type guard function: Element implicitly has an ‘any’ type

types.ts

export type Hoge =
  | {
      a: number;
    }
  | {
      b: number;
    };

この型からts-auto-guardを使ってtype guard functionを生成すると

types.guard.ts

export function isHoge(obj: unknown): obj is Hoge {
    const typedObj = obj as Hoge
    return (
        ((typedObj !== null &&
            typeof typedObj === "object" ||
            typeof typedObj === "function") &&
            typeof typedObj["a"] === "number" ||
            (typedObj !== null &&
                typeof typedObj === "object" ||
                typeof typedObj === "function") &&
            typeof typedObj["b"] === "number")
    )
}

.ts

typedObj["a"]

ここで
TS7053: Element implicitly has an 'any' type because expression of type '"a"' can't be used to index type 'Hoge'.   Property 'a' does not exist on type 'Hoge'.
が出てしまう。（typedObj["b"] も同様）

次の様にするとエラーが消える

types.ts

export type Hoge =
  | {
      a: number;
      b: never;
    }
  | {
      a: never;
      b: number;
    };

types.guard.ts

export function isHoge(obj: unknown): obj is Hoge {
    const typedObj = obj as Hoge
    return (
        ((typedObj !== null &&
            typeof typedObj === "object" ||
            typeof typedObj === "function") &&
            typeof typedObj["a"] === "number" ||
            (typedObj !== null &&
                typeof typedObj === "object" ||
                typeof typedObj === "function") &&
            typeof typedObj["b"] === "number")
    )
}