TypeScript: Type 'X' is not assignable to type 'X'.

Bug Report

Since 4.6 TS is complaining about that false is not assignable to boolean, when it is “wrapped by interface”.

🔎 Search Terms

4.6 type

🕗 Version & Regression Information

This is a compile problem
This changed between versions 4.5.5 and 4.6

⏯ Playground Link

Playground Link

💻 Code

interface Observable<T>
{
  (): T;
  (value: T): any;
}

function observable<T>(value: T): Observable<T>
{
  return undefined as any;  // the implementation is not important
}

const x: Observable<boolean> = observable(false);

🙁 Actual behavior

Type 'Observable<false>' is not assignable to type 'Observable<boolean>'.
  Types of parameters 'value' and 'value' are incompatible.
    Type 'boolean' is not assignable to type 'false'.

🙂 Expected behavior

No compiler problem as in previous versions.

About this issue

Original URL
State: closed
Created 2 years ago
Comments: 26 (7 by maintainers)

Most upvoted comments

For the record, the reason for the type error:

const oFalse: Observable<false> = observable(false);
const oBool: Observable<boolean> = oFalse;
oBool(true);
// now Observable<false> has value true

fatcerberus on Mar 7, 2022

@maskmaster to be honest I am not sure. I only know that what worked previously does not work now. As @whzx5byb pointed out, the whole thing may be “reversed” - it was working because of less perfect previous TS version that allowed this and now it is not working because simply it should not work in the first place. I am not good at type theory at all, so anybody else have to say funded words. Anyway, I am reopening this in hope for clarification.

tomhanax on Mar 17, 2022