extendSchema: Do not modify standard directives

src/utilities/__tests__/extendSchema-test.ts

@@ -17,7 +17,7 @@ import {
   assertScalarType,
   assertUnionType,
 } from '../../type/definition';
-import { assertDirective } from '../../type/directives';
+import { assertDirective, specifiedDirectives } from '../../type/directives';
 import {
   GraphQLBoolean,
   GraphQLFloat,
@@ -85,6 +85,34 @@ describe('extendSchema', () => {
     });
   });
 
+  it('Do not modify built-in types and directives', () => {
+    const schema = buildSchema(`
+      type Query {
+        str: String
+        int: Int
+        float: Float
+        id: ID
+        bool: Boolean
+      }
+    `);
+
+    const extensionSDL = dedent`
+      extend type Query {
+        foo: String
+      }
+    `;
+    const extendedSchema = extendSchema(schema, parse(extensionSDL));
+
+    // Built-ins are used
+    expect(extendedSchema.getType('Int')).to.equal(GraphQLInt);
+    expect(extendedSchema.getType('Float')).to.equal(GraphQLFloat);
+    expect(extendedSchema.getType('String')).to.equal(GraphQLString);
+    expect(extendedSchema.getType('Boolean')).to.equal(GraphQLBoolean);
+    expect(extendedSchema.getType('ID')).to.equal(GraphQLID);
+
+    expect(extendedSchema.getDirectives()).to.have.members(specifiedDirectives);
+  });
+
   it('extends objects by adding new fields', () => {
     const schema = buildSchema(`
       type Query {

src/utilities/extendSchema.ts

@@ -67,6 +67,7 @@ import {
   GraphQLDeprecatedDirective,
   GraphQLDirective,
   GraphQLSpecifiedByDirective,
+  isSpecifiedDirective,
 } from '../type/directives';
 import { introspectionTypes, isIntrospectionType } from '../type/introspection';
 import { isSpecifiedScalarType, specifiedScalarTypes } from '../type/scalars';
@@ -236,6 +237,11 @@ export function extendSchemaImpl(
   }
 
   function replaceDirective(directive: GraphQLDirective): GraphQLDirective {
+    if (isSpecifiedDirective(directive)) {
+      // Builtin directives are not extended.
+      return directive;
+    }
+
     const config = directive.toConfig();
     return new GraphQLDirective({
       ...config,