I think it works! Need to clean code comments.

Author: microly

R/geom-sf.R

@@ -197,11 +197,12 @@ geom_sf <- function(mapping = aes(), data = NULL, stat = "sf",
       mapping = mapping,
       stat = stat,
       position = position,
-      show.legend = if (is.character(show.legend)) TRUE else show.legend,
+      #show.legend = if (is.character(show.legend)) TRUE else show.legend,
+      show.legend = show.legend,
       inherit.aes = inherit.aes,
       params = list(
         na.rm = na.rm,
-        legend = if (is.character(show.legend)) show.legend else "polygon",
+        #legend = if (is.character(show.legend)) show.legend else "polygon",
         ...
       )
     ),

R/layer-sf.R

@@ -40,13 +40,15 @@ LayerSf <- ggproto("LayerSf", Layer,
     if (is.na(self$show.legend) || isTRUE(self$show.legend)) {
       if (is_sf(data)) {
         geometry_type <- sf_geometry_type(data)
-        if (geometry_type %in% c("POINT", "MULTIPOINT"))
+        if (sf_types[geometry_type] == "point")
           self$geom_params$legend <- "point"
-        else if (geometry_type %in% c("LINESTRING", "MULTILINESTRING",
-                                               "CIRCULARSTRING", "COMPOUNDCURVE",
-                                               "CURVE", "MULTICURVE"))
+        else if (sf_types[geometry_type] == "line")
           self$geom_params$legend <- "line"
+        else self$geom_params$legend <- "polygon"
       }
+    } else if (is.character(self$show.legend)) {
+      self$geom_params$legend = self$show.legend
+      self$show.legend = TRUE
     }
     data
   }
@@ -78,6 +80,6 @@ scale_type.sfc <- function(x) "identity"
 # helper function to determine the geometry type of sf object
 sf_geometry_type <- function(sf) {
   geometry_type <- unique(as.character(sf::st_geometry_type(sf)))
-  if (length(geometry_type) != 1)  geometry_type <- "other"
+  if (length(geometry_type) != 1)  geometry_type <- "GEOMETRY"
   geometry_type
 }

R/layer.r

@@ -81,8 +81,12 @@ layer <- function(geom = NULL, stat = NULL,
     params$show_guide <- NULL
   }
   if (!is.logical(show.legend)) {
-    warning("`show.legend` must be a logical vector.", call. = FALSE)
-    show.legend <- FALSE
+    # a charater vector have to be allowed for LayerSf$show.legend
+    # in order to set its legend type.
+    if (!inherits(layer_class, "LayerSf")) {
+      warning("`show.legend` must be a logical vector.", call. = FALSE)
+      show.legend <- FALSE
+    }
   }
 
   # we validate mapping before data because in geoms and stats

R/stat-sf.R

@@ -28,11 +28,12 @@ stat_sf <- function(mapping = NULL, data = NULL, geom = "rect",
     mapping = mapping,
     geom = geom,
     position = position,
-    show.legend = if (is.character(show.legend)) TRUE else show.legend,
+    #show.legend = if (is.character(show.legend)) TRUE else show.legend,
+    show.legend = show.legend,
     inherit.aes = inherit.aes,
     params = list(
       na.rm = na.rm,
-      legend = if (is.character(show.legend)) show.legend else "polygon",
+      #legend = if (is.character(show.legend)) show.legend else "polygon",
       ...
     )
   )