From c819880b91bf5ff1c5aa71afd314ae7cae241065 Mon Sep 17 00:00:00 2001
From: Matthieu Riegler <kyro38@gmail.com>
Date: Sun, 16 Aug 2026 12:26:25 +0200
Subject: [PATCH] fix(forms): report forbidden 2way bindings on when
 `FormField` is applied

We were already reporting regular bindings but not 2way.

fixes #70219
---
 .../src/ngtsc/typecheck/src/oob.ts            |  2 ++
 .../test/ngtsc/signal_forms_spec.ts           | 25 +++++++++++++++++++
 .../src/typecheck/ops/signal_forms.ts         |  2 ++
 3 files changed, 29 insertions(+)

diff --git a/packages/compiler-cli/src/ngtsc/typecheck/src/oob.ts b/packages/compiler-cli/src/ngtsc/typecheck/src/oob.ts
index 88360173b93..d06e1fea9ab 100644
--- a/packages/compiler-cli/src/ngtsc/typecheck/src/oob.ts
+++ b/packages/compiler-cli/src/ngtsc/typecheck/src/oob.ts
@@ -625,6 +625,8 @@ export class OutOfBandDiagnosticRecorderImpl implements OutOfBandDiagnosticRecor
         name = `[${node.name}]`;
       } else if (node.type === BindingType.Attribute) {
         name = `[attr.${node.name}]`;
+      } else if (node.type === BindingType.TwoWay) {
+        name = `[(${node.name})]`;
       } else {
         // We shouldn't hit this, but we have this logic as a fallback.
         name = node.name;
diff --git a/packages/compiler-cli/test/ngtsc/signal_forms_spec.ts b/packages/compiler-cli/test/ngtsc/signal_forms_spec.ts
index c7f2d3c12f3..0b86c0be83b 100644
--- a/packages/compiler-cli/test/ngtsc/signal_forms_spec.ts
+++ b/packages/compiler-cli/test/ngtsc/signal_forms_spec.ts
@@ -480,6 +480,31 @@ runInEachFileSystem(() => {
       );
     });
 
+    it('should report unsupported two-way bindings on a field', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<input type="number" [formField]="f" [(max)]="maxLength"/>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal(''));
+            maxLength = 10;
+          }
+        `,
+      );
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(extractMessage(diags[0])).toBe(
+        `Binding to '[(max)]' is not allowed on nodes using the '[formField]' directive`,
+      );
+    });
+
     it('should report unsupported property bindings on a field with a custom control', () => {
       env.write(
         'test.ts',
diff --git a/packages/compiler/src/typecheck/ops/signal_forms.ts b/packages/compiler/src/typecheck/ops/signal_forms.ts
index 610816c5121..39ce12f8ba3 100644
--- a/packages/compiler/src/typecheck/ops/signal_forms.ts
+++ b/packages/compiler/src/typecheck/ops/signal_forms.ts
@@ -429,6 +429,8 @@ export function checkUnsupportedFieldBindings(
       unsupportedBindingFields.has(input.name.toLowerCase())
     ) {
       tcb.oobRecorder.formFieldUnsupportedBinding(tcb.id, input);
+    } else if (input.type === BindingType.TwoWay && unsupportedBindingFields.has(input.name)) {
+      tcb.oobRecorder.formFieldUnsupportedBinding(tcb.id, input);
     }
   }