fix(assertions): nested stacks inside non-root stages don't resolve t…

packages/aws-cdk-lib/assertions/lib/template.ts

@@ -296,12 +296,12 @@ export interface TemplateParsingOptions {
 }
 
 function toTemplate(stack: Stack): any {
-  const root = stack.node.root;
-  if (!Stage.isStage(root)) {
+  const stage = Stage.of(stack);
+  if (!Stage.isStage(stage)) {
     throw new Error('unexpected: all stacks must be part of a Stage or an App');
   }
 
-  const assembly = root.synth();
+  const assembly = stage.synth();
   if (stack.nestedStackParent) {
     // if this is a nested stack (it has a parent), then just read the template as a string
     return JSON.parse(fs.readFileSync(path.join(assembly.directory, stack.templateFile)).toString('utf-8'));

packages/aws-cdk-lib/assertions/test/template.test.ts

@@ -1,4 +1,16 @@
-import { App, CfnCondition, CfnMapping, CfnOutput, CfnParameter, CfnResource, Fn, LegacyStackSynthesizer, NestedStack, Stack } from '../../core';
+import {
+    App,
+    CfnCondition,
+    CfnMapping,
+    CfnOutput,
+    CfnParameter,
+    CfnResource,
+    Fn,
+    LegacyStackSynthesizer,
+    NestedStack,
+    Stack,
+    Stage
+} from '../../core';
 import { Construct } from 'constructs';
 import { Capture, Match, Template } from '../lib';
 
@@ -1353,6 +1365,29 @@ describe('Template', () => {
       });
     }).not.toThrow(/dependency cycle/);
   });
+
+    test('nested stack inside a Stage in an App', () => {
+        const app = new App();
+        const stage = new Stage(app, 'Stage');
+        const stack = new Stack(stage);
+        const nested = new NestedStack(stack, 'MyNestedStack');
+        new CfnResource(nested, 'Bar', {
+            type: 'Bar::Baz',
+            properties: {
+                Qux: 'Foo',
+            },
+        });
+        const template = Template.fromStack(nested);
+
+        expect(template.toJSON()).toEqual({
+            Resources: {
+                Bar: {
+                    Type: 'Bar::Baz',
+                    Properties: { Qux: 'Foo' },
+                },
+            },
+        });
+    });
 });
 
 function expectToThrow(fn: () => void, msgs: (RegExp | string)[]): void {