geom_sf seems to break when used with ggmap

[yeedle]

This might be an issue with ggmap or an issue with how geom_sf handles inherited aesthetic mappings. I'm not sure, so I'm submitting it here and to ggmap

library(sf)
library(ggmap)

nc_map <- get_map(c(-81, 35), zoom = 6) 
#> Map from URL : http://maps.googleapis.com/maps/api/staticmap?center=35,-81&zoom=6&size=640x640&scale=2&maptype=terrain&language=en-EN&sensor=false
ggmap(nc_map)

nc <- st_read(system.file("shape/nc.shp", package="sf"))
ggplot(nc) + geom_sf()

ggmap(nc_map) + geom_sf(data = nc)
#> Error in FUN(X[[i]], ...): object 'lon' not found

[hadley]

I think this is a design infelicity in ggmap. Really it should provide geom_ggmap() which works similar to geom_sf().