title: S4 exports specified in 'NAMESPACE' but not defined in package
body: <p>I just finished some minor updates to an R package I've had on CRAN for a few years (haven't had to update in a while).  My package passes Check on my local machine and on winbuilder, but it just bounced back from CRAN with the message</p>
  
  <pre><code>checking whether package 'riverdist' can be installed ... WARNING
  Found the following significant warnings:
    Warning: S4 exports specified in 'NAMESPACE' but not defined in package 'riverdist'
  </code></pre>
  
  <p>I use roxygen2 to build my package NAMESPACE and don't have any S4 exports that I'm aware of.</p>
  
  <p>Perhaps this might be related to changes in R version 4.0.0?</p>
  
  <p>If anyone's encountered this error, I'd love to hear how you were able to resolve it.  Thanks!</p>