fix(core): fix detection of references in Fn.join (#3569)

References used in Fn.join() will now be properly substituted if
they're cross-stack references.

Fixes #3554.

packages/@aws-cdk/core/lib/cfn-fn.ts

@@ -1,6 +1,7 @@
 import { ICfnConditionExpression } from './cfn-condition';
 import { minimalCloudFormationJoin } from './private/cloudformation-lang';
 import { Intrinsic } from './private/intrinsic';
+import { Reference } from './reference';
 import { IResolvable, IResolveContext } from './resolvable';
 import { captureStackTrace } from './stack-trace';
 import { Token } from './token';
@@ -637,8 +638,6 @@ class FnJoin implements IResolvable {
 
   private readonly delimiter: string;
   private readonly listOfValues: any[];
-  // Cache for the result of resolveValues() - since it otherwise would be computed several times
-  private _resolvedValues?: any[];
 
   /**
    * Creates an ``Fn::Join`` function.
@@ -682,9 +681,7 @@ class FnJoin implements IResolvable {
    * generate shorter output.
    */
   private resolveValues(context: IResolveContext) {
-    if (this._resolvedValues) { return this._resolvedValues; }
-
-    const resolvedValues = this.listOfValues.map(context.resolve);
-    return this._resolvedValues = minimalCloudFormationJoin(this.delimiter, resolvedValues);
+    const resolvedValues = this.listOfValues.map(x => Reference.isReference(x) ? x : context.resolve(x));
+    return  minimalCloudFormationJoin(this.delimiter, resolvedValues);
   }
 }

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

@@ -118,8 +118,8 @@ export abstract class Resource extends Construct implements IResource {
    * @experimental
    */
   protected getResourceNameAttribute(nameAttr: string) {
-    return Token.asString({
-      resolve: (context: IResolveContext) => {
+    return Lazy.stringValue({
+      produce: (context: IResolveContext) => {
         const consumingStack = Stack.of(context.scope);
 
         if (this.stack.environment !== consumingStack.environment) {

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

@@ -1,7 +1,7 @@
 import fc = require('fast-check');
 import _ = require('lodash');
 import nodeunit = require('nodeunit');
-import { Fn, Stack, Token } from '../lib';
+import { App, CfnOutput, Fn, Stack, Token } from '../lib';
 import { Intrinsic } from '../lib/private/intrinsic';
 
 function asyncTest(cb: (test: nodeunit.Test) => Promise<void>): (test: nodeunit.Test) => void {
@@ -139,6 +139,34 @@ export = nodeunit.testCase({
         ]
       });
     }),
+
+    'cross-stack FnJoin elements are properly resolved': asyncTest(async (test) => {
+      // GIVEN
+      const app = new App();
+      const stack1 = new Stack(app, 'Stack1');
+      const stack2 = new Stack(app, 'Stack2');
+
+      // WHEN
+      new CfnOutput(stack2, 'Stack1Id', {
+        value: Fn.join(' = ', [ 'Stack1Id', stack1.stackId ])
+      });
+
+      // THEN
+      const template = app.synth().getStack('Stack2').template;
+
+      test.deepEqual(template, {
+        Outputs: {
+          Stack1Id: {
+            Value: {
+              'Fn::Join': [' = ', [
+                'Stack1Id',
+                { 'Fn::ImportValue': 'Stack1:ExportsOutputRefAWSStackIdB2DD5BAA' }
+              ]]
+            }
+          }
+        }
+      });
+    }),
   },
 });