Specify maximum package version

[ms609]

Is your feature request related to a problem? Please describe.
I'd like to use 'testthat' to test a package on R 3.6, but the latest version of 'testthat' has a subdependency ('evaluate') that requires R4.0

Describe the solution you'd like
Ideally, the use the most recent version of a package that is compatible with the version of R being used.

Less ideally, manually specify a maximum version of a package for each testing environment

Describe alternatives you've considered
I think a maximum version of a package can be specified in the DESCRIPTION file, but this doesn't help if I also want to fully test the package against the latest version of R.

Possibly as in #727 I could specify a URL to a particular archived version of the package on CRAN – but this would require me to set up and maintain a separate workflow for each version of R, rather than using a simple matrix.

[gaborcsardi]

Ideally, the use the most recent version of a package that is compatible with the version of R being used.

This is not possible with CRAN-like repositories.

Less ideally, manually specify a maximum version of a package for each testing environment

This is also not possible.

One solution that is certainly possible, is to duplicate the setup-r-dependencies step in your workflow, and specify a different testthat version for R 3.6.x.

Btw. setup-r-dependencies@v2 will also require R 4.0.0 at some point.

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