I started learning TS, and I can't understand what is wrong with this class method:

class HoldAnything<T> {
  data: T

  add(a: T): T {
    return this.data + a
  }
}

VS code underlines return this.data + a, and it says:

Operator + cannot be applied to types T and T

I can't understand why it's showing me this error. Probably, it was answered somewhere already, however, I can't find it. I'd appreciate if someone could explain what is wrong with the method or point me to the answer.