Return type inference limitation

[aquapi]

🔎 Search Terms

Return type, inference, cotravariant

🕗 Version & Regression Information

It seems to be in any version on the playground.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3#code/PTAEFEA8EMFsAcA2BTUBGAUAFwJ71QMID2WATtAG7SkCW0AdlgDwAaAfKALygAUAxgC5QLAJRcOFIjQAmAbgwZpyPomqoAZgFd6fLDSL1QpIpqzImAJWQBHTTVLIAzmx5YhAb1B8DZyG9DEZJTUdIyWNnYOjqAAZKCeAEZCjmQ09ADmoAC+HFkA2gC6IkIMOPKKyqoOoFo6egY19EwAKqDIfsj00tHuWQA0oBYu6kL8Qs1inBwWxfFePu3+geRUtAzMzWwD6vSjgqAT4oPZ5cam5p7QyakZ2S4YoKB5O-xH9MgA7oNO8AaOyPwAHTQUAAai8gISIhEBQwIgUZzMPAeTxefDen2+jl+9H+QKhMLhGCAA

💻 Code

// Example 1
type Cotravariant<X> = (c: X) => void;

declare function route<Requires>(t: { context: Cotravariant<Requires & { b: string }> }[]): any;

declare function fn<T extends {}, R>(f: (c: T) => R): { context: Cotravariant<T>, fn: (c: T) => R };

route<{ a: string }>(
  [fn(c => new Response(c.a + c.b))] // This infers b
)

route(
  [fn(c => new Response(c.b))] // This doesn't infer b
)

🙁 Actual behavior

The second callback doesn't infer c.b.

🙂 Expected behavior

The second callback should infer c.b.

Additional information about the issue

I have no idea if this is working as intended or not, however it would be really nice if it works as it allows inference for non-chaining API, something like:

init(
  '/:id', // Inferred c.params.id from here
  [send], // Inferred c.send from here
  [route('GET', '/', (c) => c.send.body(c.params.id))]
)

My implementation looks like this: https://github.com/mapljs/framework/blob/main/src/index.ts#L65

[RyanCavanaugh]

Inference can't go back and forth in repeated rounds between the outer call's contextual type and the inner call's context-sensitive expression and try to resolve both inferences simultaneously, which is what would have to happen here. This example would basically require a unification-based algorithm, which is unlikely to happen (for performance/reason-ability reasons).