Regression: Mapped types are no longer homomorphic when wrapped in certain conditional types

@ahejlsberg is already working on this.

Since Feb 13, 2026.

### 🔎 Search Terms

"homomorphic" "keyof" "extends"

### 🕗 Version & Regression Information

- This changed between versions 3.8.3 and 3.9.7

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBA0hIFkCGYA8AVAfFAvFA1vAPYBmU6UEAHsBAHYAmAzlAAoBORk7ocIUAfigBvKAG0YUAJZ0CxMugC6ALlhQAvlFV0IANwjsA3AChjoSFAAauWPGRo6AVwC2AIwOZDQA

### 💻 Code

```ts
type KeyMap = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;

type X = KeyMap;
```

### 🙁 Actual behavior

`X` is evaluated to

```ts
type X = {
toString: "toString";
toFixed: "toFixed";
toExponential: "toExponential";
toPrecision: "toPrecision";
valueOf: "valueOf";
toLocaleString: "toLocaleString";
}
```

### 🙂 Expected behavior

`X` should be evaluated to `number` because according to [the FAQ](https://github.com/Microsoft/TypeScript/wiki/FAQ#common-bugs-that-arent-bugs),

> Mapped types declared as `{ [ K in keyof T ]: U }` where T is a type parameter are known as `homomorphic mapped types`, which means that the mapped type is a structure preserving function of `T`. When type parameter `T` is instantiated with a primitive type the mapped type evaluates to the same primitive.

### Additional information about the issue

Replacing `PropertyKey` with either `unknown` or `any` fixes the issue for some reason.