Need way to express hybrid types that are indexable for a subset of properties

Edit by @DanielRosenwasser: This might be thought of as a "rest index signature" or a catch-all index signature.

This is a feature request.

TypeScript Version: 2.4

Code

interface CSSProperties {
  marginLeft?: string | number
  [key: string]: CSSProperties
}

Based on the docs, this is not allowed:

While string index signatures are a powerful way to describe the “dictionary” pattern, they also enforce that all properties match their return type. This is because a string index declares that obj.property is also available as obj[“property”]. In the following example, name’s type does not match the string index’s type, and the type-checker gives an error:

Unfortunately, it seems to make this type (which is common amongst jss-in-css solutions) not expressible. Coming from flow, which handles index types by assuming they refer to the properties that are not explicitly typed, this is frustrating.

As it stands, you can workaround it with:

interface CSSProperties {
  marginLeft?: string | number
  [key: string]: CSSProperties | string | number
}

But this is not sound. It allows this:

const style: CSSProperties = {
  margarineLeft: 3
}

This could potentially be solved with subtraction types if they allowed subtracting from string (and you were allowed to specify key types in this way):

interface CSSProperties {
  marginLeft?: string | number
}

interface NestedCSSProperties extends CSSProperties {
  [key: string - keyof CSSProperties]: CSSProperties
}

I asked about this on stackoverflow to confirm that I wasn't missing something. It seems I'm not, so I guess I'd consider this a suggestion/discussion starter, since it's probably not a "bug". Thanks!

CSSProperties の例とリンクされた handbook のガイダンスから始め、明示的なプロパティと index signature を組み合わせた場合の現在の type-checker の動作を再現してください。要求されたハイブリッド型を表現でき、互換性のない型を持つ未知のプロパティを拒否できるようになり、unsound な union workaround に依存しない状態になれば完了です。