Flow: Missing type annotation error when using immutable Map merge

[luca-borrione]

I have a case in one of my reducers which is using merge from immutable Map:

case ADD_QUESTION: {
  const { question }: { question:Question} = action.payload;
  const questionMap: QuestionMap = ((fromJS(question): any): QuestionMap);
  return state.merge({ [question.id]: questionMap });
}

All is working, and unit tests are fulfilled, however flow complains about something:
Missing type annotation forK.Kis a type parameter declared inMap[1] and was implicitly instantiated at call of methodmerge[2]. (questions.js:22:14)flow Missing type annotation forK_.K_is a type parameter declared in function type [1] and was implicitly instantiated at call of methodmerge[2].

I can't figure out how to fix it:

• I saw a similar question in stackoverflow with no answers. Apparently the same error is thrown when using update and setIn as well
• It appears to be related with a fix landed on 0.85 see Asking for required annotations

[luca-borrione]

I am getting a similar error whe using mergeIn and updateIn too:

case ADD_USER_TO_QUESTION_VOTES: {
  const { payload }: { payload:AddUserToQuestionVotesPayload } = action;
  const { authedUserId, questionId, optionKey } = payload;
  return state.mergeIn(
    [questionId, optionKey, 'votes'],
    authedUserId,
  );
}

Missing type annotation forK.Kis a type parameter declared inMap[1] and was implicitly instantiated at call of methodmergeIn[2]. (questions.js:28:14)flow Missing type annotation forV.Vis a type parameter declared inMap[1] and was implicitly instantiated at call of methodmergeIn[2]. (questions.js:28:14)

[jbrown215]

Answered on SO. Flow is asking you to specify an explicit type argument. Here's a simpler example:
Missing annotation error
Fixed