TupleGenerator

[zojize]

TupleGenerator

This is an experiment attempting to add support to #42033 using the syntax @thehappycheese brought up. It adds a new type:

interface TupleGenerator<Tuple extends any[] = unknown[], TReturn = any, TNext = any> 
    extends Generator<Tuple extends (infer T)[] ? T : unknown, TReturn, TNext> {}

When a custom iterable or builtin generator is spread explicitly using this type in a call expression or array literal, it should infer the correct arity and types instead of reporting an error.

Example

declare class Vec {
  [Symbol.iterator](): TupleGenerator<[number, number]>
}
declare function foo(x: number, y: number): void
declare const p: Vec2

const x: [number, number] = [...p] as const
// before: Type 'any[]' is not assignable to type '[number, number]'.
// after: no error
foo(...p)
// before: A spread argument must either have a tuple type or be passed to a rest parameter.ts(2556)
// after: no error

Limitations

• Union of tuples won't work Error when spreading a union of tuples in a call #42508
• Type inference not supported, TupleGenerator must be explicitly declared
• Extending TupleGenerator not supported (seems doable though)

Alternatives

• just make a toTuple(): [number, number] function
• *[] a dedicated syntax for generator tuples with a leading *, needs change to parser
• intrinsic TupleGenerator type instead of just interface?

Can someone help pack this to playground 😃?

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