当您将凭据与currentUser链接时，就已经登录了，不需要:

When you link the credential with the currentUser you are logged already, don't need for:

FIRAuth.auth()!.signIn

您保留匿名用户的UID.

You keep the UID of the anonymous user.

如果用户已经有一个帐户，在这种情况下，您需要:

If user already have an account, in this case you need to:

currentUser.delete()

然后使用提供的凭据登录

And then logIn with the credentials provided