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

[aweebit]

🔎 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

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

type X = KeyMap<number>;

🙁 Actual behavior

X is evaluated to

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,

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.

[Andarist]

The above repro might feel artificial and not real-world but I believe this to be a genuine problem. Homomorphic mapped types distribute and this one doesn't (given the compiler doesn't see it as homomorphic): TS playground

The reason it's not treated as a homomorphic mapped types is that keyof T becomes a substitution type in the true type of this conditional.