@exportS3Method fails when package not imported or already in global namespace

[NikNakk]

With the move to R 4.0.0, I've come across an issue with one of my packages where I wanted to include S3 methods without introducing a dependency on the package that defines the generic. I'd previously done this by @exporting the function without an @importFrom, which meant that an export rather than S3method directive was written to NAMESPACE. However, now that R 4.0.0 doesn't look in the global namespace for S3 methods, this no longer worked.

As per duckmayr's answer to my StackOverflow question, the answer comes from using @exportS3Method package::generic_function without an @importFrom. However, this fails unless the relevant package has already been loaded into the global namespace before a call to roxygen2::roxygenise. Within RStudio, this means that the Document option in the Build pane fails. Is there any chance of modifying the check that leads to the error @exportS3Method and @exportS3Method generic must be used with an S3 method' and therefore fails to include the relevant lines in NAMESPACE?

[hadley]

Can you please provide a reprex following the advice in https://github.com/r-lib/roxygen2/blob/master/.github/SUPPORT.md#making-a-reprex?

[NikNakk]

Thanks @hadley. I couldn't make this work using roc_proc_text so I've created a minimal package which is here on GitHub.

There is only a single R file which has the contents:

#' @exportS3Method knitr::knit_print
knit_print.s3example <- function(x, ...) {
}

There are three workarounds. Firstly, if the second (optional) parameter is provided to @exportS3Method, i.e. @exportS3Method knitr::knit_print s3example, then it seems to work ok. Secondly, if the package is in the global namespace and devtools::document() is called from within that R session (rather than through the RStudio build tab), then it works fine. Thirdly, if the package is added to the 'Imports' section of DESCRIPTION and the `@importFrom' directive is used, then it also works (but in the process creates a dependency on the package providing the generic).

It may be that the best option for now is to direct package authors to use the two parameter form of @exportS3Method when they don't want to import the generic package. However, I would have thought it should be possible for roxygen2 to work out the correct S3method directive for NAMESPACE without needing to have the package loaded.

[hadley]

(Note to self: when fixing this, also make sure to update namespace vignette with use of exportS3method)

[hadley]

Minimal reprex:

library(roxygen2)
roc_proc_text(namespace_roclet(), "
  #' @exportS3Method knitr::knit_print
  knit_print.s3example <- function(x, ...) {
  }
")
#> Warning: [<text>:2] @exportS3Method `@exportS3Method` and `@exportS3Method
#> generic` must be used with an S3 method
#> character(0)

^{Created on 2022-04-06 by the reprex package (v2.0.1)}

Need to use the generic in the tag to find the class name from the generic.