Given the following firewall config:

        main:
            pattern:   ^/
            stateless: true
            http_basic:
                provider: in_memory
            http_basic_ldap:
                provider:     ldap
                service:      Symfony\Component\Ldap\Ldap
                dn_string:    "%env(LDAP_USER_BASE_DN)%"
                query_string: "sAMAccountName={username}"

This config causes following deprecation notice:
Firewall "main" has no "provider" set but multiple providers exist. Using the first configured provider (in_memory) is deprecated since 3.4 and will throw an exception in 4.0, set the "provider" key on the firewall instead.
Because there is no "default" provider set as a child of the main key.

However each authenticator has a provider set explicitly, so there is no need for the "default" provider.

It would be nice, if this kind of config is allowed (no deprecation and working in 4.0)