Failed to parse usage when S4 method has "function" argument type

[andrewmarx]

Not sure if this is a pkgdown or roxygen issue, but I have an S4 method where one of the input types is a "function" and I'm getting a warning "Failed to parse usage" when trying to build a new version of my site for this particular method. Here's a reproducible example:

#' testfun
#'
#' @param fun A function
#'
#' @export

setGeneric(
  "testfun",
  function(fun) {
    standardGeneric("testfun")
  })


#' @rdname testfun
setMethod(
  "testfun",
  signature(fun = "function"),
  function(fun) {
    return(0)
  })

I'm not sure when this issue started showing up; my last published version 6 months ago worked fine, but pkgdown and roxygen may not have been up to date at the time.