Generic function passed to generic function inferred correctly only with spread

🔎 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#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-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

Start with the linked TypeScript Playground reproduction and compare the inferred types for the two generic calls. Trace how the callback's return type and parameters contribute to inferring Result. Done means baaaaaar(input, callback) infers Result as string[] without explicit type arguments or callback specialization.