For typing something like underscore's _.chain, it would be helpful if interfaces could be extended based on the shape of their type parameters.

For example:

// These methods are available for any T.
declare interface Chainable<T> {
  value(): T;
}

// These methods are only available for array types.
declare interface Chainable<T[]> {
  map<U>(mapFn: (t: T) => U): Chainable<U[]>;
}

Existing type definitions try to infer whether T is an array, object, array of objects, etc. But this is cumbersome and they'll never be able to do it as well as TypeScript itself.

The snippet above doesn't parse with TypeScript 2.1.5:

[ts] Operator '>' cannot be applied to types 'undefined[]' and '{ map<U>(mapFn: (t: any) => U): Chainable<U[]>; }'.