Functions with union Promised return types "Promise<void> | void" will fail eslint checks.

[RonaldZielaznicki]

ESlint is not able to handle conditional returns like this in terms of awaiting.

Looks like folks over at typescript-eslint suggest to always mark functions that can return a promise with async, even if it returns immediately.

typescript-eslint/typescript-eslint#6421

Ran into this problem when awaiting drain, eslint threw back this error in response.

Unexpected `await` of a non-Promise (non-"Thenable") value. [Error/@typescript-eslint/await-thenable]
Placing a void expression inside another expression is forbidden. Move it to its own statement instead. [Error/@typescript-eslint/no-confusing-void-expression]

[RonaldZielaznicki]

After some more digging, I found the PR comment that notes that this is intended.