I am a beginner in Typescript. I am trying to learn generics.

type Summable = string | number;

interface Calculation<Type> {
  number1: Type;
  number2: Type;
  operation(): Type;
}

class Addition<Summable> implements Calculation<Summable> {
  number1: Summable;
  number2: Summable;

  constructor(number1: Summable, number2: Summable) {
    this.number1 = number1;
    this.number2 = number2;
  }

  operation(): Summable {
    console.log(`Calculating Addition .... between ${this.number1} and ${this.number2}`);

    let val: Summable = this.number1 + this.number2;

    return val;
  }
}

function main() {
  let adder: Addition<number> = new Addition<number>(2, 3);
  console.log(adder.operation());
}

main();

I am getting the following error –

Operator '+' cannot be applied to types 'Summable' and 'Summable'.ts(2365)

I researched a bit and found out a github issue regarding it. Can anyone please tell why I am getting this error as + operator is valid in both number and string?