Discriminated Unions Require a Common Literal Tag Field

Union types don't narrow correctly when the compiler can't distinguish members, leading to 'Property does not exist on type' errors when accessing variant-specific fields.

Add a shared literal-typed tag field ('kind', 'type', or '_tag') to each union member. TypeScript narrows the union automatically inside if/switch blocks.

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'rect'; width: number; height: number };

function area(s: Shape): number {
  switch (s.kind) {
    case 'circle': return Math.PI * s.radius ** 2;
    case 'rect':   return s.width * s.height;
  }
}

TypeScript's control-flow analysis tracks the narrowed type inside each branch. A literal discriminant gives the compiler a concrete equality check it can reason about.