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type MergeAll<T> = Reduce<<X, V>(V) => X => { ...$Exact<X>, ...$Exact<V> }, {}, T>

declare function mergeAll<A>(A): MergeAll<A>

const x = mergeAll([{ a: 1 }, { b: 1 }, { c: 1 }, { a: 2 }])
const xx = mergeAll([{ a: 1 }, { b: 1 }, { c: 1 }, { a: 2 }])

// try to comment this line, the error disappears
;(x: { a: 2, b: 1, c: 1 }) // rightly

// or try to comment this line, the error disappears too
;(xx: { a: 2, b: 1, c: 1 }) // error?!

type GetUserInfoArgs = {| user_id: number |};
type GetUserInfo = (GetUserInfoArgs) => { first_name: string, last_name: string };
type DerivedArg = $ElementType<Arguments<GetUserInfo>, 0>; // should be equivalent to `GetUserInfoArgs`?

/**
 * Using Arguments<GetUserInfo>:
 * - `a` fails type checking as expected
 * - `b` does not fail type checking :( 
 */
const a: DerivedArg = { foo: 'bar' };
const b: {| ...DerivedArg |} = { foo: 'bar' };

/**
 * Without using Arguments<GetUserInfo>:
 * - `c` fails type checking as expected
 * - `d` fails type checking as expected 
 */
const c: GetUserInfoArgs = { foo: 'bar' };
const d: {| ...GetUserInfoArgs |} = { foo: 'bar' };