefinition } from '../hints';
 import { Sources } from './merge';
@@ -182,7 +182,6 @@ export class MismatchReporter {
       }
     }
     const supergraphMismatch = (supergraphElement && mismatchAccessor(supergraphElement, true)) ?? '';
-    assert(distributionMap.size > 1, () => `Should not have been called for ${supergraphElement}`);
     const distribution = [];
     // We always add the "supergraph" first (proper formatting of hints rely on this in particular).
     const subgraphsLikeSupergraph = distributionMap.get(supergraphMismatch);