nominative.ts

type A = {
  value: string;
  kind: "a"; // here's the tag
};
type B = {
  value: string;
  kind: "b";
};
function my_func(a: A): string {
  return a.value;
}
my_func({value: "hi", kind: "a"}); // no error, unsurprisingly
my_func({value: "hi", kind: "b"}); // now this *is* an error
my_func({value: "hi"}); // also an error