Combining `importHelpers: true` and `module: preserve` results in TS2354

[jgoz]

🔎 Search Terms

TS2354 This syntax requires an imported helper but module 'tslib' cannot be found.
importHelpers
module
preserve

🕗 Version & Regression Information

Occurs in 5.4.2 and in ^5.5.0-dev.20240307.

• This changed between versions 5.3.3 and 5.4.2

^ This is not technically true because "module": "preserve" wasn't added until 5.4.2.

⏯ Playground Link

https://github.com/jgoz/typescript-module-preserve-issue

💻 Code

export class Foo {
  #foo: boolean;

  constructor() {
    this.#foo = false; // This syntax requires an imported helper but module 'tslib' cannot be found. ts(2354)
  }

  foo(): boolean {
    return this.#foo;
  }
}

{
  "compilerOptions": {
    "moduleResolution": "bundler",
    "module": "preserve",
    "importHelpers": true,
    "outDir": "dist",
    "target": "es2021"
  }
}

🙁 Actual behavior

Error message indicates that tslib is not present, but it is.

🙂 Expected behavior

No error.

Additional information about the issue

Not sure if this is a TS issue or an issue with the export maps in tslib.

[andrewbranch]

Have a fix on the way from the report at #58325