Generic function passed to generic function inferred correctly only with spread

[webstrand]

🔎 Search Terms

generic function spread regression inference

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241121#code/KYDwDg9gTgLgBDAnmYcByEAmqC8cDecAvgNwBQZ2AxgDYCGUwAkAGYCuAdlTAJYQdwWEYSIgAeACoAaOACVgAZzY14oGMA6YFcAIJQodRABkeAa2BjOpjhADuHAHwyAogDceNOGo1a4jOpj8NIhwVjb2ANoAug4AFGRMPBxgbDAAXHDSCVR0NDQARnRUphmxSSnpmTJgjK4Z8koqMgB0rcDuNBluHgCUcDgOcorKMFJkPfXDKhTU9IysnNx8AoV0a2tQkjINI14g6praegbGZhZhdo5xCeWpGVlMOXmFxaW3ldJwNe2TjTB9AyGfzGEyBIxmwFoDFQ7C4vH4cCeBSKpkk1zgGLg73uY0xX1qGQUMCgSQA5tEQYTiWTouQyKBILBBIt4QJQHQALZgGgWCRxbGZPr4Ch4gD0orgAEkOCxgFBtEgUF8GJzgOooGCVIjoIxuMEEkJRMIyslUjIkS9TD1yGKJQA9AD8Isx4rgADE6B4FRAsTK5QhkKgwCqOWr-TstVQdZCYMFfUTgAFfbL5QgABaoKMcIkGJIwBKrdYME0Vc25ZHFa1kW1wR3OjGugBCyDoCgUZOTco0VGAMhgGeZcOWiPLWO0Uf0MermML602nxzNJiJbNI+eKKr04bEulKYVgeVBlD6s18AnutjiC3cFnGxXozXFdRfKrNbrRDIQA

💻 Code

export type Node = { };
declare function fooooooo<T, Result extends ArrayLike<unknown>, Evil extends readonly unknown[]>(
    input: T,
    callback: (input: T, prev: Result, ...evil: Evil) => Result,
): Result

declare function baaaaaar<T, Result extends ArrayLike<unknown>>(
    input: T,
    callback: (input: T, prev: Result) => Result,
): Result

declare function callback<T>(
    input: T,
    prev: string[],
): string[];

export function example<T>(input: T) {
    // Infers type parameter Result correctly
    fooooooo(input, callback);
    // ^? function fooooooo<T, string[], []>(input: T…

    // Fails to infer type parameter Result correctly instead infers the constraint
    baaaaaar(input, callback);
    // ^? function baaaaaar<T, ArrayLike<unknown>>(in…



    // Bypassing inference, the function call is correct
    baaaaaar<T, string[]>(input, callback);

    // Infers type parameter Result correctly
    baaaaaar(input, callback<T>);
    // ^? function baaaaaar<T, string[]>(input: T, ca…
}

🙁 Actual behavior

baaaaaar(input, callback) infers the constraint of Result = ArrayLike<unknown>

🙂 Expected behavior

baaaaaar(input, callback) should infer Result = string[] from the return type or parameter of callback.

Additional information about the issue

No response

[Andarist]

I believe that at the implementation level the difference boils down to this line of code:
https://github.dev/microsoft/TypeScript/blob/d85767abfd83880cea17cea70f9913e9c4496dcc/src/compiler/checker.ts#L35096

In the case that doesn't work a fixing mapper is used and that prevents inference candidates that are discovered later from being chosen.