A number of packages on CRAN use Suggests: casually.
They list other packages as "not required" in Suggests: -- as opposed to absolutely required via Imports: or the older Depends: -- yet do not test for their use in either examples or, more commonly, unit tests.
So e.g. the unit tests are bound to fail because, well, Suggests != Depends.
This has been accomodated for many years by all parties involved by treating Suggests as a Depends and installing unconditionally. As I understand it, CRAN appears to flip a switch to automatically install all Suggests from major repositories glossing over what I consider to be a packaging shortcoming. (As an aside, treatment of Additonal_repositories: is indeed optional; Brooke Anderson and I have a fine paper under review on this)
I spend a fair amount of time with reverse dependency ("revdep") checks of packages I maintain, and I will no longer accomodate these packages.
These revdep checks take long enough as it is, so I will now blacklist these packages that are guaranteed to fail when their "optional" dependencies are not present.
Writing R Extensions says in Section 1.1.3
All packages that are needed10 to successfully run R CMD check on the package must be listed in one of ‘Depends’ or ‘Suggests’ or ‘Imports’. Packages used to run examples or tests conditionally (e.g. via
if(require(pkgname)))should be listed in ‘Suggests’ or ‘Enhances’. (This allows checkers to ensure that all the packages needed for a complete check are installed.)In particular, packages providing “only” data for examples or vignettes should be listed in ‘Suggests’ rather than ‘Depends’ in order to make lean installations possible.
[...]
It used to be common practice to use require calls for packages listed in ‘Suggests’ in functions which used their functionality, but nowadays it is better to access such functionality via :: calls.
and continues in Section 1.1.3.1
Note that someone wanting to run the examples/tests/vignettes may not have a suggested package available (and it may not even be possible to install it for that platform). The recommendation used to be to make their use conditional via
if(require("pkgname"))): this is fine if that conditioning is done in examples/tests/vignettes.
I will now exercise my option to use 'lean installations' as discussed here. If you want your package included in tests I run, please make sure it tests successfully when only its required packages are present.