Disjoint unions fail when destructuring tagging property

[avaly]

The following typing triggers an error in the reducerFails reducer where we access the action.type after a destructuring operation. Flow should apply the same logic as it does for the case in reducerPasses.

type Action = 
  | { type: 'FOO' }
  | { type: 'BAR', payload: {} };

function reducerPasses(action: Action) {
  if (action.type === 'BAR') {
    return { ...action.payload };
  }
  return action;
}

function reducerFails(action: Action) {
  const { type } = action;
  if (type === 'BAR') {
    return { ...action.payload };
  }
  return action;
}

See example code

Reference docs: https://flow.org/en/docs/types/unions/#toc-disjoint-unions

[vkurchatkin]

This is expected. At this point type is basically 'FOO' | 'BAR'. Flow doesn't keep track of the fact, that it is somehow related to Action.

[avaly]

It's weird that Flow can not make the connection between the desctructured type and action.type and figure out what action is at that point.

But if this is the intended use case for disjoint unions and refinements, at least this can be highlighted in the docs section for disjoint unions (I can't find any mention of this requirement), in order to not confuse other folks. @vkurchatkin what do you think?

[mgtitimoli]

@vkurchatkin is there any near plan to support destructuring for disjoint unions, should a proposal be created for this?