Fix label names when internalSignature has optional params

ark/repo/scratch.ts

@@ -10,3 +10,18 @@ const darkColorLiteral = type.enumerated(
 )
 
 const darkColorRegex = type(`/^(${join(options, "|")})Dark$/`)
+
+const literalValues = <t>(t: type.Any<t>): t[] => {
+	const literals: unknown[] = []
+	for (const branch of t.internal.branches) {
+		if (branch.hasKind("unit")) {
+			literals.push(branch.unit)
+		}
+	}
+	return literals as never
+}
+
+const palette = type.enumerated("red", "blue")
+
+// ["blue", "red"]
+literalValues(palette)

ark/type/__tests__/fn.test.ts

@@ -402,6 +402,22 @@ contextualize(() => {
 		})
 	})
 
+	// https://github.com/arktypeio/arktype/issues/1614
+	it("labels an implementation that annotates its optional parameter", () => {
+		// labels come from the implementation's parameter names, which are only
+		// cosmetic. contextually inferred parameters already labelled correctly;
+		// it is annotating the optional one that stopped applyElementLabels from
+		// matching, dropping every label rather than just that one:
+		// (args_0?: number | undefined) => number | undefined
+		const f = type.fn("number?")((n?: number) => n)
+
+		attest(f).type.toString.snap(`TypedFn<
+	(n?: number | undefined) => number | undefined,
+	{},
+	{}
+>`)
+	})
+
 	describe("scoped", () => {
 		it("scoped param and return", () => {
 			const $ = type.scope({

ark/type/fn.ts

@@ -38,7 +38,7 @@ export type BaseFnParser<$ = {}> = <
 	externalSignature extends Fn = (
 		...args: applyElementLabels<
 			distill.In<paramsT>,
-			Parameters<internalSignature>
+			Required<Parameters<internalSignature>>
 		>
 	) => args extends readonly [...unknown[], ":", unknown] ? distill.Out<returnT>
 	:	ReturnType<internalSignature>