@export generates a wrong NAMESPACE directive for S3 methods of generics imported from a different package

[klmr]

When exporting a new S3 method for a generic imported from another package, ‘roxygen2’ 7.3.1 generates an incorrect NAMESPACE directive; namely, it generates export() rather than S3method(). However, this only happens when both of the following conditions are fulfilled:

1. if the imported package is not attached, and
2. when there is no NAMESPACE file, i.e. only the first time round: when rerunning roxygen2::roxygenize(), it “corrects” the NAMESPACE file (intermittently I’ve also managed to produce this with an existing NAMESPACE file, but the exact circumstances are unclear to me, and I no longer manage the right circumstances to reproduce this).

Here’s a minimal example: a package without NAMESPACE file with a single implementation file R/foo.r (complete MWE at the end in the details):

#' @importFrom utils head
#' @export
head.foo = function (x, ...) {
  x$head
}

#' @export
foo = function () {
  structure(list(head = 'caput'), class = 'foo')
}

Now run the following code to generate the NAMESPACE file:

detach('package:utils')

roxygen2::roxygenize()
writeLines(readLines('NAMESPACE'))

roxygen2::roxygenize()
writeLines(readLines('NAMESPACE'))

ℹ Loading pkg
Writing 'NAMESPACE'
# Generated by roxygen2: do not edit by hand

export(foo)
export(head.foo)
importFrom(utils,head)
ℹ Loading pkg
Writing 'NAMESPACE'
# Generated by roxygen2: do not edit by hand

S3method(head,foo)
export(foo)
importFrom(utils,head)

---

Complete MWE

dir.create('pkg/R', recursive = TRUE)

desc = matrix(
  c(
    'pkg', 'Package', 'What the Package Does (Title Case)', '0.1.0',
    'person(\'First\', \'Last\', email = \'last@example.com\', role = c(\'aut\', \'cre\'))',
    'More about what it does.', 'GPL', 'utils', 'UTF-8'
  ),
  ncol = 9L,
  dimnames = list(
    NULL, c('Package', 'Type', 'Title', 'Version', 'Authors@R', 'Description', 'License', 'Imports', 'Encoding')
  )
)
write.dcf(desc, 'pkg/DESCRIPTION')

code = r"[#' @importFrom utils head
#' @export
head.foo = function (x, ...) {
  x$head
}

#' @export
foo = function () {
  structure(list(head = 'caput'), class = 'foo')
}
]"
writeLines(code, 'pkg/R/foo.r')

###

setwd('pkg')
detach('package:utils')

roxygen2::roxygenize()
writeLines(readLines('NAMESPACE'))

roxygen2::roxygenize()
writeLines(readLines('NAMESPACE'))