Calculation fails when global aesthetic values contain something not supported by geom

[atusy]

See example below.
It works if removing shape = legend from aes() or gghighlight::gghighlight(legend == 'a') from layers.

reprex

library(dplyr, warn.conflicts = FALSE)
library(ggplot2)
data.frame(
    X = c(1, 2, 1, 2),
    Y = c(1, 1, 2, 2),
    legend = c('a', 'a', 'b', 'b')
) %>%
    ggplot() +
    aes(X, Y, color = legend, shape = legend) +
    geom_line() +
    gghighlight::gghighlight(legend == 'a')
#> Warning: Tried to calculate with group_by(), but the calculation failed.
#> Falling back to ungrouped filter operation...
#> label_key: legend
#> Don't know how to automatically pick scale for object of type function. Defaulting to continuous.
#> Error: A continuous variable can not be mapped to shape

^{Created on 2020-07-25 by the reprex package (v0.3.0)}

[atusy]

Let me add a background.

I wanted to add geom_point(), which uses the shape.
A workaround is to map shape within geom_point.

[Hiroaki Yutani]

Confirmed, thanks.