implicit 'any' in method with parameter of inferred generic type

🔎 Search Terms

implicit 'any' method parameter inferred generic type

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?suppressImplicitAnyIndexErrors=true&ts=5.5.0-dev.20240531#code/C4TwDgpgBAwg9gOwM7AE4FcDGw6oDwAqAfFALxQDeAUFFAhAO5QAUAdOwIaoDmSAXHQgA3CKgDaAXQCUAggG4qAXwVUAlgmCiAZh0zQA4hHqpVmAApcOAWySES1WgCMuA5sFlSyJIXFUATBUUqKlBIKAAZVRRCKAgAD00EPyQoVAgOP0QAGxBKRUkScgcoMQBpKHUoAGsIEDgtKAIJATF4ZDQsHHwCMokiABooQ2NTC1RrWx7SvolAlT8ITCyuaC10BGxVRCgtODgY+MTk1PTMhBy8guYwSxsBSOixdlYmohkoH38VJY4kFIAxPaUGhQTQoZieChBII-P5QABCXDywQA9CioABJBBaURGPQ7DiqLJIKi7ODMMRUNEAPQA-CCxIC4IMKM5UG5IcFaIpFBJ+gzEagWWyOcCQTyJFRpKj0VicWkNtAGLgqiSyRSGUzhVxRcVaMBWGDgBCFNzeVKpMF1JpUDp8QA5dBWRyiMYTOzApwuFjuOhOl2oTykby+AJKYKhaAPYAAJgOCSMxzSGWyuShBTInpK5UqNTqDSaLTaKAw2FwhF6Aygjudrtuk0rsyU80WyzSO3Wm22ZLjBFiCaSKWTZwu6b613r9yisbwT3Yr3enzDVNl2NxiqgytQqtJexjGrRUDpmr22vZwE54t5-NoYkFZ91XKgEotQA

💻 Code

type Constructor<T> = {
  new (...args: never[]): T;
};

interface GenericParams<T> {
  bar: (t: T) => void;
}

type List<T extends readonly {}[]> = {
  [K in keyof T]: [Constructor<T[K]>, GenericParams<T[K]>];
};

declare function foo<T extends readonly {}[]>(params: List<[...T]>): void;

class Foo {
  test() {}
}
class Bar {}

// Inference fails
foo([
//^?
  [Foo, {bar(t) {

  }}],
  [Bar, {bar(t) {

  }}]
])

// Inference works
foo([
  [Foo, {bar(t) {
    t.test();
  }}]
])

interface NumberParams<T> {
  bar: (t: number) => void;
}

type List2<T extends readonly {}[]> = {
  [K in keyof T]: [Constructor<T[K]>, NumberParams<T[K]>];
};

declare function foo2<T extends readonly {}[]>(params: List2<[...T]>): void;

// Inference works
foo2([
// ^?
  [Foo, {bar(t) {

  }}],
  [Bar, {bar(t) {

  }}]
])

🙁 Actual behavior

In the foo example, the parameter t in the associated bar methods are inferred as type any. It seems that TypeScript should be able to infer the actual types of the parameters.

🙂 Expected behavior

t is inferred as type T in each case, i.e. t should be Foo in the Foo case, and Bar in the Bar case.

There are two additional cases provided here: one with a single element in the provided array, and one where the parameters are not generic (number). In both cases, type inference does assign the correct non-any type. It's unclear what exactly the issue is in the first case.

Additional information about the issue

It's possible this is related to #47599, though I don't have enough knowledge of the internals to say whether or not it's the same class of bug. I haven't found another issue reporting this same behaviour.

Start with the linked TypeScript Playground reproduction and compare the generic foo calls with the single-element and non-generic foo2 cases. Trace the type inference behavior around the mapped List type; done means each bar parameter is inferred as Foo or Bar rather than any, while the other cases remain correct.