fix(core): multiple library copies lead to 'Assets must be defined within Stage or App' error

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

@@ -7,6 +7,8 @@ import { synthesize } from './private/synthesis';
 // eslint-disable-next-line
 import { Construct as CoreConstruct } from './construct-compat';
 
+const STAGE_SYMBOL = Symbol.for('@aws-cdk/core.Stage');
+
 /**
  * Initialization props for a stage.
  */
@@ -85,7 +87,7 @@ export class Stage extends CoreConstruct {
    * @experimental
    */
   public static isStage(x: any ): x is Stage {
-    return x !== null && x instanceof Stage;
+    return x !== null && typeof(x) === 'object' && STAGE_SYMBOL in x;
   }
 
   /**
@@ -137,6 +139,8 @@ export class Stage extends CoreConstruct {
       throw new Error(`invalid stage name "${id}". Stage name must start with a letter and contain only alphanumeric characters, hypens ('-'), underscores ('_') and periods ('.')`);
     }
 
+    Object.defineProperty(this, STAGE_SYMBOL, { value: true });
+
     this.parentStage = Stage.of(this);
 
     this.region = props.env?.region ?? this.parentStage?.region;

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

@@ -280,6 +280,35 @@ nodeunitShim({
     test.throws(() => new Stage(app, 'mystage', { outdir: '/tmp/foo/bar' }), /"outdir" cannot be specified for nested stages/);
     test.done();
   },
+
+  'Stage.isStage indicates that a construct is a stage'(test: Test) {
+    // WHEN
+    const app = new App();
+    const stack = new Stack();
+    const stage = new Stage(app, 'Stage');
+
+    // THEN
+    test.ok(Stage.isStage(stage));
+    test.ok(Stage.isStage(app));
+    test.ok(!Stage.isStage(stack));
+    test.done();
+  },
+
+  'Stage.isStage indicates that a construct is a stage based on symbol'(test: Test) {
+    // WHEN
+    const app = new App();
+    const stage = new Stage(app, 'Stage');
+
+    const externalStage = {};
+    const STAGE_SYMBOL = Symbol.for('@aws-cdk/core.Stage');
+    Object.defineProperty(externalStage, STAGE_SYMBOL, { value: true });
+
+    // THEN
+    test.ok(Stage.isStage(stage));
+    test.ok(Stage.isStage(app));
+    test.ok(Stage.isStage(externalStage));
+    test.done();
+  },
 });
 
 class TouchingAspect implements IAspect {