Unable to read properties off object spread in generic function parameter (regression with 4.6.1-rc)

[danvk]

Bug Report

I noticed a regression between TypeScript 4.6.0-beta and 4.6.1-rc in my repo where parameters to generic functions that use object spread sometimes get an unusable type.

🔎 Search Terms

• object spread
• 4.6.1-rc

🕗 Version & Regression Information

• This changed between versions 4.6.0-beta and 4.6.1-rc.

⏯ Playground Link

Playground link with relevant code

💻 Code

(see comment below for a simpler repro: #47865 (comment))

/** Extract params from an express path (e.g. '/students/:studentId'). */
export type ExtractRouteParams<T extends string> = string extends T
  ? Record<string, string>
  : T extends `${infer _Start}:${infer Param}/${infer Rest}`
  ? {[k in Param | keyof ExtractRouteParams<Rest>]: string}
  : T extends `${infer _Start}:${infer Param}`
  ? {[k in Param]: string}
  : {};

declare function get<T extends string>(
  path: T,
  handler: (params: ExtractRouteParams<T>) => void,
): void;

get('/path/:foo/to/:bar', ({foo, ...slugs}) => {
  const {bar} = slugs;  // ok in TS 4.6.0-beta but not in TS 4.6.1-rc
  console.log({bar});
});

get('/path/:foo/to/:bar', slugs => {
  const {bar} = slugs;  // ok in both
  console.log({bar});
});

🙁 Actual behavior

Using object spread in a function parameter produces an unusable type in TS 4.6.1-rc.

The type of slugs in the first example is:

 slugs: { bar: string; }

in TypeScript 4.6.0-beta but:

slugs: Omit<ExtractRouteParams<T>, "foo">

in TypeScript 4.6.1-rc.

These are both the same (assuming T = "/path/:foo/to/:bar") but the latter doesn't let me read any properties off of it.

🙂 Expected behavior

I should be able to read bar off of slugs.

[danvk]

Here's a simpler repro that doesn't involve template literal types. This (correctly) passes the type checker with TS 4.5.5 but raises a spurious error with 4.6.1-rc.

interface Slugs {
  foo: string;
  bar: string;
}

function call<T extends object>(obj: T, cb: (val: T) => void) {
  cb(obj);
}

declare let obj: Slugs;
call(obj, ({foo, ...rest}) => {
  console.log(rest.bar);
  //               ~~~ Property 'bar' does not exist on type 'Omit<T, "foo">'. ts(2339)
});

[RyanCavanaugh]

Bisected to #47337

[RyanCavanaugh]

@gabritto seems like we're failing to assign the inferred type to rest here; T shouldn't be appearing at all