instantiation expression usage leading to an invalid d.ts file generation

[AlexisTessier]

🔎 Search Terms

partial function generic application, function generic variant from another function, instantiation-expressions

🕗 Version & Regression Information

• This changed between versions 5.6.2 and 5.7.2

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.2#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

💻 Code

type SomePropertyKeys = ReadonlyArray<PropertyKey>
type At<A extends ReadonlyArray<unknown>, I extends number = number> = A[I]

export function mainKeyGuard<
  K extends readonly [PropertyKey, ...SomePropertyKeys],
>(
  keys: K,
): <T extends object>(
  value: T,
) => value is Extract<T, Record<At<K, 0>, unknown>> {
  return <T extends object>(
    value: T,
  ): value is Extract<T, Record<At<K, 0>, unknown>> => {
    return keys[0] in value
  }
}

type Foo = { foo1: string, foo2: string }
type Bar = { bar1: string, bar2: string }
type FooBar = Foo | Bar
const barKeys = ['bar1', 'bar2'] as const
export const isBar = mainKeyGuard(barKeys)<FooBar>

🙁 Actual behavior

Starting with the v5.7.2, the generated d.ts file declares isBar as a function which reference a missing generic type T, so an error is raised

export declare const isBar: (value: FooBar) => value is Extract<T, Record<At<K, 0>, unknown>>;

🙂 Expected behavior

I would expect the isBar function d.ts declaration to not reference a missing generic type T.
Previously, the generated d.ts file behaved like the following

export declare const isBar: (value: FooBar) => value is Bar;

Additional information about the issue

The title mentions "instantiation expression" but I'm not totally sure about how the involved feature is actually named

[Andarist]

Bisects to #59282

[robpalme]

cc @dragomirtitian

[dragomirtitian]

A simpler repro:

function outer<K>(n: K) {
    return function inner<T>(a: [T, K]): a is [T, K] {
        return null!
    }
}

let nr = outer(1)<number>

Playground Link

The above will generate let nr: (a: [number, number]) => a is [T, K]; which seems to indicate T and K are correctly resolved for parameters but not in parameters but not for predicates. Also If the outer function does not have a type parameter everything works (Playground Link).

Looking into it as well.