Assertion function & Type guards for Record types broken on 5.0

[JeanMeche]

🔎 Search Terms

• asserts
• assertion function
• type guard
• record

🕗 Version & Regression Information

Worked on 4.9, broken since 5.0

Probably introduced by #52984

⏯ Playground Link

Playground link with relevant code

💻 Code

declare function isObject(o: unknown): o is Record<string, any>

declare function assertObject(o: unknown): asserts o is Record<string, any>  

let value: {} = null!;
if(isObject(value)) {
    value.anything // error since 5.0.2
}

assertObject(value)

value.anything // error since 5.0.2

🙁 Actual behavior

Property 'anything' does not exist on type '{}'.(2339)

🙂 Expected behavior

No error, narrowing / assertion should work fine.

[ahejlsberg]

Hmm, looks like the strict subtype relation considers {} and Record<string, any> to be mutual subtypes. That's an oversight and apparently we don't have test coverage for that combination. It should be the case that {} is a strict supertype of Record<string, any>, similar to {} being a strict supertype of Record<string, unknown> (for which we do have test coverage).