This PR aims to fix a problem I ran into during 2 previous PRs (#5167 and #4879). Instead of the pragmatic hotfixes in those PRs, this one addresses the root cause.

The problem is that calc_element() could not be used to calculate any part of the theme that had dual <character/numeric> inheritance. Under the hood, an exception was made in the validation to allow character elements to also be numeric. This was the case for legend.position that could be either "top" or c(0, 1), for example. This is all fine, however, calc_element() did not special-case the <character/numeric> dual inheritance, and would throw an error if you provide a numeric vector (see reprex).

The solution in this PR is to properly give those elements that should support dual inheritance a dual class. By doing this, we no longer need to special-case in the validator, and calc_element() gives the right answers. Also, we can let aspect.ratio inherit from <numeric> now, whereas previously it would throw uninformative errors from later in the plotting process.

A demonstration of before this PR (backtraces omitted):

library(ggplot2)

theme <- theme_get() +
  theme(
    aspect.ratio = "foobar",
    plot.tag.position = c(0, 1)
  )

calc_element("aspect.ratio", theme)
#> [1] "foobar"
calc_element("plot.tag.position", theme)
#> Error:
#> ! Theme element `plot.tag.position` must have class <character>

Whereas now:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

calc_element("aspect.ratio", theme)
#> Error:
#> ! Theme element `aspect.ratio` must have class <numeric>

calc_element("plot.tag.position", theme)
#> [1] 0 1

^{Created on 2023-02-02 with reprex v2.0.2}