fix: failure on anonymous class with a method (backport #1824) (#1846)

# Backport

This will backport the following commits from `main` to
`maintenance/v5.6`:
- [fix: failure on anonymous class with a method
(#1824)](#1824)

<!--- Backport version: 9.5.1 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

Co-authored-by: Rico Hermans <[email protected]>

Author: aws-cdk-automation

fixtures/jsii-calc/lib/decorators.ts

@@ -0,0 +1,50 @@
+type Constructor = { new (...args: any[]): {} };
+
+export function functionReturnsClasstype<T extends Constructor>(ctr: T) {
+  return class extends ctr {
+  };
+}
+
+/**
+ * A class decorator that changes inherited state and adds a readonly field to the class.
+ *
+ * This wasn't the thing that was exploding, see `function-returning-anonymous-class.ts` for that.
+ * Nevertheless, this makes for a good class decorator demo.
+ */
+export function classDecorator(x: typeof SomeDecoratedClass): typeof SomeDecoratedClass {
+  const ret = class extends x {
+    constructor() {
+      super();
+      this.state = this.state + this.state;
+    }
+  };
+
+  // This adds a field to the class, but we can't reflect that in the type because of the limitations
+  // of decorators. That's we advertise it through interface merging below.
+  (ret.prototype as any)['field'] = 'some_added_field';
+
+  return ret;
+}
+
+@classDecorator
+export class SomeDecoratedClass {
+  protected state = 'state';
+
+  public accessState() {
+    return this.state;
+  }
+}
+
+export interface SomeDecoratedClass {
+  readonly field: string;
+}
+
+/**
+ * Exercise the above code
+ */
+function tryDecoratedClass() {
+  const instance = new SomeDecoratedClass();
+  return instance.field;
+}
+// Suppress unused locals warnings
+void tryDecoratedClass;

fixtures/jsii-calc/lib/function-returning-anonymous-class.ts

@@ -0,0 +1,17 @@
+type Constructor = { new (...args: any[]): {} };
+
+/**
+ * Just the mere presence of this function is enough to break jsii, even if it's not exported from
+ * the jsii root module.
+ *
+ * The reason is that when we add deprecation warnings we visit all functions in all files.
+ */
+export function propertyInjectionDecorator<T extends Constructor>(ctr: T) {
+  // Important for the bug: the anonymous class extends something, *and*
+  // declares a method.
+  return class extends ctr {
+    public someMethod(): string {
+      return 'abc';
+    }
+  };
+}

fixtures/jsii-calc/lib/index.ts

@@ -9,6 +9,7 @@ export * from './stability';
 export * from './submodules';
 export * from './container-types';
 export * from './indirect-implementation';
+export * from './decorators';
 
 export * as submodule from './submodule';
 export * as onlystatic from './only-static';

src/common/symbol-id.ts

@@ -49,9 +49,13 @@ interface SymbolIdOptions {
  */
 export function symbolIdentifier(
   typeChecker: ts.TypeChecker,
-  sym: ts.Symbol,
+  sym: ts.Symbol | undefined,
   options: SymbolIdOptions = {},
 ): string | undefined {
+  if (!sym) {
+    return undefined;
+  }
+
   // If this symbol happens to be an alias, resolve it first
   // eslint-disable-next-line no-bitwise
   while ((sym.flags & ts.SymbolFlags.Alias) !== 0) {