tsserver hangs indefinitely when inferring return type of class method that passes this to a function with deeply nested conditional return type

まだ誰も着手していません。

### 🔎 Search Terms

hang circular

### 🕗 Version & Regression Information

This behavior is present in TypeScript 5 and TypeScript 6 beta.

### ⏯ Playground Link

N/A — requires the remeda package (npm i remeda). Reproduction is a self-contained file below.

### 💻 Code

```ts
import { pick } from "remeda";

export class MyClass {
constructor(public readonly field: number) {}

// [bad] tsserver hangs indefinitely: no explicit return type
getIdentity() {
return pick(this, ["field"]);
}

// [good] works fine: explicit return type breaks the cycle
getIdentityFixed(): Pick {
return pick(this, ["field"]);
}
}
```

### 🙁 Actual behavior

`tsserver` becomes completely unresponsive. The editor (VS Code) loses all language service features (hover types, completions, diagnostics) until tsserver is manually restarted. No error is emitted; tsserver simply loops forever.

The cause is an undetected circular type inference chain. To infer the return type of `getIdentity()`, TypeScript must evaluate `PickFromArray` (remeda's `pick` return type). That involves `IsBoundedRecord` → `IsBounded>`. `KeysOfUnion` uses `UnionToIntersection`, which places `MyClass` in a contravariant function parameter position — forcing eager, non-deferred evaluation of the full structural type of `MyClass`. The full type of `MyClass` includes `getIdentity: () => `, whose inferred return type is what we started trying to compute. Because the cycle passes through ~5 distinct type alias instantiations, TypeScript's cycle-detection heuristics never fire, and the checker loops indefinitely instead of emitting a "circularly references itself" error.

### 🙂 Expected behavior

TypeScript should detect the circular inference and either:

- Emit a "Return type annotation circularly references itself" error (as it does for simpler cycles), or
- Fall back to any with a warning.

Either outcome is acceptable. The checker should never hang.

### Additional information about the issue

Workaround: add an explicit return type annotation to the method. This gives TypeScript the type of `MyClass.getIdentity()` upfront, preventing the cycle from forming.

まず remeda をインストールし、tsserver で自己完結型の TypeScript 再現コードを実行します。ハングする推論戻り値のケースと、明示的な Pick アノテーションを比較し、その後、報告されている PickFromArray、IsBoundedRecord、IsBounded、KeysOfUnion、UnionToIntersection の連鎖における循環性の処理を追跡します。tsserver がハングしなくなり、循環戻り値型エラーを報告するか、警告付きで any にフォールバックすれば完了です。

索引モデルが issue の本文から書いたものです。