From 6af7ca7d9fde230342d27ad5a75a09a58c022974 Mon Sep 17 00:00:00 2001
From: Josh Goldberg <joshuakgoldberg@outlook.com>
Date: Mon, 1 Nov 2021 00:59:29 -0400
Subject: [PATCH] fix(eslint-plugin): skip seenTypes for unions in
 isTypeReadonly (#4043)

---
 packages/eslint-plugin/src/util/isTypeReadonly.ts    |  6 ++++--
 .../rules/prefer-readonly-parameter-types.test.ts    | 12 +++++++++++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/packages/eslint-plugin/src/util/isTypeReadonly.ts b/packages/eslint-plugin/src/util/isTypeReadonly.ts
index 8b3efc27856..8231455c294 100644
--- a/packages/eslint-plugin/src/util/isTypeReadonly.ts
+++ b/packages/eslint-plugin/src/util/isTypeReadonly.ts
@@ -185,8 +185,10 @@ function isTypeReadonlyRecurser(
 
   if (isUnionType(type)) {
     // all types in the union must be readonly
-    const result = unionTypeParts(type).every(t =>
-      isTypeReadonlyRecurser(checker, t, options, seenTypes),
+    const result = unionTypeParts(type).every(
+      t =>
+        seenTypes.has(t) ||
+        isTypeReadonlyRecurser(checker, t, options, seenTypes),
     );
     const readonlyness = result ? Readonlyness.Readonly : Readonlyness.Mutable;
     return readonlyness;
diff --git a/packages/eslint-plugin/tests/rules/prefer-readonly-parameter-types.test.ts b/packages/eslint-plugin/tests/rules/prefer-readonly-parameter-types.test.ts
index 0a6ee103807..be50f096963 100644
--- a/packages/eslint-plugin/tests/rules/prefer-readonly-parameter-types.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-readonly-parameter-types.test.ts
@@ -280,13 +280,23 @@ ruleTester.run('prefer-readonly-parameter-types', rule, {
     `, // TSMethodSignature
 
     // https://github.com/typescript-eslint/typescript-eslint/issues/1665
-    // directly recursive
+    // directly recursive interface
     `
       interface Foo {
         readonly prop: Foo;
       }
       function foo(arg: Foo) {}
     `,
+
+    // https://github.com/typescript-eslint/typescript-eslint/issues/3396
+    // directly recursive union type
+    `
+      type MyType = string | readonly MyType[];
+
+      function foo<A extends MyType[]>(a: A): MyType[] {
+        return [];
+      }
+    `,
     // indirectly recursive
     `
       interface Foo {