load_all() error when using setOldClass() with inheritance

[famuvie]

I have prepared a MRE for this issue, as a dummy package that can be installed and tested quickly.

In summary, I need to register some S3 classes with inheritance structure using setOldClass().

#' @importFrom methods setOldClass
methods::setOldClass(c('derived_class', 'my_class'))

When I load_all() for the first time, everything goes fine. But for successive times, it fails as follows:

Loading testpkg
Error in methods::setOldClass(c("derived_class", "my_class")) (from testmethod.R#3) : 
  inconsistent old-style class information for “derived_class”; the class is defined but does not extend “my_class” and is not valid as the data part

Variations:

• The error does not occur if the class to be registered is single (i.e. setOldClass('my_class')).
• Incomprehensibly, it does not occur either if the derived class name is 'splines' (!?!?)
• Finally, the error still occurs if I register 'my_class' beforehand in a separate sentence.

> session_info()
Session info ---------------------------------------------------------------------------------------------------------------
 setting  value                       
 version  R version 3.2.0 (2015-04-16)
 system   x86_64, linux-gnu           
 ui       RStudio (0.99.441)          
 language (EN)                        
 collate  en_US.UTF-8                 
 tz       <NA>                        

Packages -------------------------------------------------------------------------------------------------------------------
 package    * version date       source        
 devtools     1.7.0   2015-01-17 CRAN (R 3.1.2)
 digest     * 0.6.8   2014-12-31 CRAN (R 3.2.0)
 memoise    * 0.2.1   2014-04-22 CRAN (R 3.1.2)
 Rcpp       * 0.11.5  2015-03-06 CRAN (R 3.1.3)
 roxygen2   * 4.1.1   2015-04-15 CRAN (R 3.2.0)
 rstudioapi * 0.3.1   2015-04-07 CRAN (R 3.2.0)
 stringr    * 0.6.2   2012-12-06 CRAN (R 3.1.2)

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