Annoying flow problem

flatmap.js

// @flow

type FlatMapMapper<In, Out> = (elem: In, index: number, arr: In[]) => Out | Out[];

function flatMap<In, Out>(arr: In[], mapFunc: FlatMapMapper<In, Out>): Out[] {
  const result = [];
  for (const [ index, elem ] of arr.entries()) {
    const x = mapFunc(elem, index, arr);
    // We allow mapFunc() to return non-Arrays
    if (Array.isArray(x)) {
      result.push(...x);
    }
    else {
      result.push(x);
    }
  }
  return result;
}

function main() {
  const input = [ 1, 2, 3 ];
  const output: string[] = flatMap(input, i => { return [ 'x' ]; });
  output.map(console.log);
}

main();

flatmap.js:22
 22:   const output: string[] = flatMap(input, i => { return [ 'x' ]; });
                                                             ^^^^^^^ array literal. This type is incompatible with
 22:   const output: string[] = flatMap(input, i => { return [ 'x' ]; });
                     ^^^^^^ string