For posterity what’s the actual Flow signature with overload that we intend?

@acdlite I think we should also add an overload that accepts void as a third arg.

That way it allows undefined to be passed. That’s useful when you’re forwarding arguments in wrappers like we do.

@sebmarkbage Good call, I edited my comment accordingly

declare function useReducer<S, A>(
  reducer: (S, A) => S,
  initialState: S,
  init: void,
): [S, Dispatch<A>];

What use case is this variation intended for?

Probably:

That way it allows undefined to be passed. That’s useful when you’re forwarding arguments in wrappers like we do.

Like if you wrap useReducer from higher level API that may or may not receive an init argument I guess?

Yeah maybe that's it. I wouldn't initially have expected that to require its own explicit variation.

Effectively, this acts as if the third argument defaults to the identity function? 🤔

declare function useReducer<S, A>(
  reducer: (state: S, action: A) => S,
  initialState: S,
  initializer?: undefined
): [S, Dispatcher<A>]
declare function useReducer<S, A, I>(
  reducer: (state: S, action: A) => S,
  initializerArg: I,
  initializer: (arg: I) => S
): [S, Dispatcher<A>]