My try on dependent types + conditional types using typescript to validate optional args based on a key

test.ts

type Routes = {
    a: {
        string: string 
    }
    b: undefined
}


type NullableKeys<T, K extends keyof T> = T[K] extends null | undefined
    ? K
    : never;

type NonNullableKeys<T, K extends keyof T> = T[K] extends null | undefined
    ? never
    : K;    

declare function teste<R extends keyof Routes>(
    a: NullableKeys<Routes, R>
)

declare function teste<R extends keyof Routes>(
    a: NonNullableKeys<Routes, R>,
    b: Routes[R]
)

teste('a') // error
teste('a', { string: 'test'}) // ok
teste('b', { string: 'test'}) // error
teste('b') // ok