fix(asg/ec2): fix value of defaultChild

packages/@aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts

@@ -459,6 +459,7 @@ export class AutoScalingGroup extends AutoScalingGroupBase implements
       resource: 'autoScalingGroup:*:autoScalingGroupName',
       resourceName: this.autoScalingGroupName
     });
+    this.node.defaultChild = this.autoScalingGroup;
 
     this.applyUpdatePolicies(props);
   }

packages/@aws-cdk/aws-autoscaling/test/test.auto-scaling-group.ts

@@ -605,7 +605,23 @@ export = {
       "Roles": ["HelloDude"]
     }));
     test.done();
-  }
+  },
+
+  'defaultChild is available on an ASG'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = mockVpc(stack);
+    const asg = new autoscaling.AutoScalingGroup(stack, 'MyStack', {
+      instanceType: ec2.InstanceType.of(ec2.InstanceClass.M4, ec2.InstanceSize.MICRO),
+      machineImage: new ec2.AmazonLinuxImage(),
+      vpc,
+    });
+
+    // THEN
+    test.notEqual(asg.node.defaultChild, undefined);
+
+    test.done();
+  },
 };
 
 function mockVpc(stack: cdk.Stack) {

packages/@aws-cdk/aws-ec2/lib/vpc.ts

@@ -1165,6 +1165,7 @@ export class Subnet extends Resource implements ISubnet {
     this.subnetAvailabilityZone = subnet.attrAvailabilityZone;
     this.subnetIpv6CidrBlocks = subnet.attrIpv6CidrBlocks;
     this.subnetNetworkAclAssociationId = subnet.attrNetworkAclAssociationId;
+    this.node.defaultChild = subnet;
 
     const table = new CfnRouteTable(this, 'RouteTable', {
       vpcId: props.vpcId,

packages/@aws-cdk/aws-ec2/test/test.vpc.ts

@@ -540,8 +540,18 @@ export = {
       }), /`vpnGatewayAsn`.+`vpnGateway`.+false/);
 
       test.done();
-    }
+    },
+
+    'Subnets have a defaultChild'(test: Test) {
+      // GIVEN
+      const stack = new Stack();
 
+      const vpc = new Vpc(stack, 'VpcNetwork');
+
+      test.notEqual(vpc.publicSubnets[0].node.defaultChild, undefined);
+
+      test.done();
+    },
   },
 
   "When creating a VPC with a custom CIDR range": {

packages/@aws-cdk/core/lib/construct.ts

@@ -125,6 +125,7 @@ export class ConstructNode {
   private readonly _references = new Set<Reference>();
   private readonly _dependencies = new Set<IDependable>();
   private readonly invokedAspects: IAspect[] = [];
+  private _defaultChild: IConstruct | undefined;
 
   constructor(private readonly host: Construct, scope: IConstruct, id: string) {
     id = id || ''; // if undefined, convert to empty string
@@ -202,6 +203,10 @@ export class ConstructNode {
    * @returns a construct or undefined if there is no default child
    */
   public get defaultChild(): IConstruct | undefined {
+    if (this._defaultChild !== undefined) {
+      return this._defaultChild;
+    }
+
     const resourceChild = this.tryFindChild('Resource');
     const defaultChild = this.tryFindChild('Default');
     if (resourceChild && defaultChild) {
@@ -211,6 +216,20 @@ export class ConstructNode {
     return defaultChild || resourceChild;
   }
 
+  /**
+   * Override the defaultChild property.
+   *
+   * This should only be used in the cases where the correct
+   * default child is not named 'Resource' or 'Default' as it
+   * should be.
+   *
+   * If you set this to undefined, the default behavior of finding
+   * the child named 'Resource' or 'Default' will be used.
+   */
+  public set defaultChild(value: IConstruct | undefined) {
+    this._defaultChild = value;
+  }
+
   /**
    * All direct children of this construct.
    */

packages/@aws-cdk/core/test/test.construct.ts

@@ -447,6 +447,15 @@ export = {
       test.same(root.node.defaultChild, defaultChild);
       test.done();
     },
+    'can override defaultChild'(test: Test) {
+      const root = new Root();
+      new Construct(root, 'Resource');
+      const defaultChild = new Construct(root, 'OtherResource');
+      root.node.defaultChild = defaultChild;
+
+      test.same(root.node.defaultChild, defaultChild);
+      test.done();
+    },
     'returns "undefined" if there is no default'(test: Test) {
       const root = new Root();
       new Construct(root, 'child1');