False positive with field nullability stored in local variable

In this case, a @Nullable field is stored to a local variable. When the field is used in a null condition an error occurs, but if the variable is used then it does not. It seems the data flow analysis does not realize they are the same.

error: [NullAway] returning @Nullable expression from method with @NonNull return type
    return (writer == null) ? CacheWriter.disabledWriter() : castedWriter;
    ^

<K1 extends K, V1 extends V> CacheWriter<K1, V1> getCacheWriter() {
  @SuppressWarnings("unchecked")
  CacheWriter<K1, V1> castedWriter = (CacheWriter<K1, V1>) writer;
  return (writer == null) ? CacheWriter.disabledWriter() : castedWriter;
}

Start by reproducing the Java example and tracing the nullability data-flow handling for the @Nullable writer field and its castedWriter local variable. Done means the analysis recognizes the field and local as equivalent in the shown condition and no longer reports the false positive; add or update a regression test for this case.