Bug in guide training

[teunbrand]

In guide training this assumption is made:

ggplot2/R/guides-.R

Lines 416 to 417 in 25f54d9

	# A strong assumption here is that `scales` is parallel to the guides
	train = function(self, scales, direction, labels) {

I found out that you can violate this assumption as follows, where the first scale has 2 aesthetics and the second scale has 1 aesthetics.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
packageVersion("ggplot2") # current main branch
#> [1] '3.4.3.9000'

df <- data.frame(x = (1:3)/3, z = c("red", "blue", "green"))

ggplot(df) +
  aes(x, z, colour = z, fill = z, size = x) +
  geom_point() +
  scale_discrete_identity(aesthetics = c("colour", "fill")) +
  scale_size_identity()
#> Warning in mapply(FUN = f, ..., SIMPLIFY = FALSE): longer argument not a
#> multiple of length of shorter

^{Created on 2023-09-18 with reprex v2.0.2}

This is probably worth fixing and guaranteeing the assumption more.