object literal. Inexact type is incompatible with exact type (no object spread)

Question

Flow works correctly with exact types in the below case:

type Something={|a: string|};
const x1: Something = {a: '42'};        // Flow is happy
const x2: Something = {};               // Flow correctly detects problem
const x3: Something = {a: '42', b: 42}; // --------||---------

… however Flow also complains at the following:

type SomethingEmpty={||};
const x: SomethingEmpty = {}; 

Message is:

object literal. Inexact type is incompatible with exact type

This is not the same case as this one as no spread is used.

Tested with the latest 0.57.3.

Answer 1

The Object literal without properties is inferred as an unsealed object type in Flow, this is to say you can add properties to such an object or deconstruct non-existing properties without errors being raised:

// inferred as...

const o = {}; // unsealed object type
const p = {bar: true} // sealed object type

const x = o.foo; // type checks
o.bar = true; // type checks

const y = p.foo; // type error
p.baz = true; // type error

Try

To type an empty Object literal as an exact type without properties you need to seal it explicitly:

type Empty = {||};
const o :Empty = Object.seal({}); // type checks

Try