fix(eslint-plugin): [prefer-readonly] correct issue with anonymus functions

packages/eslint-plugin/src/rules/prefer-readonly.ts

@@ -270,8 +270,12 @@ class ClassScope {
     classNode: ts.ClassLikeDeclaration,
     private readonly onlyInlineLambdas?: boolean,
   ) {
-    this.checker = checker;
-    this.classType = checker.getTypeAtLocation(classNode);
+    const classType = checker.getTypeAtLocation(classNode);
+    if (tsutils.isIntersectionType(classType)) {
+      this.classType = classType.types[0];
+    } else {
+      this.classType = classType;
+    }
 
     for (const member of classNode.members) {
       if (ts.isPropertyDeclaration(member)) {

packages/eslint-plugin/tests/rules/prefer-readonly.test.ts

@@ -292,6 +292,19 @@ class Foo {
     },
     {
       code: `
+function ClassWithName<TBase extends new (...args: any[]) => {}>(Base: TBase) {
+  return class extends Base {
+    private _name: string;
+
+    public test(value: string) {
+      this._name = value;
+    }
+  };
+}
+      `,
+    },
+    {
+      code: `
 class Foo {
   private value: Record<string, number> = {};
 
@@ -704,5 +717,30 @@ class Foo {
         }
       `,
     },
+    {
+      code: `
+function ClassWithName<TBase extends new (...args: any[]) => {}>(Base: TBase) {
+  return class extends Base {
+    private _name: string;
+  };
+}
+      `,
+      output: `
+function ClassWithName<TBase extends new (...args: any[]) => {}>(Base: TBase) {
+  return class extends Base {
+    private readonly _name: string;
+  };
+}
+      `,
+      errors: [
+        {
+          data: {
+            name: '_name',
+          },
+          line: 4,
+          messageId: 'preferReadonly',
+        },
+      ],
+    },
   ],
 });