Update S3 method advice

[hadley]

Update S3 method advice

[jennybc]

Specifically, we're talking about this passage:

r-pkgs/dependencies-in-practice.Rmd

Lines 733 to 782 in 3b8ffd3

	The last and trickiest case is when your package offers a method for a generic "owned" by a package you've listed in `Suggests`.
	The basic idea is that you want to register the availability of your S3 method conditionally, when your package is being loaded.
	If the suggested package is present, your S3 method should be registered, but otherwise it should not.
	We'll illustrate this with an example.
	Within the tidyverse, the glue package is managed as a low-level package that should have minimal dependencies (@sec-dependencies-tidyverse).
	Glue functions generally return a character vector that also has the `"glue"` S3 class.
	```{r}
	library(glue)
	name <- "Betty"
	(ret <- glue('My name is {name}.'))
	class(ret)
	```
	The motivation for this is that it allows glue to offer special methods for `print()`, the `+` operator, and subsetting via `[` and `[[`.
	One downside, though, is that this class attribute complicates string comparisons:
	```{r}
	identical(ret, "My name is Betty.")
	all.equal(ret, "My name is Betty.")
	```
	Therefore, for testing, it is helpful if glue offers a method for `testthat::compare()`, which explains why this expectation succeeds:
	```{r}
	testthat::expect_equal(ret, "My name is Betty.")
	```
	But glue can't list testthat in `Imports`!
	It must go in `Suggests`.
	The solution is to register the method conditionally when glue is loaded.
	Here is a redacted version of glue's `.onLoad()` function, where you'll see that it conditionally registers some other methods as well:
	```{r}
	#| eval: false
	.onLoad <- function(...) {
	s3_register("testthat::compare", "glue")
	s3_register("waldo::compare_proxy", "glue")
	s3_register("vctrs::vec_ptype2", "glue.glue")
	...
	invisible()
	}
	```
	The `s3_register()` function comes from the vctrs package.
	If you don't have an organic need to depend on vctrs, it is common (and encouraged) to simply inline [the `s3_register()` source](https://github.com/r-lib/vctrs/blob/main/R/register-s3.R) into your own package.
	You can't always copy code from other people's packages and paste it into yours, but you can in this case.
	This usage is specifically allowed by the license of the source code of `s3_register()`.
	This provides a great segue into @sec-license, which is all about licensing.