[core] SynthUtils.toCloudFormation(stack) fails on stacks defined in external project

[tobias-bardino]

SynthUtils.toCloudFormation(stack) fails with Unable to find artifact with id when stack is defined in an external project.

Reproduction Steps

I've created an example to reproduces the error here: https://github.com/gwriss/instanceof-stack-bug

What did you expect to happen?

SynthUtils.toCloudFormation(stack) should spit out a CF stack

What actually happened?

SynthUtils.toCloudFormation(stack) fails with Unable to find artifact with id "MyTestStack291FA567"

All our stack-tests fails after upgrade from 1.54.0 -> 1.65.0 (we have snapshot tests on all external stacks using toCloudFormation)

Environment

• CLI Version : 1.65.0
• Framework Version: 1.65.0
• Node.js Version: v12.18.4
• OS : OSX Catalina 10.15.7 (19H2)
• Language (Version): TypeScript (3.9.7)

Other

It seems to be related to #1249 where instanceof Stack does not work as expected and should be replaced with isStack(stack)

It used to work in CDK 1.54.0 (there is a branch in my test-repo where the test runs successfully on 1.54.0)

A search for the instanceof Stack in the repo shows another occurrence. Maybe it would be beneficial to fix that too?

➜ ag "instanceof Stack"
CHANGELOG.md
5101:// must be before defining the table (this instanceof Stack)

packages/@aws-cdk/core/lib/stack-synthesizers/_shared.ts
91:    if (node instanceof Stack && node.nestedStackParent === undefined) {

---

This is 🐛 Bug Report

[tobias-bardino]

I think this would fix the error but I am not able to write a test for it, because of the requirement of another project (another nodes_modules dir) to reproduce the problem

diff --git a/packages/@aws-cdk/core/lib/private/synthesis.ts b/packages/@aws-cdk/core/lib/private/synthesis.ts
index c8243ec03..e93df721a 100644
--- a/packages/@aws-cdk/core/lib/private/synthesis.ts
+++ b/packages/@aws-cdk/core/lib/private/synthesis.ts
@@ -153,11 +153,11 @@ function synthesizeTree(root: IConstruct, builder: cxapi.CloudAssemblyBuilder) {
       assembly: builder,
     };
 
-    if (construct instanceof Stack) {
+    if (Stack.isStack(construct)) {
       construct.synthesizer.synthesize(session);
     } else if (construct instanceof TreeMetadata) {
       construct._synthesizeTree(session);

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