We get "bug reports" of this nature very often:

I wrote this code

interface Window {
  myPlugin: any;
}
let x = window.myPlugin;
class Foo { }

then I added 'export' and I get compile errors, wat?

interface Window {
  myPlugin: any;
}
let x = window.myPlugin;
export class Foo { }

Our current answer today is "Move your stuff to another .d.ts file". This is an OK workaround, but not great. The real sticker is when there's a type defined inside an external module -- it's impossible to use those types to augment a global interface, even though this is a thing that actually happens. It gets even trickier because this encourages people to move types into the global namespace when they didn't want to in the first place, exacerbating the problem.

We need some proposal for how to make this work.