Type union collapses into the general type in condition

[FlandreDaisuki]

Bug Report

🔎 Search Terms

type union collapses

🕗 Version & Regression Information

• I've tried from 3.3.3 to 5.2.0-beta and the result are the same.

⏯ Playground Link

Playground link

💻 Code

const a = (Math.random() > 0.5)
  ? Object.freeze({ok: false, hello: 'world'})
  : Object.freeze({ok: false});

🙁 Actual behavior

type A = typeof a;
//   ^? shows type A = {readonly ok: false}

🙂 Expected behavior

type A = typeof a;
//   ^? shows type A = Readonly<{ok: false}> | Readonly<{ok: false, hello: 'world'}>

[MartinJohns]

This is working as intended and happens because of type reduction. See #50171 and many many others.

Also: https://www.typescriptlang.org/docs/handbook/type-inference.html#best-common-type

[FlandreDaisuki]

@MartinJohns thanks your help and reference links.

[Jack-Works]

but I don't think this reduction is meaningful to the developers. it lost information that should be there.

[fatcerberus]

@Jack-Works Variations of this same issue have been raised many, many times in the past and the response by maintainers is consistently that subtype reduction, while sometimes inconvenient, is not only not considered to be a bug, but completely by design, so it's very unlikely you're going to get much traction on this. #53425 (comment)