Declaration emit fails to import symbol on function auto-property

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🔎 Search Terms

mixin symbol not imported function property

🕗 Version & Regression Information

• This changed between version 3.7.7 and 3.8.1, it went from working to being an error.
• This changed between version 4.2.4 and 4.3.2, it went from being an error to invalid.

⏯ Playground Link

https://www.typescriptlang.org/dev/bug-workbench/?#code/PTAEAEBMFMGMBsCGAnRAXAlgewHYC5Q1kBXaAKBAgGcALLAdwFEBbDNCscWhlttaSADEM8aAQw4YADwB0kGWipkMzAA5ZkaUAG9QrKRNABfUADNkWZqABEM4MminrAbjJkEiKlVCCsWUNBS-JLe+hIAFB5eOkYAlDpkoEmgZEZugeqaOj5+xm6U4KYi0DiIzGKgDqYKSu64VFpUAJ5WALygAMotAEZY8OGxZKbEOLCYuHoYBjgAPAAqAUElkN440PSg4TLbKADmVASIOE0A2gC68a0AfKBY3QBWcGhX4d2eFXPx2onJiN0NqDGoCi3gAsotgitQG8qNAEskEaATmEcDJmswzqB2kRSD8kmkEQ40MRkDhQKDUmQUWiWljQOjXGQMhotLoUcYgA

💻 Code

import { mixin } from "./ref";

class Foo extends mixin(class {}) {
 
}

export { Foo }

// @filename: ref.ts

const sym = Symbol()
function mixin<T extends new (...args: any[]) => object>(base: T) {
 abstract class M extends base {
 [mixin.sym] = true
 }
 return M
}
mixin.sym = sym;

export { mixin }

🙁 Actual behavior

The symbol sym is not referenced properly in the emitted .d.ts it is an undefined name, nor is an error thrown:

import { mixin } from "./ref";
declare const Foo_base: (abstract new (...args: any[]) => {
 [sym]: boolean;
}) & {
 new (): {};
};
declare class Foo extends Foo_base {
}
export { Foo };

🙂 Expected behavior

The symbol sym should be referenced as it's used mixin.sym

import { mixin } from "./ref";
declare const Foo_base: (abstract new (...args: any[]) => {
 [mixin.sym]: boolean;
}) & {
 new (): {};
};
declare class Foo extends Foo_base {
}
export { Foo };

or an error should be thrown.

Additional information about the issue

From v3.3.3 to v3.7.7 this technique worked as expected. From v3.8.1 to v4.2.4 this technique resulted in a compiler error. And from v4.3.2 onward this technique has emitted invalid code.

This bug surfaced trying to workaround the fact that const sym is not exported and is a private name.