📝 Docs: Note that & why Twoslash and `$ExpectType` have different behavior

[OliverJAsh]

🐛 Bug Report

• eslint-plugin-expect-type version: 0.2.2
• ESLint version: 8.36.0
• Node version: 16.3.0

Actual Behavior

Full reduced test case here: https://github.com/OliverJAsh/eslint-plugin-expect-type-issue

Contents inlined below.

package.json:

{
  "dependencies": {
    "@typescript-eslint/parser": "^5.55.0",
    "eslint": "^8.36.0",
    "eslint-plugin-expect-type": "^0.2.2",
    "typescript": "^5.0.2"
  }
}

tsconfig.json:

{
  "compilerOptions": {
    "strict": true
  }
}

.eslintrc.js:

const path = require("path");

module.exports = {
  parser: "@typescript-eslint/parser",
  parserOptions: {
    project: path.join(__dirname, "./tsconfig.json"),
  },
  plugins: ["eslint-plugin-expect-type"],
  rules: {
    "expect-type/expect": "error",
  },
};

source.ts:

export type Compact<A> = A extends Function ? A : { [K in keyof A]: A[K] } & {};

test.ts:

import { Compact } from "./source";

declare const any: any;

const x: Compact<Record<"a" | "b", number> | Record<"a" | "c", number>> = any;

// ❌ Error:
//  Expected type to be: { a: number; b: number; } | { a: number; c: number; }, got: Compact<Record<"a" | "b", number> | Record<"a" | "c", number>>
// $ExpectType { a: number; b: number; } | { a: number; c: number; }
type Test1 = typeof x;

// ✅ No error
type Test2 = typeof x;
//   ^? type Test2 = {
//          a: number;
//          b: number;
//      } | {
//          a: number;
//          c: number;
//      }

• $ExpectType expects: Compact<Record<"a" | "b", number> | Record<"a" | "c", number>>
• Twoslash expects:

     type Test2 = {
      a: number;
      b: number;
  } | {
      a: number;
      c: number;
  }

I believe the reason for this difference is because Twoslash and $ExpectType use different mechanisms to generate the "actual" value:

• Twoslash: languageService.getQuickInfoAtPosition
• $ExpectType: checker.typeToString

This particular issue started after we upgraded to TypeScript 5. Prior to this, $ExpectType had the same behavior as Twoslash in this particular example. I'm not sure why this has changed.

Expected Behavior

Twoslash queries and $ExpectType should behave the same.

Reproduction

https://github.com/OliverJAsh/eslint-plugin-expect-type-issue

[danvk]

Thanks for the report @OliverJAsh. Do you think its expected (by the TS team) that the behaviors of getQuickInfoAtPosition and typeToString have diverged? Should we file an upstream issue about it?

Assuming this is intentional… while it is curious, I'm not sure this is undesirable behavior? I would expect $ExpectType to behave the same way as dtslint. And I'd expect twoslash-style assertions to behave the same way they do in the TypeScript playground. Which I believe is the case right now, even though those two have diverged.

[OliverJAsh]

Do you think its expected (by the TS team) that the behaviors of getQuickInfoAtPosition and typeToString have diverged?

Good question. I honestly have no idea. Up until now, my mental model for $ExpectType has always been it will use the same printed string as what you see when you hover over a type or use Twoslash, but maybe I was wrong to think that.

[JoshuaKGoldberg]

Coming back >1.5 years later: I think this is more of a documentation issue at this point. We have the two approaches with different root ideals:

• $ExpectType: Goes directly to the TypeScript's stringification
• ^?: Instead mirrors the visual display