Use overloads for .readonly()

src/index.ts

@@ -23,14 +23,7 @@ import {
   OverloadReturnTypes,
   OverloadsNarrowedByParameters,
 } from './overloads'
-import type {
-  StrictEqualUsingTSInternalIdenticalToOperator,
-  AValue,
-  MismatchArgs,
-  Extends,
-  IsNever,
-  SetReadonly,
-} from './utils'
+import type {StrictEqualUsingTSInternalIdenticalToOperator, AValue, MismatchArgs, Extends, SetReadonly} from './utils'
 
 export * from './branding' // backcompat, consider removing in next major version
 export * from './utils' // backcompat, consider removing in next major version
@@ -704,12 +697,51 @@ export interface BaseExpectTypeOf<Actual, Options extends {positive: boolean}> {
    *
    * @since 1.0.0
    */
-  readonly<PropertiesToMakeReadonly extends keyof Actual = never>(
-    propertiesToMakeReadonly?: PropertiesToMakeReadonly,
-  ): ExpectTypeOf<
-    IsNever<PropertiesToMakeReadonly> extends true ? Readonly<Actual> : SetReadonly<Actual, PropertiesToMakeReadonly>,
-    Options
-  >
+  readonly<PropertiesToMakeReadonly extends keyof Actual>(
+    propertiesToMakeReadonly: PropertiesToMakeReadonly,
+  ): ExpectTypeOf<SetReadonly<Actual, PropertiesToMakeReadonly>, Options>
+
+  /**
+   * Converts specified properties of an object to `readonly`.
+   * If no properties are specified, it defaults
+   * to making all properties `readonly`, similar to the native
+   * TypeScript {@linkcode Readonly} utility type.
+   *
+   * @example
+   * <caption>#### Make all properties `readonly` (default behavior)</caption>
+   *
+   * ```ts
+   * import { expectTypeOf } from 'expect-type'
+   *
+   * type Post = {
+   *   title: string
+   *   content: string
+   * }
+   *
+   * expectTypeOf<Post>().readonly().toEqualTypeOf<Readonly<Post>>()
+   * ```
+   *
+   * @example
+   * <caption>#### Make specific properties `readonly`</caption>
+   *
+   * ```ts
+   * import { expectTypeOf } from 'expect-type'
+   *
+   * type Post = {
+   *   title: string
+   *   content: string
+   * }
+   *
+   * expectTypeOf<Post>()
+   *   .readonly('title')
+   *   .toEqualTypeOf<{ readonly title: string; content: string }>()
+   * ```
+   *
+   * @returns the type with the specified properties made `readonly`. If no properties are specified, all properties will be made `readonly`, behaving like the TypeScript {@linkcode Readonly} utility.
+   *
+   * @since 1.0.0
+   */
+  readonly(): ExpectTypeOf<Readonly<Actual>, Options>
 
   /**
    * Extracts a certain function argument with `.parameter(number)` call to