fix(forms): Improve a very commonly viewed error message by adding a guide

aio/content/errors/NG1203.md

@@ -0,0 +1,28 @@
+@name Missing value accessor
+@category forms
+@shortDescription You must register an `NgValueAccessor` with a custom form control
+
+@description
+For all custom form controls, you must register a value accessor.
+
+Here's an example of how to provide one:
+
+```typescript
+providers: [
+  {
+    provide: NG_VALUE_ACCESSOR,
+    useExisting: forwardRef(() => MyInputField),
+    multi: true,
+  }
+]
+```
+
+@debugging
+As described above, your control was expected to have a value accessor, but was missing one. However, there are many different reasons this can happen in practice. Here's a listing of some known problems leading to this error.
+
+1. If you **defined** a custom form control, did you remember to provide a value accessor?
+1. Did you put `ngModel` on an element with no value, or an **invalid element** (e.g. `<div [(ngModel)]="foo">`)?
+1. Are you using a custom form control declared inside an `NgModule`? if so, make sure you are **importing** the `NgModule`.
+1. Are you using `ngModel` with a third-party custom form control? Check whether that control provides a value accessor. If not, use **`ngDefaultControl`** on the control's element.
+1. Are you **testing** a custom form control? Be sure to configure your testbed to know about the control. You can do so with `Testbed.configureTestingModule`.
+1. Are you using **Nx and Module Federation** with Webpack? Your `webpack.config.js` may require [extra configuration](https://github.com/angular/angular/issues/43821#issuecomment-1054845431) to ensure the forms package is shared.

goldens/public-api/forms/errors.md

@@ -25,6 +25,8 @@ export const enum RuntimeErrorCode {
     // (undocumented)
     NAME_AND_FORM_CONTROL_NAME_MUST_MATCH = 1202,
     // (undocumented)
+    NG_MISSING_VALUE_ACCESSOR = -1203,
+    // (undocumented)
     NG_VALUE_ACCESSOR_NOT_PROVIDED = 1200,
     // (undocumented)
     NGMODEL_IN_FORM_GROUP = 1350,

goldens/size-tracking/integration-payloads.json

@@ -48,8 +48,8 @@
   "animations": {
     "uncompressed": {
       "runtime": 1070,
-      "main": 155920,
-      "polyfills": 33814
+      "main": 156355,
+      "polyfills": 33897
     }
   },
   "standalone-bootstrap": {

packages/forms/src/directives/shared.ts

@@ -65,7 +65,7 @@ export function setUpControl(
     callSetDisabledState: SetDisabledStateOption = setDisabledStateDefault): void {
   if (typeof ngDevMode === 'undefined' || ngDevMode) {
     if (!control) _throwError(dir, 'Cannot find control with');
-    if (!dir.valueAccessor) _throwError(dir, 'No value accessor for form control with');
+    if (!dir.valueAccessor) _throwMissingValueAccessorError(dir);
   }
 
   setUpValidators(control, dir);
@@ -322,6 +322,12 @@ function _describeControlLocation(dir: AbstractControlDirective): string {
   return 'unspecified name attribute';
 }
 
+function _throwMissingValueAccessorError(dir: AbstractControlDirective) {
+  const loc = _describeControlLocation(dir);
+  throw new RuntimeError(
+      RuntimeErrorCode.NG_MISSING_VALUE_ACCESSOR, `No value accessor for form control ${loc}.`);
+}
+
 function _throwInvalidValueAccessorError(dir: AbstractControlDirective) {
   const loc = _describeControlLocation(dir);
   throw new RuntimeError(

packages/forms/src/errors.ts

@@ -31,6 +31,7 @@ export const enum RuntimeErrorCode {
   NG_VALUE_ACCESSOR_NOT_PROVIDED = 1200,
   COMPAREWITH_NOT_A_FN = 1201,
   NAME_AND_FORM_CONTROL_NAME_MUST_MATCH = 1202,
+  NG_MISSING_VALUE_ACCESSOR = -1203,
 
   // Template-driven Forms errors (1350-1399)
   NGMODEL_IN_FORM_GROUP = 1350,

packages/forms/test/directives_spec.ts

@@ -184,7 +184,8 @@ class CustomValidatorDirective implements Validator {
           dir.name = 'login';
 
           expect(() => form.addControl(dir))
-              .toThrowError(new RegExp(`No value accessor for form control with name: 'login'`));
+              .toThrowError(new RegExp(
+                  `NG01203: No value accessor for form control name: 'login'. Find more at https://angular.io/errors/NG01203`));
         });
 
         it('should throw when no value accessor with path', () => {
@@ -195,7 +196,7 @@ class CustomValidatorDirective implements Validator {
 
           expect(() => form.addControl(dir))
               .toThrowError(new RegExp(
-                  `No value accessor for form control with path: 'passwords -> password'`));
+                  `NG01203: No value accessor for form control path: 'passwords -> password'. Find more at https://angular.io/errors/NG01203`));
         });
 
         it('should set up validators', fakeAsync(() => {
@@ -582,15 +583,16 @@ class CustomValidatorDirective implements Validator {
         namedDir.name = 'one';
 
         expect(() => namedDir.ngOnChanges({}))
-            .toThrowError(new RegExp(`No value accessor for form control with name: 'one'`));
+            .toThrowError(new RegExp(
+                `NG01203: No value accessor for form control name: 'one'. Find more at https://angular.io/errors/NG01203`));
       });
 
       it('should throw when no value accessor with unnamed control', () => {
         const unnamedDir = new NgModel(null!, null!, null!, null!);
 
         expect(() => unnamedDir.ngOnChanges({}))
-            .toThrowError(
-                new RegExp(`No value accessor for form control with unspecified name attribute`));
+            .toThrowError(new RegExp(
+                `NG01203: No value accessor for form control unspecified name attribute. Find more at https://angular.io/errors/NG01203`));
       });
 
       it('should set up validator', fakeAsync(() => {

packages/forms/test/reactive_integration_spec.ts

@@ -5093,7 +5093,9 @@ const ValueAccessorB = createControlValueAccessor('[cva-b]');
         const fixture = initTest(NoCVAComponent);
         expect(() => {
           fixture.detectChanges();
-        }).toThrowError('No value accessor for form control with name: \'control\'');
+        })
+            .toThrowError(
+                `NG01203: No value accessor for form control name: 'control'. Find more at https://angular.io/errors/NG01203`);
 
         // Making sure that cleanup between tests doesn't cause any issues
         // for not fully initialized controls.