Completion list should include property names of constraining type when writing object type literals in type argument positions

[jcalz]

Search Terms

completion list, IntelliSense, keys, members, generic constraint,

Suggestion with Example

It would be nice to get some IntelliSense help when specifying the members of a constrained object type. For example, given these types:

interface Foo {
  apple: string;
  banana: number;
  cherry: boolean;
}

interface SomeGeneric<F extends Foo> {
  foo: F;
}

I was kind of expecting to be given some hints about what keys I needed when writing this:

declare const s: SomeGeneric<{
  apple: "hey";
  banana: 123;
  // <-- no IntelliSense here for anything but "readonly"
}>;

Instead, I only seem to get a suggestion for readonly. It would be very helpful if apple, banana, and cherry were part of the completion list.

Use Cases

I would think this is useful anytime you're specifying a constrained generic parameter, or possibly even extending an existing interface. Its absence is painful in any case where the generic type has lots of members, as in this Stack Overflow question.

Related Issues

Possibly related to #1506?

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[williamluke4]

@DanielRosenwasser If someone can point me in the right direction. I don't mind having a go at implementing this

[DanielRosenwasser]

@williamluke4 sorry I missed your message. There is currently another community PR that seems like it's on the right track if you're still interested in contributing and want to get a good handle on what the work here involves.

[DanielRosenwasser]

Fixed by #43526. Thanks @tiagovtristao!