[gen-ts-declarations] Transitive mixins from external packages are not discovered

[web-padawan]

The following code results in a warning and does not produce correct output.

import { DirMixin } from '@vaadin/vaadin-element-mixin/vaadin-dir-mixin.js';

/**
 * @polymerMixin
 * @mixes DirMixin
 */
export const ItemMixin = superClass => class VaadinItemMixin extends DirMixin(superClass) {
  // mixin contents
}

Warning

export const ItemMixin = superClass => class VaadinItemMixin extends DirMixin(superClass) {
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

warning [GEN_TYPESCRIPT_DECLARATIONS_WARNING] - Found 0 features for mixin DirMixin, expected 1.

Expected Output

declare function ItemMixin<T extends new (...args: any[]) => {}>(base: T): T & ItemMixinConstructor & DirMixinConstructor;

interface ItemMixin extends DirMixin {
  // mixin contents
}

Actual Output

declare function ItemMixin<T extends new (...args: any[]) => {}>(base: T): T & ItemMixinConstructor;

interface ItemMixin {
  // mixin contents
}

Note

I was able to track this down to this method call:

tools/packages/gen-typescript-declarations/src/gen-ts.ts

Line 746 in 2f57496

private transitiveMixins(

And now I feel like 3rd party mixins are not picked up by analyzePackage() 🤔

@aomarks any suggestions?

[stale]

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