1 files changed, 8 insertions, 1 deletions
diff --git a/spec/ruby/core/kernel/printf_spec.rb b/spec/ruby/core/kernel/printf_spec.rb
index d8f93ce429..50939b3794 100644
--- a/spec/ruby/core/kernel/printf_spec.rb
+++ b/spec/ruby/core/kernel/printf_spec.rb
@@ -4,7 +4,7 @@ require_relative 'shared/sprintf'
 
 describe "Kernel#printf" do
   it "is a private method" do
-    Kernel.should have_private_instance_method(:printf)
+    Kernel.private_instance_methods(false).should.include?(:printf)
   end
 end
 
@@ -31,6 +31,13 @@ describe "Kernel.printf" do
     object.should_receive(:write).with("string")
     Kernel.printf(object, "%s", "string")
   end
+
+  it "calls #to_str to convert the format object to a String" do
+    object = mock('format string')
+    object.should_receive(:to_str).and_return("to_str: %i")
+    $stdout.should_receive(:write).with("to_str: 42")
+    Kernel.printf($stdout, object, 42)
+  end
 end
 
 describe "Kernel.printf" do