This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
_current,
  workInProgress,
  Component,
  nextProps,
  renderLanes,
) {
  if (_current !== null) {
    
    
    
    
    _current.alternate = null;