fix(eslint-plugin): [prefer-return-this-type] don't report an error when returning a union type that includes a classType (#11432)

* fix: Add logic when returning a union type.

* test: add test

* Math.random()

---------

Co-authored-by: Josh Goldberg <git@joshuakgoldberg.com>

Author: mdm317

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;
       });
 

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 (Math.random()) {
+      return this;
+    }
+
+    return valueUnion;
+  }
 }
     `,
   ],
@@ -395,6 +408,42 @@ class Animal<T> {
     console.log("I'm moving!");
     return this;
   }
+}
+      `,
+    },
+    {
+      code: `
+declare const valueUnion: number | string;
+
+class BaseUnion {
+  f(): BaseUnion | string {
+    if (Math.random()) {
+      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 (Math.random()) {
+      return this;
+    }
+
+    return valueUnion;
+  }
 }
       `,
     },