Revert "Fix Fiber#transfer"

This reverts commit fa8ac91e95.

Previous behavior is intentional.

cont.c

@@ -2226,8 +2226,7 @@ static VALUE
 rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fiber_value)
 {
     rb_fiber_t *fiber = fiber_ptr(fiber_value);
-    fiber_current()->transferred = 1;
-    fiber->transferred = 0;
+    fiber->transferred = 1;
     return fiber_switch(fiber, argc, argv, 0, PASS_KW_SPLAT);
 }
 

spec/ruby/library/fiber/transfer_spec.rb

@@ -42,18 +42,9 @@ describe "Fiber#transfer" do
     fiber2.transfer.should == [:fiber2_start, :fiber1, :fiber2_end]
   end
 
-  ruby_version_is ''...'2.7' do
-    it "raises a FiberError when transferring to a Fiber which resumes itself" do
-      fiber = Fiber.new { fiber.resume }
-      -> { fiber.transfer }.should raise_error(FiberError)
-    end
-  end
-
-  ruby_version_is '2.7' do
-    it "allows transferring to a Fiber which resumes itself" do
-      fiber = Fiber.new { fiber.resume 1 }
-      fiber.transfer.should == 1
-    end
+  it "raises a FiberError when transferring to a Fiber which resumes itself" do
+    fiber = Fiber.new { fiber.resume }
+    -> { fiber.transfer }.should raise_error(FiberError)
   end
 
   it "works if Fibers in different Threads each transfer to a Fiber in the same Thread" do

test/ruby/test_fiber.rb

@@ -222,8 +222,8 @@ class TestFiber < Test::Unit::TestCase
   end
 
   def test_resume_self
-    f = Fiber.new {f.resume 1}
-    assert_equal(1, f.transfer)
+    f = Fiber.new {f.resume}
+    assert_raise(FiberError, '[ruby-core:23651]') {f.transfer}
   end
 
   def test_fiber_transfer_segv
@@ -289,12 +289,14 @@ class TestFiber < Test::Unit::TestCase
     assert_raise(FiberError){
       g=nil
       f=Fiber.new{
-        g.transfer
+        g.resume
+        g.resume
       }
       g=Fiber.new{
         f.resume
+        f.resume
       }
-      f.resume
+      f.transfer
     }
   end