Toy

ext/pathname/lib/pathname.rb

@@ -305,8 +305,8 @@ def ascend
   # This method doesn't access the file system; it is pure string manipulation.
   #
   def +(other)
-    other = Pathname.new(other) unless Pathname === other
-    Pathname.new(plus(@path, other.to_s))
+    other = self.class.new(other) unless Pathname === other
+    self.class.new(plus(@path, other.to_s))
   end
 
   def plus(path1, path2) # -> path
@@ -476,9 +476,9 @@ def relative_path_from(base_directory)
     base_names.fill('..')
     relpath_names = base_names + dest_names
     if relpath_names.empty?
-      Pathname.new('.')
+      self.class.new('.')
     else
-      Pathname.new(File.join(*relpath_names))
+      self.class.new(File.join(*relpath_names))
     end
   end
 end

test/pathname/test_pathname.rb

@@ -30,6 +30,9 @@ def self.defassert(name, result, *args)
   DOSISH_DRIVE_LETTER = File.dirname("A:") == "A:."
   DOSISH_UNC = File.dirname("//") == "//"
 
+  class SuperPathname < Pathname
+  end
+
   def cleanpath_aggressive(path)
     Pathname.new(path).cleanpath.to_s
   end
@@ -204,6 +207,10 @@ def plus(path1, path2) # -> path
 
   defassert(:plus, 'a//b/d//e', 'a//b/c', '../d//e')
 
+  define_assertion(:plus) {
+    assert_instance_of(SuperPathname, SuperPathname.new('a') + 'b')
+  }
+
   def test_parent
     assert_equal(Pathname("."), Pathname("a").parent)
   end
@@ -306,6 +313,10 @@ def self.defassert_raise(name, exc, *args)
   defassert_raise(:relative_path_from, ArgumentError, "a", "..")
   defassert_raise(:relative_path_from, ArgumentError, ".", "..")
 
+  define_assertion(:relative_path_from) {
+    assert_instance_of(SuperPathname, SuperPathname.new('a').relative_path_from(SuperPathname.new('b')))
+  }
+
   def with_tmpchdir(base=nil)
     Dir.mktmpdir(base) {|d|
       d = Pathname.new(d).realpath.to_s