feat(forms): formControlName also accepts a number (#30606)

This commit relaxes the type of the `formControlName` input to accept both a `string` and a `number`.

Currently, when using a `FormArray`, most templates look like:

```
<div formArrayName="tags">
  <div *ngFor="let tag of tagsArray.controls; index as i">
    <input [formControlName]="i">
  </div>
</div>
```

Here `formControlName` receives a number whereas its input type is a string.

This is fine for VE and `fullTemplateTypeCheck`, but not for Ivy which does a more thorough type checking on inputs with `fullTemplateTypeCheck` enabled and throws `Type 'number' is not assignable to type 'string'`. It is fixable by using `formControlName="{{i}}"` but you have to know the difference between `a="{{b}}"` and `[a]="b"` and change it all over the application codebase. This commit allows the existing code to still type-check.

PR Close #30606

packages/forms/src/directives/ng_control.ts

@@ -36,7 +36,7 @@ export abstract class NgControl extends AbstractControlDirective {
    * @description
    * The name for the control
    */
-  name: string|null = null;
+  name: string|number|null = null;
 
   /**
    * @description

packages/forms/src/directives/reactive_directives/form_control_name.ts

@@ -145,9 +145,13 @@ export class FormControlName extends NgControl implements OnChanges, OnDestroy {
    * @description
    * Tracks the name of the `FormControl` bound to the directive. The name corresponds
    * to a key in the parent `FormGroup` or `FormArray`.
+   * Accepts a name as a string or a number.
+   * The name in the form of a string is useful for individual forms,
+   * while the numerical form allows for form controls to be bound
+   * to indices when iterating over controls in a `FormArray`.
    */
   // TODO(issue/24571): remove '!'.
-  @Input('formControlName') name !: string;
+  @Input('formControlName') name !: string | number | null;
 
   /**
    * @description
@@ -238,7 +242,9 @@ export class FormControlName extends NgControl implements OnChanges, OnDestroy {
    * Returns an array that represents the path from the top-level form to this control.
    * Each index is the string name of the control on that level.
    */
-  get path(): string[] { return controlPath(this.name, this._parent !); }
+  get path(): string[] {
+    return controlPath(this.name == null ? this.name : this.name.toString(), this._parent !);
+  }
 
   /**
    * @description

packages/forms/src/directives/shared.ts

@@ -28,8 +28,8 @@ import {SelectMultipleControlValueAccessor} from './select_multiple_control_valu
 import {AsyncValidator, AsyncValidatorFn, Validator, ValidatorFn} from './validators';
 
 
-export function controlPath(name: string, parent: ControlContainer): string[] {
+export function controlPath(name: string | null, parent: ControlContainer): string[] {
-  return [...parent.path !, name];
+  return [...parent.path !, name !];
 }
 
 export function setUpControl(control: FormControl, dir: NgControl): void {

tools/public_api_guard/forms/forms.d.ts

@@ -255,7 +255,7 @@ export declare class FormControlName extends NgControl implements OnChanges, OnD
     readonly formDirective: any;
     isDisabled: boolean;
     /** @deprecated */ model: any;
-    name: string;
+    name: string | number | null;
     readonly path: string[];
     /** @deprecated */ update: EventEmitter<any>;
     readonly validator: ValidatorFn | null;
@@ -353,7 +353,7 @@ export declare const NG_VALUE_ACCESSOR: InjectionToken<ControlValueAccessor>;
 
 export declare abstract class NgControl extends AbstractControlDirective {
     readonly asyncValidator: AsyncValidatorFn | null;
-    name: string | null;
+    name: string | number | null;
     readonly validator: ValidatorFn | null;
     valueAccessor: ControlValueAccessor | null;
     abstract viewToModelUpdate(newValue: any): void;