Default label is ignored when a mapping is overwritten by a parameter

[yutannihilation]

Related to #5879

In the code below, the legend title is chosen differently depending on whether the same aes variable is specified by a parameter outside of aes(). In the second one, I think the title should be either

• <<< G >>>, because it's specified by the label attribute
• G, because the second layer is what the color scale is actually for

library(ggplot2)

d <- data.frame(x = 1, y = 2, g = 3)
d2 <- d
colnames(d2) <- c("X", "Y", "G")

attr(d[[1L]], "label") <- "<<< X >>>"
attr(d[[2L]], "label") <- "<<< Y >>>"
attr(d[[3L]], "label") <- "<<< G >>>"

ggplot() +
  geom_point(data = d, aes(x, y, colour = g)) +
  geom_point(data = d2, aes(X, Y, colour = G))

ggplot() +
  geom_point(data = d, aes(x, y, colour = g), colour = "grey") + 
  #                                           ^^^^^^^^^^^^^^^
  #                                             the difference
  geom_point(data = d2, aes(X, Y, colour = G))

^{Created on 2024-07-17 with reprex v2.1.0}

[teunbrand]

Thanks for posting this! I think the label should be G in this case.
In the first layer, colour = "grey" should rule out any colour-label coming from the first layer.
Thus, the second layer should decide and since d2$G has no label-attribute, it should be G.