class JsonRequest extends Illuminate\Foundation\Http\FormRequest
{
 public function rules()
 {
  $method = $this->method();
  
  assert(in_array($method, [static::METHOD_POST, static::METHOD_PUT, static::METHOD_PATCH], true));
  
  $controller = $this->route()->getController();
  $rules  = $controller::RULES;

  return ($rules[$this->method()] ?? []) + ($rules['*'] ?? []);
 }

 public function authorize()
 {
  return true;
 }
}

use Illuminate\Http\Request;

final class AccountController extends \App\Http\Controllers\Controller
{
 public const RULES = [
  Request::METHOD_POST => [
   'bank_account' => 'required_if:type,bank',
   'loan_account' => 'required_if:type,loan',
  ],
  Request::METHOD_PUT => [
   // ...
  ],
  '*' => [
   // ...
  ],
 ];
}

{
 "name": "lx1036",
 "type": "loan",
 "bank_account": {
  "source": "bank",
 }
}

{
 "name": "lx1036",
 "type": "bank",
 "loan_account": {
  "source": "loan",
 }
}

 public function boot()
 {
  // \Illuminate\Foundation\Http\FormRequest use 了 ValidatesWhenResolvedTrait，extends 了 \Illuminate\Contracts\Validation\ValidatesWhenResolved
  $this->app->afterResolving(ValidatesWhenResolved::class, function ($resolved) {
   $resolved->validate();
  });

  // ...
 }