fix(openapi-typescript-helpers): preserve built-in objects in Readable/Writable types for TS6

In TypeScript 6, `Date extends object` evaluates to `true` (changed from
TS5). This caused `Readable<T>` and `Writable<T>` to structurally expand
Date, RegExp, and function fields instead of preserving them.

Add a guard clause for built-in objects (Date, RegExp, functions) before
the `T extends object` branch in both type utilities.

Ref: #2723

Author: benjamin.eckstein

.changeset/widen-typescript-peer-dep.md

@@ -1,5 +1,9 @@
 ---
 "openapi-typescript": patch
+"openapi-typescript-helpers": patch
 ---
 
-feat(openapi-typescript): add TypeScript 6 support
+feat: add TypeScript 6 support
+
+- Widen peer dep to `^5.x || ^6.x`
+- Fix `Readable<T>` and `Writable<T>` to preserve built-in objects (Date, RegExp, functions) that now match `extends object` in TS6

packages/openapi-typescript-helpers/src/index.ts

@@ -223,9 +223,11 @@ export type Readable<T> =
       ? Readable<U>
       : T extends (infer E)[]
         ? Readable<E>[]
-        : T extends object
-          ? { [K in keyof T as NonNullable<T[K]> extends $Write<any> ? never : K]: Readable<T[K]> }
-          : T;
+        : T extends Date | RegExp | ((...args: never[]) => unknown)
+          ? T
+          : T extends object
+            ? { [K in keyof T as NonNullable<T[K]> extends $Write<any> ? never : K]: Readable<T[K]> }
+            : T;
 
 /**
  * Resolve type for writing (requests): strips $Read properties, unwraps $Write
@@ -240,8 +242,10 @@ export type Writable<T> =
       ? Writable<U>
       : T extends (infer E)[]
         ? Writable<E>[]
-        : T extends object
-          ? { [K in keyof T as NonNullable<T[K]> extends $Read<any> ? never : K]: Writable<T[K]> } & {
-              [K in keyof T as NonNullable<T[K]> extends $Read<any> ? K : never]?: never;
-            }
-          : T;
+        : T extends Date | RegExp | ((...args: never[]) => unknown)
+          ? T
+          : T extends object
+            ? { [K in keyof T as NonNullable<T[K]> extends $Read<any> ? never : K]: Writable<T[K]> } & {
+                [K in keyof T as NonNullable<T[K]> extends $Read<any> ? K : never]?: never;
+              }
+            : T;