fix(zod): tighten guards in tryBuildDiscriminatedUnion and buildDiscriminatorExpression

CallumJHays · CallumJHays · commit 640eb2aaadf6 · 2026-05-13T14:05:22.000+10:00
- Add `items?.length` check before `every()` to prevent vacuous-truth on
  empty `or.items` array (would have emitted invalid `z.enum([])`)
- Add `value.length &gt; 0` guard in `buildDiscriminatorExpression` for the
  same reason &mdash; function is exported so the guard is worth keeping locally
- Collapse single-element arrays to `z.literal(value[0])` ahead of the
  array branches (more ergonomic, consistent with scalar path)

All three cases are unreachable today (IR collapses single-item `or` and
only emits `or` when `valueSchemas.length &gt; 1`), but tightening keeps the
invariants local to the functions rather than relying on upstream guarantees.
diff --git a/packages/openapi-ts/src/plugins/zod/shared/discriminated-union.ts b/packages/openapi-ts/src/plugins/zod/shared/discriminated-union.ts
@@ -48,7 +48,8 @@ export function tryBuildDiscriminatedUnion({
       discriminatedValue = discriminatorProp.const;
     } else if (
       discriminatorProp?.logicalOperator === 'or' &&
-      discriminatorProp.items?.every((item) => item.const !== undefined)
+      discriminatorProp.items?.length &&
+      discriminatorProp.items.every((item) => item.const !== undefined)
     ) {
       // OR-of-consts &mdash; the discriminator maps multiple values to one schema.
       // Every item must have a const; a future IR change that violates this would
@@ -99,15 +100,21 @@ export function tryBuildDiscriminatedUnion({
  * ZodUnion of literals). The shape to emit depends on the value:
  *
  *   "cat"       &rarr; z.literal("cat")
- *   ["a", "b"]  &rarr; z.enum(["a", "b"])    all strings: enum is more ergonomic than union
+ *   ["x"]       &rarr; z.literal("x")         single-element: literal is more ergonomic
+ *   ["a", "b"]  &rarr; z.enum(["a", "b"])     all strings: enum is more ergonomic than union
  *   [1, 2]      &rarr; z.union([z.literal(1), z.literal(2)])
  */
 export function buildDiscriminatorExpression(z: Symbol, value: unknown): Chain {
   if (!Array.isArray(value)) {
     return $(z).attr(identifiers.literal).call($.fromValue(value));
   }
 
-  if (value.every((v) => typeof v === 'string')) {
+  // Single-element array: prefer z.literal over a single-item enum/union
+  if (value.length === 1) {
+    return $(z).attr(identifiers.literal).call($.fromValue(value[0]));
+  }
+
+  if (value.length > 0 && value.every((v) => typeof v === 'string')) {
     return $(z).attr(identifiers.enum).call($.fromValue(value));
   }
 
