A switch dispatches over a discriminated union's tag (a literal-typed property like shape.kind) but doesn't handle every variant, and has no default case. Adding a new variant to the union later silently falls through unhandled instead of failing loudly — the whole point of a discriminated union is that the compiler (and a reader) can trust every case is covered.

Type-aware: resolves the discriminant's type through the ts-morph type host. Only fires when the discriminant's type is a literal-only union (every member is a string/number/boolean literal) — a plain string discriminant, or a union that includes non-literal members, is skipped entirely rather than guessed at.

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number }
  | { kind: "triangle"; base: number; height: number };

function area(shape: Shape) {
  switch (shape.kind) {          // flagged — "triangle" isn't handled, no default
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.side ** 2;
  }
}

// fix: handle every variant, or add an explicit default
function area(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.side ** 2;
    case "triangle":
      return (shape.base * shape.height) / 2;
  }
}

A default case of any content is treated as an intentional catch-all and suppresses the finding — this doesn't require the const _exhaustive: never = shape idiom (which tsc itself already flags in strict mode where applicable); requiring it here would double-report what the compiler already catches.

Scoped to switch statements only in this version — exhaustive if/else if chains over a discriminant are a known gap, not yet detected. Case labels are matched against string, numeric, and boolean literals; a case test that isn't a literal at all (e.g. a computed expression) can't be attributed to a member and is ignored rather than treated as covering anything.

# cofferdam.toml
[checks."Design.UnionExhaustivenessGap"]
severity = "low"   # demote if your project relies heavily on partial handling by convention