fix: check directive isNode on merge if inherited from obj (#5973)

packages/merge/src/typedefs-mergers/merge-nodes.ts

@@ -5,6 +5,7 @@ import {
   SchemaDefinitionNode,
   SchemaExtensionNode,
 } from 'graphql';
+import { isNode } from 'graphql/language/ast.js';
 import { collectComment, NamedDefinitionNode } from '@graphql-tools/utils';
 import { mergeDirective } from './directives.js';
 import { mergeEnum } from './enum.js';
@@ -104,6 +105,14 @@ export function mergeGraphQLNodes(
             );
             break;
           case Kind.DIRECTIVE_DEFINITION:
+            if (mergedResultMap[name]) {
+              const isInheritedFromPrototype = name in {};
+              if (isInheritedFromPrototype) {
+                if (!isNode(mergedResultMap[name])) {
+                  mergedResultMap[name] = undefined as any;
+                }
+              }
+            }
             mergedResultMap[name] = mergeDirective(nodeDefinition, mergedResultMap[name] as any);
             break;
         }

packages/merge/tests/merge-nodes.spec.ts

@@ -280,6 +280,27 @@ describe('Merge Nodes', () => {
         'Unable to merge GraphQL type "A": Field "f1" already defined with a different type. Declared as "String", but you tried to override with "Int"',
       );
     });
+
+    it('Should merge GraphQL Types and merge directives (when the directive is inherited from the Object prototype)', () => {
+      const type1 = parse(/* GraphQL */ `
+        type A @toString {
+          f1: String
+        }
+      `);
+      const type2 = parse(/* GraphQL */ `
+        type A @test2 {
+          f2: Int
+        }
+      `);
+      const merged = mergeGraphQLNodes([...type1.definitions, ...type2.definitions]);
+      const type = merged['A'];
+      assertObjectTypeDefinitionNode(type);
+      assertSome(type.directives);
+
+      expect(type.directives.length).toBe(2);
+      expect(type.directives[0].name.value).toBe('toString');
+      expect(type.directives[1].name.value).toBe('test2');
+    });
   });
 
   describe('enum', () => {