test: add type test for OptionalParameter transform

Author: kuhe

packages/types/src/util.spec.ts

@@ -0,0 +1,40 @@
+import type { Exact, OptionalParameter } from "./util";
+
+type Assignable<LHS, RHS> = [RHS] extends [LHS] ? true : false;
+
+type OptionalInput = {
+  key?: string;
+  optional?: string;
+};
+
+type RequiredInput = {
+  key: string | undefined;
+  optional?: string;
+};
+
+{
+  // optional parameter transform of an optional input is not equivalent to exactly 1 parameter.
+  type A = [...OptionalParameter<OptionalInput>];
+  type B = [OptionalInput];
+  type C = [OptionalInput] | [];
+
+  const assert1: Exact<A, B> = false as const;
+  const assert2: Exact<A, C> = true as const;
+
+  const assert3: Assignable<A, []> = true as const;
+  const assert4: A = [];
+
+  const assert5: Assignable<A, [{ key: "" }]> = true as const;
+  const assert6: A = [{ key: "" }];
+}
+
+{
+  // optional parameter transform of a required input is equivalent to exactly 1 parameter.
+  type A = [...OptionalParameter<RequiredInput>];
+  type B = [RequiredInput];
+
+  const assert1: Exact<A, B> = true as const;
+  const assert2: Assignable<A, []> = false as const;
+  const assert3: Assignable<A, [{ key: "" }]> = true as const;
+  const assert4: A = [{ key: "" }];
+}