fix(kernel): incorrectly scoped FQN resolutions

The registration was done on the object's prototype, and tthe value from
the constructor was always used, even if that was inherited, such that
if a parent type ahd already been resolved previously, all child types
would use the parent's FQN instead of their own.

Addressed this by instead storing the associations in an external
WeakMap, and added a test case to validate correct behavior.

Fixes aws/aws-cdk#26604

packages/@jsii/kernel/src/objects.test.ts

@@ -1,4 +1,4 @@
-import { ObjectTable } from './objects';
+import { ObjectTable, jsiiTypeFqn } from './objects';
 
 const mockResolve = jest.fn();
 
@@ -24,3 +24,20 @@ test('registered objects have clean JSON.Stringify', () => {
 
   expect(JSON.stringify(obj)).toEqual(expected);
 });
+
+test('FQN resolution is not broken by inheritance relationships', () => {
+  const rtti = Symbol.for('jsii.rtti');
+
+  class Parent {
+    public static readonly [rtti] = { fqn: 'phony.Parent' };
+  }
+  class Child extends Parent {
+    public static readonly [rtti] = { fqn: 'phony.Child' };
+  }
+
+  const parent = new Parent();
+  expect(jsiiTypeFqn(parent, () => true)).toBe(Parent[rtti].fqn);
+
+  const child = new Child();
+  expect(jsiiTypeFqn(child, () => true)).toBe(Child[rtti].fqn);
+});

packages/@jsii/kernel/src/objects.ts

@@ -15,31 +15,15 @@ const OBJID_SYMBOL = Symbol.for('$__jsii__objid__$');
  */
 const IFACES_SYMBOL = Symbol.for('$__jsii__interfaces__$');
 
-/**
- * Symbol we use to tag constructors that are exported from a jsii module.
- */
-const JSII_TYPE_FQN_SYMBOL = Symbol('$__jsii__fqn__$');
-
 /**
  * Symbol under which jsii runtime type information is stored.
  */
 const JSII_RTTI_SYMBOL = Symbol.for('jsii.rtti');
 
 /**
- * Exported constructors processed by the jsii compiler have runtime type
- * information woven into them under the `[JSII_RTTI]` symbol property.
+ * Cache for resolved associations between constructors and FQNs.
  */
-interface MaybeManagedConstructor {
-  readonly [JSII_RTTI_SYMBOL]?: {
-    /** The fully qualified name of the jsii type. */
-    readonly fqn: spec.FQN;
-    /** The version of the assembly that declared this type. */
-    readonly version: string;
-  };
-
-  /** The resolved JSII type FQN for this type. This is set only after resolution */
-  readonly [JSII_TYPE_FQN_SYMBOL]?: spec.FQN;
-}
+const RESOLVED_TYPE_FQN = new WeakMap<object, spec.FQN>();
 
 /**
  * Get the JSII fqn for an object (if available)
@@ -55,11 +39,11 @@ export function jsiiTypeFqn(
   obj: any,
   isVisibleType: (fqn: spec.FQN) => boolean,
 ): spec.FQN | undefined {
-  const ctor = obj.constructor as MaybeManagedConstructor;
+  const ctor = obj.constructor;
 
   // We've already resolved for this type, return the cached value.
-  if (ctor[JSII_TYPE_FQN_SYMBOL] != null) {
-    return ctor[JSII_TYPE_FQN_SYMBOL];
+  if (RESOLVED_TYPE_FQN.has(ctor)) {
+    return RESOLVED_TYPE_FQN.get(ctor)!;
   }
 
   let curr = ctor;
@@ -138,22 +122,19 @@ function tagObject(obj: unknown, objid: string, interfaces?: string[]) {
  * Set the JSII FQN for classes produced by a given constructor
  */
 export function tagJsiiConstructor(constructor: any, fqn: spec.FQN) {
-  if (Object.prototype.hasOwnProperty.call(constructor, JSII_TYPE_FQN_SYMBOL)) {
+  const existing = RESOLVED_TYPE_FQN.get(constructor);
+
+  if (existing != null) {
     return assert.strictEqual(
-      constructor[JSII_TYPE_FQN_SYMBOL],
+      existing,
       fqn,
-      `Unable to register ${constructor.name} as ${fqn}: it is already registerd with FQN ${constructor[JSII_TYPE_FQN_SYMBOL]}`,
+      `Unable to register ${constructor.name} as ${fqn}: it is already registerd with FQN ${existing}`,
     );
   }
 
   // Mark this constructor as exported from a jsii module, so we know we
   // should be considering it's FQN as a valid exported type.
-  Object.defineProperty(constructor, JSII_TYPE_FQN_SYMBOL, {
-    configurable: false,
-    enumerable: false,
-    writable: false,
-    value: fqn,
-  });
+  RESOLVED_TYPE_FQN.set(constructor, fqn);
 }
 
 /**