an emitter with 239 events breaks type check, throw ts2590 Expression produces a union type that is too complex to represent

Bug Report

I wrote a framework (namely koishi) which uses a fully-typed event emitter implemented by itself. However when I create more events (totally 239), the type check just broke with an error ts2590 (Expression produces a union type that is too complex to represent). It seems that the error was thrown here:

I know 239 events is a lot, but I still cannot figure out how the 239 events became "100000 type checks".

Can you help prevent this error either by optimize type check performance or by improve my code? Thanks.

🔎 Search Terms

union type, interface, event emitter, ts2590

🕗 Version & Regression Information

I tried with v4.1.3, v4.1.5 & v4.2.1-rc. They all failed.

⏯ Playground Link

This playground will not work because it imports a dependency @octokit/webhooks-definitions (it's just a simple collection of github webhooks type definitions, but it is too big (over 5000 lines) to be included as I thought).

Playground link with a dependency

And the dependency code can be found here: https://unpkg.com/@octokit/webhooks-definitions@3.60.0/schema.d.ts

💻 Code

interface EventMap extends SessionEventMap, WebhookEventMap {}

declare function on<K extends keyof EventMap>(name: K, listener: EventMap[K]): () => boolean

on('message', () => {})

🙁 Actual behavior

TS2590 error: Expression produces a union type that is too complex to represent.

🙂 Expected behavior

No compiler error

Start with the linked Playground and the minimal EventMap/on example, then inspect the referenced @octokit/webhooks-definitions schema.d.ts dependency to reproduce TS2590 with roughly 239 events. Compare the reported TypeScript versions and confirm that the example type-checks without the union-complexity error.