Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the rule's documentation

https://typescript-eslint.io/rules/no-duplicate-type-constituents/

Description

I propose that no-duplicate-type-constituents supports a checkOptionalProperties option.

Similar to #9203 - when a property is marked as optional with ?, it implicitly includes undefined in the type definition, so there is no need to explicitly add an | undefined. I'd like to detect these cases and also autofix away the | undefined.

Note that this is explicitly incompatible with the exactOptionalPropertyTypes compiler option, that the rule otherwise is intended to not conflict with (see e.g. #5344). The documentation should clearly state that this option is intended for projects where exactOptionalPropertyTypes is NOT enabled.

Fail

type T = {
  a?: string | undefined
}

Pass

type T = {
  a?: string
}

Additional Info

I would be more than happy to create a PR that implements this :)