% svn diff --diff-cmd diff -x '-u -p'
Index: io.c
===================================================================
--- io.c	(revision 49185)
+++ io.c	(working copy)
@@ -4415,11 +4415,18 @@ rb_io_close(VALUE io)
  *
  *  If <em>ios</em> is opened by <code>IO.popen</code>,
  *  <code>close</code> sets <code>$?</code>.
+ *
+ *  Calling this method on closed IO object is just ignored since Ruby 2.3.
  */
 
 static VALUE
 rb_io_close_m(VALUE io)
 {
+    rb_io_t *fptr = RFILE(io)->fptr;
+    rb_io_check_initialized(fptr);
+    if (fptr->fd < 0) {
+        return Qnil;
+    }
     rb_io_check_closed(RFILE(io)->fptr);
     rb_io_close(io);
     return Qnil;
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb	(revision 49185)
+++ test/ruby/test_io.rb	(working copy)
@@ -3159,4 +3159,17 @@ End
       end
     end
   end
+
+  def test_close_twice
+    open(__FILE__) {|f|
+      assert_equal(nil, f.close)
+      assert_equal(nil, f.close)
+    }
+  end
+
+  def test_close_uninitialized
+    io = IO.allocate
+    assert_raise(IOError) { io.close } 
+  end
+
 end
Index: test/socket/test_basicsocket.rb
===================================================================
--- test/socket/test_basicsocket.rb	(revision 49185)
+++ test/socket/test_basicsocket.rb	(working copy)
@@ -9,7 +9,7 @@ class TestSocket_BasicSocket < Test::Uni
     sock = Socket.new(Socket::AF_INET, Socket::SOCK_STREAM, 0)
     yield sock
   ensure
-    assert_raise(IOError) {sock.close}
+    assert(sock.closed?)
   end
 
   def test_getsockopt
Index: test/zlib/test_zlib.rb
===================================================================
--- test/zlib/test_zlib.rb	(revision 49185)
+++ test/zlib/test_zlib.rb	(working copy)
@@ -929,7 +929,7 @@ if defined? Zlib
         f = open(t.path)
         f.binmode
         assert_equal("foo", Zlib::GzipReader.wrap(f) {|gz| gz.read })
-        assert_raise(IOError) { f.close }
+        assert(f.closed?)
       }
     end