Error reported on computed property key that was refined from union type to literal type

[IsaacPacht]

Flow version: 0.119.0

Expected behavior

Given this code:

function f1(key: 'a' | 'b') {
  if (key === 'a') {
    const o = { [key]: 'x' };
  }
}

I would have expected no error. Even though flow doesn't allow union types for computed property keys, at the point where the error occurs, the type of "key" has already been refined to the literal type 'a'.

Actual behavior

I'm getting the error message:

Cannot use key [1] as a computed property. Computed properties may only be primitive literal values, but this one may be either string literal a [2] or string literal a [3]. Can you add a literal type annotation to key [1]? See https://flow.org/en/docs/types/literals/ for more information on literal types.

• Link to Try-Flow or Github repo: https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swoBGACgGsBTATwC4wByAQxbAB9WAjFgJRgA3qjBhCUMDQZgAvAtYcho8eNwkAzvjBx5IsAG069ALrMWAD04BfANxiwN1M6A