unique symbol + interface + mixin causes TS4058 ("but cannot be named")

🔎 Search Terms

"unique symbol", "interface", "computed property", "mixin", "TS4058"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs

⏯ Playground Link

No response

💻 Code

// @filename: interface.ts
export const someUniqueSymbol = Symbol.for("some-unique-symbol");

export interface ISomeInterface {
  [someUniqueSymbol]: string;
}

// @filename: class.ts
import { someUniqueSymbol, ISomeInterface } from "./interface";

export class SomeClass implements ISomeInterface {
  [someUniqueSymbol] = "some-value";
}

// @filename: mixin.ts
import { SomeClass } from "./class";

type Constructor<T> = abstract new (...args: any[]) => T;

// An error occurs here:
// Return type of exported function has or is using name 'someUniqueSymbol'
// from external module "file:///class" but cannot be named.(4058)
export function SomeMixin<T extends Constructor<SomeClass>>(base: T) {
  abstract class SomeMixin extends base {}
  return SomeMixin;
}

Workbench Repro

🙁 Actual behavior

The compiler emits an error at the definition of function SomeMixin(), saying:

Return type of exported function has or is using name 'someUniqueSymbol'
from external module "file:///class" but cannot be named.(4058)

The problem disappears if you change the definition of someUniqueSymbol to:

export const someUniqueSymbol = "some-string";

I believe this is wrong for 3 reasons:

• There should be no reason why the type of someUniqueSymbol cannot be named because it's exported by interface.ts.
• The message is wrong. It's from interface.ts but not class.ts.
• The function SomeMixin() does not mention someUniqueSymbol, or anything from interface.ts at all, so it would be wrong to emit the error at the definition site of the said function.

🙂 Expected behavior

Compiles with no errors.

Additional information about the issue

This is similar to #37888 but I'm not sure if it's the same issue.

Start with the three-file reproduction: interface.ts, class.ts, and mixin.ts, then inspect the compiler path that reports TS4058 for the exported SomeMixin return type. Compare the unique-symbol case with the string-literal case and the related issue #37888. Done means the reproduction compiles without the erroneous diagnostic and existing declaration-emission behavior remains covered.