plumber fails to run when type is already defined in the global namespace

[LTLA]

System details

Output of sessioninfo::session_info()():

─ Session info ───────────────────────────────────────────────────────────────
 setting  value
 version  R version 4.0.3 Patched (2021-01-08 r79815)
 os       macOS Catalina 10.15.5
 system   x86_64, darwin19.5.0
 ui       X11
 language (EN)
 collate  en_US.UTF-8
 ctype    en_US.UTF-8
 tz       America/Los_Angeles
 date     2021-03-25

─ Packages ───────────────────────────────────────────────────────────────────
 package     * version     date       lib source
 assertthat    0.2.1       2019-03-21 [1] CRAN (R 4.0.3)
 callr         3.5.1       2020-10-13 [1] CRAN (R 4.0.3)
 cli           2.3.0       2021-01-31 [1] CRAN (R 4.0.3)
 crayon        1.4.1       2021-02-08 [1] CRAN (R 4.0.3)
 curl          4.3         2019-12-02 [1] CRAN (R 4.0.3)
 glue          1.4.2       2020-08-27 [1] CRAN (R 4.0.3)
 jsonlite      1.7.2       2020-12-09 [1] CRAN (R 4.0.3)
 later         1.1.0.1     2020-06-05 [1] CRAN (R 4.0.3)
 lifecycle     1.0.0       2021-02-15 [1] CRAN (R 4.0.3)
 magrittr      2.0.1       2020-11-17 [1] CRAN (R 4.0.3)
 pkgbuild      1.2.0       2020-12-15 [1] CRAN (R 4.0.3)
 plumber     * 1.0.0.90002 2021-03-25 [1] Github (rstudio/plumber@28bc58c)
 prettyunits   1.1.1       2020-01-24 [1] CRAN (R 4.0.3)
 processx      3.4.5       2020-11-30 [1] CRAN (R 4.0.3)
 promises      1.2.0.1     2021-02-11 [1] CRAN (R 4.0.3)
 ps            1.5.0       2020-12-05 [1] CRAN (R 4.0.3)
 R6            2.5.0       2020-10-28 [1] CRAN (R 4.0.3)
 Rcpp          1.0.6       2021-01-15 [1] CRAN (R 4.0.3)
 remotes       2.2.0       2020-07-21 [1] CRAN (R 4.0.3)
 rlang         0.4.10      2020-12-30 [1] CRAN (R 4.0.3)
 rprojroot     2.0.2       2020-11-15 [1] CRAN (R 4.0.3)
 rstudioapi    0.13        2020-11-12 [1] CRAN (R 4.0.3)
 sessioninfo   1.1.1       2018-11-05 [1] CRAN (R 4.0.3)
 stringi       1.5.3       2020-09-09 [1] CRAN (R 4.0.3)
 swagger       3.33.1      2020-10-02 [1] CRAN (R 4.0.3)
 webutils      1.1         2020-04-28 [1] CRAN (R 4.0.3)
 withr         2.4.1       2021-01-26 [1] CRAN (R 4.0.3)

[1] /Users/luna/Software/R/R-4-0-branch/library

Example application or steps to reproduce the problem

The plumber script is as follows:

library(BiocGenerics)

#' @get /
#' @serializer html
function() {
    "YAY"
}

Running this gives me (omitting BiocGenerics start-up messages):

p <- plumber::plumb("plumber.R")
p$run()
## Running plumber API at http://127.0.0.1:7515
## Error in nchar(ctype) :
##   cannot coerce type 'closure' to vector of type 'character'

Describe the problem in detail

BiocGenerics defines a type() function that is inadvertently found by plumber::responsesSpecification at

plumber/R/openapi-spec.R

Lines 68 to 71 in 28bc58c

	ctype <- tryCatch({
	get("type", envir =
	environment(get("serialize_fn", envir =
	environment(endpts$serializer))))},

This is unlikely to be an issue with BiocGenerics, but rather, a general problem that would occur with any package that defines a type() function. In fact, the same error can be obtained after omitting the library(BiocGenerics) by doing:

type <- function(x) x
p <- plumber::plumb("plumber.R")
p$run()