Typescript 4.9: Type 'NonNullable<K>' may represent a primitive value, which is not permitted as the right operand of the 'in' operator.(2638)

Question

Typescript 4.9 features some changes to how the in operator works.

In the code snippet below, how do I convince tsc that id is not a primitive value?

export type KeyOrId =
  | {
      key: string;
    }
  | {
      id: string;
    };

export default function foo<K extends KeyOrId | null>(
  keyOrId: K,
) {
 
    if (!keyOrId) {
      return undefined;
    }

    // Type 'NonNullable<K>' may represent a primitive value, which is not permitted as the right operand of the 'in' operator.(2638)
    return 'id' in keyOrId;
}

I've tried a guard with typeof keyOrId !== 'object'. No dice.

Playground example

Answer 1

This line is causing the issue - K extends KeyOrId | null .

K extends KeyOrId | null = K extends KeyOrId | K extends null .

The following should work -

export type KeyOrId =
  | {
      key: string;
    }
  | {
      id: string;
    };


export default function foo<K extends KeyOrId>(
  keyOrId: K | null,
) {
 
    if (!keyOrId) {
      return undefined;
    }

    return 'id' in keyOrId;
}