* enum.c: Support for enumerators created by Enumerable with forwarding:

find_all, reject, ...
  [Feature #6636]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37507 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_enumerator.rb

@@ -10,6 +10,10 @@ class TestEnumerator < Test::Unit::TestCase
         a.each {|x| yield x }
       end
     end
+    @sized = @obj.clone
+    def @sized.size
+      42
+    end
   end
 
   def enum_test obj
@@ -429,6 +433,16 @@ class TestEnumerator < Test::Unit::TestCase
     end
   end
 
+  def test_size_for_enum_created_from_enumerable
+    %i[find_all reject map flat_map partition group_by sort_by min_by max_by
+      minmax_by each_with_index reverse_each each_entry].each do |method|
+      assert_equal nil, @obj.send(method).size
+      assert_equal 42, @sized.send(method).size
+    end
+    assert_equal nil, @obj.each_with_object(nil).size
+    assert_equal 42, @sized.each_with_object(nil).size
+  end
+
   def check_consistency_for_combinatorics(method)
     [ [], [:a, :b, :c, :d, :e] ].product([-2, 0, 2, 5, 6]) do |array, arg|
       assert_equal array.send(method, arg).to_a.size, array.send(method, arg).size,