Opaque type projection does not maintain exactness

tagged.flowspec.js

/* @flow */

import {
  type Tagged, // eslint-disable-line import/named
  tagged
} from './tagged';

// I can create tagged values...
const taggedString: Tagged<string> = tagged('asdf');
// ...and those values can be used as the type that was provided as the argument to `Tagged`:
const untaggedString: string = taggedString;

// This works for object types too:
const taggedObject: Tagged<{ works: boolean }> = tagged({ works: true });
const untaggedObject: { works: boolean } = taggedObject;

// But fails to work for object types flagged exact:
const taggedExactObject: Tagged<{| works: boolean |}> = tagged({
  works: false
});
// $FlowFixMe
const untaggedExactObject: {| works: boolean |} = taggedExactObject;

// Cannot assign taggedExactObject to untaggedExactObject because inexact Tagged [1] is incompatible with
// exact object type [2].
// 
// [1] 14│ const taggedExactObject: Tagged<{| works: boolean |}> = tagged({
//     15│   works: false
//     16│ });
// [2] 17│ const untaggedExactObject: {| works: boolean |} = taggedExactObject;
//     18│

tagged.js

/* @flow */

export opaque type Tagged<A>: A = A;

export const tagged = <A>(value: A): Tagged<A> => value;