Raise an error when opening a file which does not exist in read-write ('r+') mode

lib/fakefs/file.rb

@@ -108,7 +108,7 @@ module FakeFS
       @file = FileSystem.find(path)
       @open = true
 
-      check_file_existence! if read_only?
+      check_file_existence! if !file_creation_mode?
     end
 
     def close
@@ -157,6 +157,14 @@ module FakeFS
       @mode == READ_ONLY
     end
 
+    def file_creation_modes
+      MODES - [READ_ONLY, READ_WRITE]
+    end
+
+    def file_creation_mode?
+      file_creation_modes.include?(@mode)
+    end
+
     def check_mode(mode)
       if !MODES.include?(mode)
         raise ArgumentError, "illegal access mode #{mode}"

test/fakefs_test.rb

@@ -120,6 +120,12 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_raises_error_when_cannot_find_file_in_read_write_mode
+    assert_raises(Errno::ENOENT) do
+      File.open("does_not_exist", "r+")
+    end
+  end
+
   def test_can_read_files_once_written
     path = '/path/to/file.txt'
     File.open(path, 'w') do |f|