Generic Extract alone cannot be used from uninstantiated generic parameter

[erwanvivien]

Bug Report

🔎 Search Terms

• Extract
• Generic
• Type union

🕗 Version & Regression Information

• This is a bug
• This is the behavior in every version I tried, even nightly

⏯ Playground Link

Playground link

💻 Code

type ApiPost =
    | {
        path: "/something1";
        body: {};
    }
    | {
        path: "/something2";
        body: { name: string; };
    }

type PostPath = ApiPost["path"];
type PostBody<PATH extends PostPath> = Extract<ApiPost, { path: PATH }>["body"];
// type PostBody<PATH extends PostPath> = Extract<ApiPost, { path: PATH }>["body"] & []; // works
// type PostBody<PATH extends PostPath> = Extract<ApiPost, { path: PATH }>["body"] & object; // works
// type PostBody<PATH extends PostPath> = Extract<ApiPost, { path: PATH }>["body"] & { weird: "bug" }; //works
// type PostBody<PATH extends PostPath> = (ApiPost & { path: PATH })["body"] // works

const post = async <PATH extends PostPath>(
    path: PATH,
    {body, ...options}: Omit<RequestInit, 'body'> & {body: PostBody<PATH>}
): Promise<null> => {
    return null
}

const tmp = <PATH extends PostPath>(
  path: PATH,
  body: PostBody<PATH>
) => {
  post<PATH>(path, { body })
}

🙁 Actual behavior

It brings an error where it SHOULD accept the type, adding random types makes the error disappear

🙂 Expected behavior

I should be able to use the first type as it does the same as every other types

[ahejlsberg]

The issue here appears to be that the type PostBody<PATH> diverges when normalized for reading vs. writing--effectively, the type ends up not being assignable to itself! This happens early on in isRelatedTo, before we even check if the source and target types have identical object identities. We definitely need an identity check to happen first.

[ahejlsberg]

Simpler repro:

function fx1<P extends PostPath>(x: { body: PostBody<P> }, y: { body: PostBody<P> }) {
    x = y;  // Error, but shouldn't be
}