Improve same directory detection in FileUtils

Closes: ruby/ruby#1425

Author: presidentbeef

lib/fileutils.rb

@@ -1563,10 +1563,13 @@ def join(dir, base)
     else
       DIRECTORY_TERM = "(?=/|\\z)"
     end
-    SYSCASE = File::FNM_SYSCASE.nonzero? ? "-i" : ""
 
     def descendant_directory?(descendant, ascendant)
-      /\A(?#{SYSCASE}:#{Regexp.quote(ascendant)})#{DIRECTORY_TERM}/ =~ File.dirname(descendant)
+      if File::FNM_SYSCASE.nonzero?
+        File.expand_path(File.dirname(descendant)).casecmp(File.expand_path(ascendant)) == 0
+      else
+        File.expand_path(File.dirname(descendant)) == File.expand_path(ascendant)
+      end
     end
   end   # class Entry_
 

test/fileutils/test_fileutils.rb

@@ -381,6 +381,16 @@ def test_cp_r
     assert_same_file 'tmp/cpr_src/b', 'tmp/cpr_dest/b'
     assert_same_file 'tmp/cpr_src/c', 'tmp/cpr_dest/c'
     assert_directory 'tmp/cpr_dest/d'
+    assert_raise(ArgumentError) do
+      cp_r 'tmp/cpr_src', './tmp/cpr_src'
+    end
+    assert_raise(ArgumentError) do
+      cp_r './tmp/cpr_src', 'tmp/cpr_src'
+    end
+    assert_raise(ArgumentError) do
+      cp_r './tmp/cpr_src', File.expand_path('tmp/cpr_src')
+    end
+
     my_rm_rf 'tmp/cpr_src'
     my_rm_rf 'tmp/cpr_dest'