Installation on R < 4.0 fails on `evaluate` now requiring R >= 4.0

[lachlansimpson]

Seen in zendesk ticket 106807. Mentioned here: r-lib/evaluate#173 (comment)

Solved by installation of older version using devtools

install.package('devtools')
require('devtools')
install_version("evaluate", version = "0.23", repos = "http://cran.us.r-project.org")
install.packages('testthat')

Ideally testthat installation candidate will install the version of evaluate required per the version of R being run, or testthat have a requirement of R>=4.0 as well.

[hadley]

That version of evaluate hasn't been released yet, so I'm surprised it's causing problems.

[hadley]

Oh it is in the released version; ooops.

[hadley]

Having read through that support log, I'm pretty confident the right fix is to ensuring we're requiring 4.0.0 for testthat, which I believe will happen naturally as part of our release process.

[hadley]

Given that it seems like 3.6 is going to be around for a while, we want to at least reserve the possibility of continuing test some packages on 3.6. That means we should formally declare 3.6 support in testthat and add it to our testing matrix.

[hadley]

After more time has elapsed it seems like requiring R 4.0.0 caused few problems (especially once we changed our GitHub action to install older versions if necessary).