test: add test for UnionKeys

packages/type-plus/src/union_keys.spec.ts

@@ -0,0 +1,28 @@
+import { it } from '@jest/globals'
+import { testType, type UnionKeys } from './index.js'
+
+it('gets the keys of an object', () => {
+	testType.equal<UnionKeys<{ a: 1, b: 2 }>, 'a' | 'b'>(true)
+})
+
+
+it('returns known keys', () => {
+	type Foo = {
+		a: string
+		b: string
+	}
+
+	// @ts-ignore
+	function foo<T>(input: UnionKeys<Foo & T>): void {
+		input = 'a'
+		input = 'b'
+		// @ts-expect-error
+		input = 'c'
+
+		let r: keyof (Foo & T)
+		r = 'a'
+		r = 'b'
+		// @ts-expect-error
+		r = 'c'
+	}
+})