Weird typescript behavior

export class Attributes<T> {
    constructor(private data: T) {}

    get<K extends keyof T>(key: K): T[K] {
        return this.data[key];
    }

    set(update: T): void {
        Object.assign(this.data, update);
    }
}

Screenshot of an error
Error

and text version

No overload matches this call.
  Overload 1 of 4, '(target: {}, source: T): {} & T', gave the following error.
    Argument of type 'T' is not assignable to parameter of type '{}'.
  Overload 2 of 4, '(target: object, ...sources: any[]): any', gave the following error.
    Argument of type 'T' is not assignable to parameter of type 'object'.ts(2769)
Attributes.ts(1, 25): This type parameter might need an `extends {}` constraint.
Attributes.ts(1, 25): This type parameter might need an `extends object` constraint.

>Solution :

As error says:

number.ts(1, 25): This type parameter might need an `extends object` constraint.

So you should constraint T to extend the object, which will means that it is not a primitive:

class Attributes<T extends object> {
  constructor(private data: T) {}

  get<K extends keyof T>(key: K): T[K] {
    return this.data[key];
  }

  set(update: T): void {
    Object.assign(this.data, update); // no error
  }
}