Allow login using username or email in Laravel 5.4

I think a simpler way is to just override the username method in LoginController:

public function username()
{
   $login = request()->input('login');
   $field = filter_var($login, FILTER_VALIDATE_EMAIL) ? 'email' : 'username';
   request()->merge([$field => $login]);
   return $field;
}

Follow instructions from this link: https://laravel.com/docs/5.4/authentication#authenticating-users

Then you can check for the user input like this

$username = $request->username; //the input field has name='username' in form

if(filter_var($username, FILTER_VALIDATE_EMAIL)) {
    //user sent their email 
    Auth::attempt(['email' => $username, 'password' => $password]);
} else {
    //they sent their username instead 
    Auth::attempt(['username' => $username, 'password' => $password]);
}

//was any of those correct ?
if ( Auth::check() ) {
    //send them where they are going 
    return redirect()->intended('dashboard');
}

//Nope, something wrong during authentication 
return redirect()->back()->withErrors([
    'credentials' => 'Please, check your credentials'
]);

This is just a sample. THere are countless various approaches you can take to accomplish the same.