Two levels of generic nesting and Parameters<T> results in a TypeScript error. But one level works, and no Parameter<T> works. Why?

TypeScript doesn't always handle constraints on these nested indexes well. You can get around it by declaring a version of Parameters without the constraint (which will just evaluate to never when applied to a non-function type):

type ParametersUnconstrained<T> = T extends (...args: infer P) => any ? P : never

const callAPI = <
    Category extends keyof API,
    Name extends keyof API[Category],
>(category: Category, name: Name, param: ParametersUnconstrained<API[Category][Name]>[0]) => {
    console.log(`Param to ${name}: ${param}`);
};

or you could add a conditional type to remind TypeScript of the constraint: API[Category][Name] extends (...args: any) => any ? API[Category][Name] : never, which corresponds to Extract<API[Category][Name], (...args: any) => any>:

const callAPI = <
    Category extends keyof API,
    Name extends keyof API[Category],
>(category: Category, name: Name, param: Parameters<Extract<API[Category][Name], (...args: any) => any>>[0]) => {
    console.log(`Param to ${name}: ${param}`);
};

And a third option would be to write a conditional type to do the indexing in API:

type IndexAPI<Category, Name> = 
  Category extends keyof API ? Name extends keyof API[Category] ? API[Category][Name] : never : never

which makes callAPI look like this:

const callAPI = <
    Category extends keyof API,
    Name extends keyof API[Category],
>(category: Category, name: Name, param: Parameters<IndexAPI<Category,Name>>[0]) => {
    console.log(`Param to ${name}: ${param}`);
};

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Two levels of generic nesting and Parameters<T> results in a TypeScript error. But one level works, and no Parameter<T> works. Why?

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