// Created on savesnippets.com · https://savesnippets.com/Fi9Gzcf4DjzZGB
export function assertNever(x: never): never {
  throw new Error(
    `Unhandled discriminant: ${typeof x === 'object' ? JSON.stringify(x) : String(x)}`
  );
}

type Shape =
  | { kind: 'circle';    radius: number }
  | { kind: 'square';    size: number }
  | { kind: 'rectangle'; w: number; h: number };

function area(s: Shape): number {
  switch (s.kind) {
    case 'circle':    return Math.PI * s.radius ** 2;
    case 'square':    return s.size * s.size;
    case 'rectangle': return s.w * s.h;
    default:          return assertNever(s);   // catches new variants
  }
}