This snippet is not compilable:

typedef tls-client-verify {
    type enumeration {
        enum require {
        }
        enum optional {
        }
        enum none {
        }
    }
}

With error

[ERROR] some/path/target/generated-sources/mdsal-binding/org/opendaylight/yang/gen/v1/urn/opendaylight/jsonrpc/authsec/rev180607/TlsClientVerify.java:[74,24] cannot find symbol
[ERROR]   symbol:   method ofNullable(org.opendaylight.yang.gen.v1.urn.opendaylight.jsonrpc.authsec.rev180607.TlsClientVerify)
[ERROR]   location: variable Optional of type org.opendaylight.yang.gen.v1.urn.opendaylight.jsonrpc.authsec.rev180607.TlsClientVerify

Because of generated code:

Required(0, "required"),
Optional(1, "optional"),
None(2, "none")
;

public static Optional<TlsClientVerify> forName(String name) {
    return Optional.ofNullable(NAME_MAP.get(Objects.requireNonNull(name)));
}

Optional is referring to both enum value and java.util.Optional class.