From b3d80751788778cd471d58656150c7267d7cfd9e Mon Sep 17 00:00:00 2001
From: mdm317 <dbswh1008@gmail.com>
Date: Wed, 30 Jul 2025 00:09:03 +0900
Subject: [PATCH 1/2] fix: Add logic when returning a union type.

---
 .../src/rules/prefer-return-this-type.ts            |  9 +++++++++
 .../tests/rules/prefer-return-this-type.test.ts     | 13 +++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/packages/eslint-plugin/src/rules/prefer-return-this-type.ts b/packages/eslint-plugin/src/rules/prefer-return-this-type.ts
index 7d7b14044cfb..ef5733fd47ff 100644
--- a/packages/eslint-plugin/src/rules/prefer-return-this-type.ts
+++ b/packages/eslint-plugin/src/rules/prefer-return-this-type.ts
@@ -1,6 +1,7 @@
 import type { TSESTree } from '@typescript-eslint/utils';
 
 import { AST_NODE_TYPES } from '@typescript-eslint/utils';
+import { isUnionType } from 'ts-api-utils';
 import * as ts from 'typescript';
 
 import { createRule, forEachReturnStatement, getParserServices } from '../util';
@@ -116,6 +117,14 @@ export default createRule({
           return;
         }
 
+        if (
+          isUnionType(type) &&
+          type.types.some(typePart => typePart === classType)
+        ) {
+          hasReturnClassType = true;
+          return true;
+        }
+
         return;
       });
 
diff --git a/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts b/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
index 5b372350059f..6e6cab78bc45 100644
--- a/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
@@ -98,6 +98,19 @@ class Foo {
     `
 class Foo {
   f?: string;
+}
+    `,
+    `
+declare const valueUnion: BaseUnion | string;
+
+class BaseUnion {
+  f(): BaseUnion | string {
+    if (hidden) {
+      return this;
+    }
+
+    return valueUnion;
+  }
 }
     `,
   ],

From 602a349921fda458763c0bfbcebb76c56a439b14 Mon Sep 17 00:00:00 2001
From: mdm317 <dbswh1008@gmail.com>
Date: Wed, 30 Jul 2025 00:16:11 +0900
Subject: [PATCH 2/2] test: add test

---
 .../rules/prefer-return-this-type.test.ts     | 36 +++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts b/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
index 6e6cab78bc45..7d8748086bc0 100644
--- a/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-return-this-type.test.ts
@@ -408,6 +408,42 @@ class Animal<T> {
     console.log("I'm moving!");
     return this;
   }
+}
+      `,
+    },
+    {
+      code: `
+declare const valueUnion: number | string;
+
+class BaseUnion {
+  f(): BaseUnion | string {
+    if (hidden) {
+      return this;
+    }
+
+    return valueUnion;
+  }
+}
+      `,
+      errors: [
+        {
+          column: 8,
+          endColumn: 17,
+          line: 5,
+          messageId: 'useThisType',
+        },
+      ],
+      output: `
+declare const valueUnion: number | string;
+
+class BaseUnion {
+  f(): this | string {
+    if (hidden) {
+      return this;
+    }
+
+    return valueUnion;
+  }
 }
       `,
     },