fix(forms): Allow NonNullableFormBuilder to be injected. (#45904)

Based on early feedback, calling `fb.nonNullable.group(...)` continues to be clunky for a form with many such groups. Allowing `NonNullableFormBuilder` to be directly injected enables the following:

```
constructor(private fb: NonNullableFormBuilder) {}
```

PR Close #45904

goldens/public-api/forms/index.md

@@ -706,12 +706,16 @@ export class NgSelectOption implements OnDestroy {
 }
 
 // @public
-export interface NonNullableFormBuilder {
-    array<T>(controls: Array<T>, validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null, asyncValidator?: AsyncValidatorFn | AsyncValidatorFn[] | null): FormArray<ɵElement<T, never>>;
-    control<T>(formState: T | FormControlState<T>, validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null, asyncValidator?: AsyncValidatorFn | AsyncValidatorFn[] | null): FormControl<T>;
-    group<T extends {}>(controls: T, options?: AbstractControlOptions | null): FormGroup<{
+export abstract class NonNullableFormBuilder {
+    abstract array<T>(controls: Array<T>, validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null, asyncValidator?: AsyncValidatorFn | AsyncValidatorFn[] | null): FormArray<ɵElement<T, never>>;
+    abstract control<T>(formState: T | FormControlState<T>, validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null, asyncValidator?: AsyncValidatorFn | AsyncValidatorFn[] | null): FormControl<T>;
+    abstract group<T extends {}>(controls: T, options?: AbstractControlOptions | null): FormGroup<{
         [K in keyof T]: ɵElement<T[K], never>;
     }>;
+    // (undocumented)
+    static ɵfac: i0.ɵɵFactoryDeclaration<NonNullableFormBuilder, never>;
+    // (undocumented)
+    static ɵprov: i0.ɵɵInjectableDeclaration<NonNullableFormBuilder>;
 }
 
 // @public

packages/forms/src/form_builder.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injectable} from '@angular/core';
+import {inject, Injectable, InjectionToken} from '@angular/core';
 
 import {AsyncValidatorFn, ValidatorFn} from './directives/validators';
 import {ReactiveFormsModule} from './form_providers';
@@ -315,13 +315,17 @@ export class FormBuilder {
  *
  * @publicApi
  */
-export interface NonNullableFormBuilder {
+@Injectable({
+  providedIn: ReactiveFormsModule,
+  useFactory: () => inject(FormBuilder).nonNullable,
+})
+export abstract class NonNullableFormBuilder {
   /**
    * Similar to {@see FormBuilder#group}, except any implicitly constructed `FormControl`
    * will be non-nullable (i.e. it will have `initialValueIsDefault` set to true). Note
    * that already-constructed controls will not be altered.
    */
-  group<T extends {}>(
+  abstract group<T extends {}>(
       controls: T,
       options?: AbstractControlOptions|null,
       ): FormGroup<{[K in keyof T]: ɵElement<T[K], never>}>;
@@ -331,15 +335,15 @@ export interface NonNullableFormBuilder {
    * will be non-nullable (i.e. it will have `initialValueIsDefault` set to true). Note
    * that already-constructed controls will not be altered.
    */
-  array<T>(
+  abstract array<T>(
       controls: Array<T>, validatorOrOpts?: ValidatorFn|ValidatorFn[]|AbstractControlOptions|null,
       asyncValidator?: AsyncValidatorFn|AsyncValidatorFn[]|null): FormArray<ɵElement<T, never>>;
 
   /**
    * Similar to {@see FormBuilder#control}, except this overridden version of `control` forces
    * `initialValueIsDefault` to be `true`, resulting in the control always being non-nullable.
    */
-  control<T>(
+  abstract control<T>(
       formState: T|FormControlState<T>,
       validatorOrOpts?: ValidatorFn|ValidatorFn[]|AbstractControlOptions|null,
       asyncValidator?: AsyncValidatorFn|AsyncValidatorFn[]|null): FormControl<T>;

packages/forms/test/form_builder_spec.ts

@@ -5,8 +5,9 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
-import {fakeAsync, tick} from '@angular/core/testing';
-import {FormBuilder, UntypedFormBuilder, Validators} from '@angular/forms';
+import {Component} from '@angular/core';
+import {fakeAsync, TestBed, tick} from '@angular/core/testing';
+import {FormBuilder, NonNullableFormBuilder, ReactiveFormsModule, UntypedFormBuilder, Validators} from '@angular/forms';
 import {of} from 'rxjs';
 
 (function() {
@@ -197,6 +198,60 @@ describe('Form Builder', () => {
     expect(a.errors).toEqual({'sync1': true, 'sync2': true});
   });
 
+  it('should be injectable', () => {
+    @Component({
+      standalone: true,
+      template: '...',
+    })
+    class MyComp {
+      constructor(public fb: FormBuilder) {}
+    }
+
+    TestBed.configureTestingModule({imports: [ReactiveFormsModule]});
+    const fixture = TestBed.createComponent(MyComp);
+
+    fixture.detectChanges();
+    expect(fixture.componentInstance.fb).toBeInstanceOf(FormBuilder);
+
+    const fc = fixture.componentInstance.fb.control('foo');
+    {
+      // Check the type of the value by assigning in each direction
+      type ValueType = string|null;
+      let t: ValueType = fc.value;
+      let t1 = fc.value;
+      t1 = null as unknown as ValueType;
+    }
+    fc.reset();
+    expect(fc.value).toEqual(null);
+  });
+
+  it('should be injectable as NonNullableFormBuilder', () => {
+    @Component({
+      standalone: true,
+      template: '...',
+    })
+    class MyComp {
+      constructor(public fb: NonNullableFormBuilder) {}
+    }
+
+    TestBed.configureTestingModule({imports: [ReactiveFormsModule]});
+
+    const fixture = TestBed.createComponent(MyComp);
+    fixture.detectChanges();
+    expect(fixture.componentInstance.fb).toBeInstanceOf(FormBuilder);
+
+    const fc = fixture.componentInstance.fb.control('foo');
+    {
+      // Check the type of the value by assigning in each direction
+      type ValueType = string;
+      let t: ValueType = fc.value;
+      let t1 = fc.value;
+      t1 = null as unknown as ValueType;
+    }
+    fc.reset();
+    expect(fc.value).toEqual('foo');
+  });
+
   describe('updateOn', () => {
     it('should default to on change', () => {
       const c = b.control('');