Unclass expression labels to allow latex2exp::Tex() to be axis labels

[yutannihilation]

Fix #6638

Quote myself in #6622 (comment):

The error happens here. key is a data frame containing a latexexpression object in .label column.

ggplot2/R/guide-.R

Line 287 in af6f4e8

key <- vec_slice(key, is.finite(oob_censor_any(breaks, range)))

The problem is, the latexexpression object cannot be subset by vec_slice() because

x <- as.list(latex2exp::TeX("$\\alpha$"))
vctrs::obj_is_vector(x)
#> [1] FALSE

However, it works if the object doesn't have S3 classes.

vctrs::obj_is_vector(unclass(x))
#> [1] TRUE

This pull request fixes the error by simply removing the class. Honestly, I'm not fully sure if this doesn't break any existing usage that actually requires the class information in labels. But, I believe this does the right thing.

---

devtools::load_all("~/GitHub/ggplot2/")
#> ℹ Loading ggplot2
#> Overwriting method +(<ggplot2::ggplot>, <ANY>)
#> 
#> Overwriting method +(<ggplot2::theme>, <ANY>)
#> 
#> Overwriting method convert(<ggplot2::ggplot>, <list>)
library(latex2exp)
ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  scale_x_continuous(breaks = 3, labels = TeX("$\\alpha$"))

^{Created on 2025-09-24 with reprex v2.1.1}