support "star projections" in generics

Suggestion

🔍 Search Terms

star projection

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

⭐ Suggestion

in kotlin, you can omit a generic from a type annotation when you don’t care what its value is:

class Foo<T: Number>

fun foo(value: Foo<*>) {}

more info: https://typealias.com/guides/star-projections-and-how-they-work/

in typescript, to do the same thing you need to use any, which is gross:

type Foo<T extends number> = T

declare function foo(value: Foo<any>): void

or simply duplicate the bound, which isn’t convenient:

type Foo<T extends number | string | boolean> = T

declare function foo(value: Foo<number | string | boolean>): void

📃 Motivating Example

it’s especially useful for types that have multiple bounded generics

type ThingWithLotsOfGenerics<Type1 extends Base1, Type2 extends Base2, Type3 extends Base3> = {}

declare function foo(value: ThingWithLotsOfGenerics<*, *, *>): void

[DetachHead]

@MartinJohns if you use any:

type Foo<T extends number> = T

declare function foo(value: Foo<any>): void

foo(1)
foo({a: "asdf"}) //no error???

if you use unknown:

//error: Type 'unknown' does not satisfy the constraint 'number'.
declare function foo(value: Foo<unknown>): void

[KotlinIsland]

You can’t use unknown because it’s wider than the bound. and you shouldn’t use any because it then allows things from outside the bound.