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type Set1<T, K1 extends keyof T> = T extends any[] ? T : Pick<T, Exclude<keyof T, K1>> & {
[SK1 in K1]-?: Required<Pick<T, SK1>>;
}[K1];
type Set2<T, K1 extends keyof T, K2 extends keyof T[K1]> = T extends any[] ? T : Pick<T, Exclude<keyof T, K1>> & {
[SK1 in K1]-?: Required<{
[key in K1]: Set1<T[K1], K2>;
}>;
}[K1];
declare function set<T, K1 extends keyof T>(source: T, path: [K1], value: T[K1]): Set1<T, K1>;
declare function set<T, K1 extends keyof T, K2 extends keyof T[K1]>(source: T, path: [K1, K2], value: T[K1][K2]): Set2<T, K1, K2>;
interface State {
a: {
b: string;
c: number;
};
d: boolean;
}
const state: State = {
a: {
b: "",
c: 0,
},
d: false,
};
const newState: State = set(state, ["a", 'b'], 'why'); // shouldn't be an error
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