compiler: better error messages for slicing invalid types

    
    Reviewed-on: https://go-review.googlesource.com/43457

From-SVN: r248082

gcc/go/gofrontend/MERGE

-d3997526dc0710e6b9b727a41184ce1770805794
+924a1fcc5658a5d66f5015921d7258e3a77519bc
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -10610,10 +10610,24 @@ Index_expression::do_lower(Gogo*, Named_object*, Statement_inserter*, int)
 	}
       return Expression::make_map_index(left, start, location);
     }
+  else if (cap != NULL)
+    {
+      go_error_at(location,
+		  "invalid 3-index slice of object that is not a slice");
+      return Expression::make_error(location);
+    }
+  else if (end != NULL)
+    {
+      go_error_at(location,
+		  ("attempt to slice object that is not "
+		   "array, slice, or string"));
+      return Expression::make_error(location);
+    }
   else
     {
       go_error_at(location,
-                  "attempt to index object which is not array, string, or map");
+                  ("attempt to index object that is not "
+		   "array, slice, string, or map"));
       return Expression::make_error(location);
     }
 }