The latest versions of AutoValue report an diagnostic:

A warning is now produced if a setX method in a Builder or its return type is marked @Nullable. Those methods always return the Builder instance, which is never null.

See also google/auto@e5b4b54

That diagnostic currently causes compilation errors in google-cloud-bigquery, e.g.:

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ConnectionSettings.java:225: error: [AutoValueBuilderSetterNullable] Setter methods always return the Builder so @Nullable is not appropriate
    public abstract Builder setUseReadAPI(Boolean useReadAPI);
                            ^