Restrict union to be used as constraint only?

[emil14]

ChatGPT provided several examples why Go team restricted union types to be used only as constraints.

1. Unintended behavior due to implicit type conversions:

function add(x: number | string, y: number | string) {
  return x + y;
}

console.log(add(1, 2)); // Returns 3
console.log(add('1', '2')); // Returns '12'
console.log(add(1, '2')); // Returns '12'

2. Difficulty in maintaining type safety:

interface User {
  name: string;
  age: number;
}

interface Product {
  name: string;
  price: number;
}

type Entity = User | Product;

function fetchEntity(id: string): Entity {
  // Fetch entity from API
}

const entity = fetchEntity('123');
console.log(entity.age); // Error: Property 'age' does not exist on type 'Entity'

3. Reduced performance due to type checking overhead:

type Status = 'Active' | 'Inactive' | 'Pending' | 'Expired';

interface User {
  id: number;
  name: string;
  status: Status;
}

function filterActiveUsers(users: User[]) {
  return users.filter(user => user.status === 'Active');
}