"This type is incompatible with null" on merging {method: 'POST'} with RequestOptions

[jdelStrother]

https://flowtype.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swAHOAZ3wApMAnGALjGvsOwHMAaMOMoiUosASgFMAjpjHUA8gOLZKASjABvVGDD0x+BqSi7cACzqNeagLa7jcACYsA5AAVZAZQAqj3gDo--QSUAX2VUINQgA

I'm trying to supply a default {method: 'POST'} option to whatwg-fetch -

/* @flow */
function post(url: string, options: RequestOptions) {
  return fetch(url, {method: 'POST', ...options})
}

3:   return fetch(url, {method: 'POST', ...options})
                                ^ string. This type is incompatible with
/static/flowlib/bom.js:641:     method?: ?MethodType;
                                          ^ null

A workaround is casting the 'POST' string with ('POST': ?MethodType), but it seems like this ought to work without, no?

[avikchaudhuri]

Hmm, looks like a spread issue.

[villesau]

I think this fixes the issue: #7298

[nmote]

I came up with a minimal repro based on this bug report, and this is fixed in master.