Prettier removes Promise<void> type and other annotations in TypeScript arrow functions

[danielbjornadal]

Summary

Prettier removes Promise type annotations in TypeScript arrow functions, even with "arrowParens": "always" configured, making it impossible to satisfy TypeScript's requirement for generic type parameters on Promises.

Steps To Reproduce:

1. Create a new TypeScript file with an interface containing Promise return types
2. Configure Prettier with "arrowParens": "always"
3. Save the file and observe Prettier's formatting
4. Note that TypeScript shows an error: "Generic type 'Promise' requires 1 type argument(s)"

Expected result

Prettier should preserve the Promise type annotation when formatting TypeScript files, especially when "arrowParens": "always" is set.

Example:

interface Example {
  myFunction: (param: string) => Promise<void>;
}

Actual result

Prettier removes the type parameter, resulting in invalid TypeScript:

interface Example {
  myFunction: (param: string) => Promise;
}

This causes TypeScript to error with "Generic type 'Promise' requires 1 type argument(s)".

Additional information

The only current workarounds are // @ts-ignore or // prettier-ignore

Neither solution is ideal for maintaining clean, type-safe code.

VS Code Version: 1.95.3

Prettier Extension Version: 11.0.0

OS and version: Ubuntu 24.04

Prettier Log Output

[INFO] Prettier Options:
{
  "plugins": ["@trivago/prettier-plugin-sort-imports"],
  "importOrder": [...],
  "importOrderSeparation": true,
  "importOrderSortSpecifiers": true,
  "printWidth": 120,
  "arrowParens": "always"
}
[INFO] Prettier Resolution: Prettier Path: /Users/[username]/project/node_modules/prettier
[INFO] Formatting file:///Users/[username]/project/src/types.ts
[INFO] Formatting completed in 0.1s