Add a `default` branch that assigns to `never` so adding a union member becomes a compile error until handled.
Without an exhaustiveness check, a new union variant slips through unhandled and silently falls to default behavior at runtime.
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Assigning the value to a never-typed variable in default makes the compiler flag any unhandled case the moment a new union member is added.
// Bad
switch (shape.kind) {
case "circle": return area(shape);
// new "square" added later — silently unhandled
}
// Good
function assertNever(x: never): never { throw new Error("Unhandled: " + JSON.stringify(x)); }
switch (shape.kind) {
case "circle": return circleArea(shape);
case "square": return squareArea(shape);
default: return assertNever(shape); // compile error if a case is missing
}