Fix setting default value for the TypeScript type (#117)

Author: yutak23

index.d.ts

@@ -11,7 +11,7 @@ Return a default type if input type is nil.
 @template T - Input type.
 @template U - Default type.
 */
-type WithDefault<T, U extends T> = T extends undefined | void | null ? U : T; // eslint-disable-line @typescript-eslint/ban-types
+type WithDefault<T, U> = T extends undefined | void | null ? U : T; // eslint-disable-line @typescript-eslint/ban-types
 
 // TODO: Replace this with https://github.com/sindresorhus/type-fest/blob/main/source/includes.d.ts
 /**
@@ -237,9 +237,9 @@ export default function camelcaseKeys<
 	options?: OptionsType
 ): CamelCaseKeys<
 T,
-WithDefault<OptionsType['deep'], false>,
-WithDefault<OptionsType['pascalCase'], false>,
-WithDefault<OptionsType['preserveConsecutiveUppercase'], false>,
-WithDefault<OptionsType['exclude'], EmptyTuple>,
-WithDefault<OptionsType['stopPaths'], EmptyTuple>
+WithDefault<'deep' extends keyof OptionsType ? OptionsType['deep'] : undefined, false>,
+WithDefault<'pascalCase' extends keyof OptionsType ? OptionsType['pascalCase'] : undefined, false>,
+WithDefault<'preserveConsecutiveUppercase' extends keyof OptionsType ? OptionsType['preserveConsecutiveUppercase'] : undefined, false>,
+WithDefault<'exclude' extends keyof OptionsType ? OptionsType['exclude'] : undefined, EmptyTuple>,
+WithDefault<'stopPaths' extends keyof OptionsType ? OptionsType['stopPaths'] : undefined, EmptyTuple>
 >;

index.test-d.ts

@@ -453,3 +453,21 @@ expectType<[
 		}),
 	]),
 );
+
+// Test for function with inferred type
+// eslint-disable-next-line @typescript-eslint/comma-dangle
+function camelcaseKeysDeep<
+	T extends Record<string, unknown> | readonly unknown[]
+>(response: T): CamelCaseKeys<T, true> {
+	return camelcaseKeys(response, {deep: true});
+}
+
+// eslint-disable-next-line @typescript-eslint/comma-dangle
+function camelcaseKeysPascalCase<
+	T extends Record<string, unknown> | readonly unknown[]
+>(response: T): CamelCaseKeys<T, false, true> {
+	return camelcaseKeys(response, {pascalCase: true});
+}
+
+expectType<{fooBar: {hogeHoge: string}}>(camelcaseKeysDeep({foo_bar: {hoge_hoge: 'hoge_hoge'}}));
+expectType<{FooBar: string}>(camelcaseKeysPascalCase({foo_bar: 'foo_bar'}));