Add specs for Kernel#warn's edge case when redefining Warning#warn

* See #2109
  and ruby/ruby@8a2a882e53

Author: eregon

spec/ruby/core/kernel/warn_spec.rb

@@ -197,4 +197,35 @@ def o.to_sym; :deprecated; end
     -> { warn(**h) }.should_not complain(verbose: true)
     -> { warn('foo', **h) }.should complain("foo\n")
   end
+
+  it "does not call Warning.warn if self is the Warning module" do
+    # RubyGems redefines Kernel#warn so we need to use a subprocess and disable RubyGems here
+    code = <<-RUBY
+    def Warning.warn(*args, **kwargs)
+      raise 'should not be called'
+    end
+    Kernel.instance_method(:warn).bind(Warning).call('Kernel#warn spec edge case')
+    RUBY
+    out = ruby_exe(code, args: "2>&1", options: "--disable-gems")
+    out.should == "Kernel#warn spec edge case\n"
+    $?.should.success?
+  end
+
+  it "avoids recursion if Warning#warn is redefined and calls super" do
+    # This works because of the spec above, which is the workaround for it.
+    # Note that redefining Warning#warn is a mistake which would naturally end in infinite recursion,
+    # Warning.extend Module.new { def warn } should be used instead.
+    # RubyGems redefines Kernel#warn so we need to use a subprocess and disable RubyGems here
+    code = <<-RUBY
+    module Warning
+      def warn(*args)
+        super
+      end
+    end
+    warn "avoid infinite recursion"
+    RUBY
+    out = ruby_exe(code, args: "2>&1", options: "--disable-gems")
+    out.should == "avoid infinite recursion\n"
+    $?.should.success?
+  end
 end

spec/tags/core/kernel/warn_tags.txt

@@ -1,2 +1,4 @@
 slow:Kernel#warn :uplevel keyword argument shows the caller of #require and not #require itself without RubyGems
 slow:Kernel#warn :uplevel keyword argument shows the caller of #require and not #require itself with RubyGems loaded
+slow:Kernel#warn does not call Warning.warn if self is the Warning module
+slow:Kernel#warn avoids recursion if Warning#warn is redefined and calls super