fix: DeepReadonly for a union with an array of itself (#306) (#310)

* fix: DeepReadonly for a union with an array of itself (#306)

* chore: fix styling according to prettier

* chore: fix handling of tuples

* chore: fix styling according to prettier

* chore: add changeset

.changeset/moody-oranges-care.md

@@ -0,0 +1,5 @@
+---
+"ts-essentials": patch
+---
+
+Fix `DeepReadonly` for a union with an array of itself

lib/types.ts

@@ -167,6 +167,10 @@ export type DeepReadonly<T> = T extends Builtin
   ? WeakSet<DeepReadonly<U>>
   : T extends Promise<infer U>
   ? Promise<DeepReadonly<U>>
+  : T extends AnyArray<infer U>
+  ? T extends IsTuple<T>
+    ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
+    : ReadonlyArray<DeepReadonly<U>>
   : T extends {}
   ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
   : IsUnknown<T> extends true

test/index.ts

@@ -315,6 +315,17 @@ function testDeepReadonly() {
     Assert<IsExact<DeepReadonly<ComplexNestedRequired>, ComplexNestedReadonly>>,
   ];
 
+  // Build-time test to ensure the fix for
+  // https://github.com/ts-essentials/ts-essentials/pull/310
+  // because IsExact<> is unable to text it.
+  {
+    type TestUnion = { value: string } | TestUnion[];
+    type ReadonlyTestUnion = { readonly value: string } | readonly ReadonlyTestUnion[];
+
+    const a: DeepReadonly<TestUnion> = [];
+    const b: ReadonlyTestUnion = a;
+  }
+
   // Build-time test to ensure the fix for
   // https://github.com/krzkaczor/ts-essentials/issues/17 remains in place.
   {