Allow augmented declaration merging of type aliases.

[nkappler]

I want to extend a type alias declared in node_modules/@types

File "index.d.ts":

interface A { /* stuff */ }
interface B { /* stuff */ }
interface C { /* stuff */ }

export type X = A | B | C;

My File:

interface D { /* stuff */ }

declare module "someModule" {
  export type X = X | D;
}

This does not work at the moment, and I don't know if there is a good workaround for what I want to achieve, because I want to call a generic function from that module which expects a parameter of type X and I want to pass it a D.

[weswigham]

@nkappler write

interface XRegistry {
  A:A
  B:B
  C:C
}
type X= XRegistry[keyof XRegistry];
//... And in the other file
declare module "someModule" {
  interface XRegistry { D:D }
}

Instead.

[nkappler]

@weswigham Thanks for that solution, I had that in mind too, however it feels a little clumsy.
Are there any disadvantages or technical issues with allowing the extension of type aliases?

[weswigham]

I'd hardly call it clumsy, afaik we use it in the Dom lib for some things. But yeah, merging aliases doesn't make nearly as much sense as interfaces. As you've written it, file parse order would matter. And generally speaking, two arbitrary types don't "merge" cleanly. Eg, what would a merge of Qux[T] and Baz extends T ? Qux : never even be?

[shaunluttin]

How would we make this approach work to extend an external type like this:

someModule.ts

export type Foo = 'bar' | 'baz';

someFile.ts

import { Foo } from 'someModule';

const: Foo = 'qux'; // how could we extend Foo to make this work?

The first commit here is my initial attempt at using the approach @weswigham recommended. The use case is related to this SO question.