"Could not decide which case to select" when making copy of union with object spread

[cakoose]

// @flow
type Union =
  {type: 'A', a: number} |
  {type: 'B', b: string};

function f(u: Union): Union {
  return {...u};  // spurious error
}

From flow.org/try:

7:   return {...u};
            ^ Could not decide which case to select. Since case 1 [1] may work but if it doesn't case 2 [2] looks promising too. To fix add a type annotation to .a [3] or to .type [3].
  References:
  3:   {type: 'A', a: number} |
       ^ [1]
  4:   {type: 'B', b: string};
       ^ [2]
  7:   return {...u};
              ^ [3]

[villesau]

This will fix the issue: #7298

[nmote]

This no longer errors in master