fix: check and complaint if @@index references fields fro m delegate base

packages/language/src/validators/attribute-application-validator.ts

@@ -290,11 +290,13 @@ export default class AttributeApplicationValidator implements AstValidator<Attri
         }
     }
 
-    @check('@@unique')
     @check('@@id')
+    @check('@@index')
+    @check('@@unique')
     // @ts-expect-error
     private _checkUnique(attr: AttributeApplication, accept: ValidationAcceptor) {
         const fields = attr.args[0]?.value;
+        const attrName = attr.decl.ref?.name;
         if (!fields) {
             accept('error', `expects an array of field references`, {
                 node: attr.args[0]!,
@@ -303,7 +305,7 @@ export default class AttributeApplicationValidator implements AstValidator<Attri
         }
         if (isArrayExpr(fields)) {
             if (fields.items.length === 0) {
-                accept('error', `\`@@unique\` expects at least one field reference`, { node: fields });
+                accept('error', `\`${attrName}\` expects at least one field reference`, { node: fields });
                 return;
             }
             fields.items.forEach((item) => {
@@ -321,7 +323,7 @@ export default class AttributeApplicationValidator implements AstValidator<Attri
                 }
 
                 if (item.target.ref.$container !== attr.$container && isDelegateModel(item.target.ref.$container)) {
-                    accept('error', `Cannot use fields inherited from a polymorphic base model in \`@@unique\``, {
+                    accept('error', `Cannot use fields inherited from a polymorphic base model in \`${attrName}\``, {
                         node: item,
                     });
                 }

tests/regression/test/issue-283.test.ts

@@ -0,0 +1,23 @@
+import { loadSchemaWithError } from '@zenstackhq/testtools';
+import { describe, it } from 'vitest';
+
+describe('Regression for issue #283', () => {
+    it('verifies issue 283', async () => {
+        await loadSchemaWithError(
+            `
+model Base {
+    id Int @id @default(autoincrement())
+    x Int
+    type String
+    @@delegate(type)
+}
+
+model Sub extends Base {
+    y Int
+    @@index([x, y])
+}
+`,
+            'Cannot use fields inherited from a polymorphic base model',
+        );
+    });
+});