`keyof` on homomorphic mapped type over an array thinks keys are function types

Bug Report

Creating a mapped type (where each value is just the key - { [K in keyof T]: K }) when applied over a generic type T, where T has been passed as an array type, somehow manages to resolve the keys of an array as functions. I've read the FAQ about homomorphic mapped types but I can't imagine the K in K in keyof T ending up as a function type - no matter what the T eventually becomes.

🔎 Search Terms

keyof, array, types, type constructors

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about homomorphic mapped types

Tested on 3.9.7, 4.2.3.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Keys<T> = {
    [K in keyof T]: K;
}[keyof T];

type GT = Keys<string[]>;
// ^ Hover here. Keys are functions ???

🙁 Actual behavior

Keys applied over an array, returns a union of functions.

🙂 Expected behavior

Keys applied over an array, should return the property names of the array.

Reproduisez le comportement dans le TypeScript Playground lié en utilisant l’exemple Keys avec string[]. Commencez par étudier comment le compilateur traite keyof et les types mappés homomorphes pour les tableaux ; c’est terminé lorsque Keys<string[]> se résout en l’union des noms de propriétés du tableau plutôt qu’en une union de fonctions.