tsserver and intellisense hang indefinitely on "loading intellisense status" and "loading..." when using assertion function and complex conditional type

Bug Report

🔎 Search Terms

tsserver typescript langauge server loading intellisense status indefinite error forever asserts assertion function vscode conditional type

🕗 Version & Regression Information

• This changed between versions 4.7.4 and 4.8.0-beta and happens in all versions up to 4.9.0-dev.20221015

⏯ Playground Link

Unfortunately, painstakingly copy-pasting all the types from the various packages into the playground doesn’t allow me to reproduce the issue. So instead, I’ve created a minimal example repository that demonstrates the issue: https://github.com/Xunnamius/mre-typescript-issue-51188

💻 Code

import { visit, SKIP, type Visitor } from 'unist-util-visit';

import type { Content as MdastContent } from 'mdast';

function assertNonNullable<T>(
  value: T,
  err: string
): asserts value is NonNullable<T> {
  if (value === null) {
    throw new Error(err);
  }
}

export function visitAndReveal<Tree extends MdastContent>(
  tree: Tree,
  visitor?: Visitor,
  reverse?: boolean
) {
  visit(
    tree,
    'hidden',
    (node, index, parent) => {
      // UNCOMMENTING THIS WILL KILL INTELLISENSE
      // assertNonNullable(
      //   index,
      //   'error while revealing hidden node: node index is missing'
      // );

      // UNCOMMENTING THIS WILL KILL INTELLISENSE
      // assertNonNullable(
      //  parent,
      //  'error while revealing hidden node: node parent is missing'
      //);

      return visitor?.(node, index, parent) ?? [SKIP, index];
    },
    reverse
  );
}

🙁 Actual behavior

Tsserver and intellisense in vscode (latest version) hang when using a TypeScript version >=4.8.0-beta. See video example below.

🙂 Expected behavior

Tsserver/intellisense does not hang in latest versions of vscode and TypeScript.

---

https://user-images.githubusercontent.com/656017/195998496-e83d5c99-dc27-4867-8bcc-aa3d6a54ff55.mp4

[ahejlsberg]

For example, sometimes TS would give me “Type instantiation is excessively deep and possibly infinite” seemingly at random

That error is caused by our type instantiation governor when a single source element gives rise to >5M type instantiations. Any code that triggers this is definitely highly suspect. It looks like you were barely managing to squeeze under the limit in 4.7, so I would strongly recommend simplifying the types. In particular, the InclusiveDescendant<...> type appears expensive. I think it is trying to reduce the set of possible descendant types. I don’t know how important that added precision is, but it clearly comes at a very high cost.

[Xunnamius]

For example, checking the visitAndReveal function in the example causes ~5.8M type instantiations in 4.7 when compiled with tsc. This shrinks to ~1.2M type instantiations in 4.8 and later. Those numbers are big warning flags that the types are too complex.

Thanks for taking a look! I had a suspicion complexity was a problem after some other small issues with the types coming from the unist-util-visit package. For example, sometimes TS would give me “Type instantiation is excessively deep and possibly infinite” seemingly at random, then I’d restart the language server, and it would return the expected type.

cc unified team: @wooorm