Wrong assignability of class types from expressions with private fields

[dragomirtitian]

🔎 Search Terms

class expression private fields assignability

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

function makeClass(v: number) {
 return class C{
 #id = v;
 static getId(o: C) {
 return o.#id;
 }
 }
}

const cls1 = makeClass(1)
const cls2 = makeClass(2)

let c1 = new cls2(); // TS thinks this is ok
cls1.getId(c1) // runtime error different versions of #id

🙁 Actual behavior

c2 is assignable to c1 even though they contain private fields that are different (even if they come from the same source code)

🙂 Expected behavior

Instances of cls2 should not be assignable to cls1 since they have different versions of the private field.

Additional information about the issue

Found as I was exploring the consequences of #56145

[fatcerberus]

I'm thinking this is probably a design limitation (i.e. nominally-typed properties are distinguished by source location or something), but yeah, ouch, that's unfortunate.

[RyanCavanaugh]

Yeah, there is zero concept of the kind of per-invocation tracking you'd need to handle this.

[jcalz]

Reminds me of #37469, where each invocation of a function spits out a new nominal type.