ng_form.ts

export const formDirectiveProvider: any = {
  provide: ControlContainer,
  useExisting: forwardRef(() => NgForm)
};

...

@Directive({
  selector: 'form:not([ngNoForm]):not([formGroup]),ngForm,[ngForm]',
  providers: [formDirectiveProvider],
  ...
  exportAs: 'ngForm'
})
export class NgForm extends ControlContainer implements Form {
  ...