[ruby/irb] Allow show_source for private methods

(https://github.com/ruby/irb/pull/589)

* Allow `show_source` for private methods

Fix https://github.com/ruby/irb/pull/577

* Pend tests on TruffleRuby

It seems `eval(..., __LINE__ + 1)` does not work.
Other similar tests are also pended on TruffleRuby, so I think it
is acceptable.

* Use `private_method_defined?` instead of `defined?`

lib/irb/cmd/show_source.rb

@@ -31,13 +31,14 @@ module IRB
           when /\A(?<owner>[A-Z]\w*(::[A-Z]\w*)*)#(?<method>[^ :.]+)\z/ # Class#method
             owner = eval(Regexp.last_match[:owner], irb_context.workspace.binding)
             method = Regexp.last_match[:method]
-            if owner.respond_to?(:instance_method) && owner.instance_methods.include?(method.to_sym)
-              file, line = owner.instance_method(method).source_location
+            if owner.respond_to?(:instance_method)
+              methods = owner.instance_methods + owner.private_instance_methods
+              file, line = owner.instance_method(method).source_location if methods.include?(method.to_sym)
             end
           when /\A((?<receiver>.+)(\.|::))?(?<method>[^ :.]+)\z/ # method, receiver.method, receiver::method
             receiver = eval(Regexp.last_match[:receiver] || 'self', irb_context.workspace.binding)
             method = Regexp.last_match[:method]
-            file, line = receiver.method(method).source_location if receiver.respond_to?(method)
+            file, line = receiver.method(method).source_location if receiver.respond_to?(method, true)
           end
           if file && line
             Source.new(file: file, first_line: line, last_line: find_end(file, line, irb_context))

test/irb/test_cmd.rb

@@ -488,6 +488,47 @@ module TestIRB
       assert_empty err
       assert_include(out, code)
     end
+
+    def test_show_source_private_instance
+      pend if RUBY_ENGINE == 'truffleruby'
+      eval(code = <<-EOS, binding, __FILE__, __LINE__ + 1)
+        class PrivateInstanceTest
+          private def show_source_test_method
+            unless true
+            end
+          end unless private_method_defined?(:show_source_test_method)
+        end
+      EOS
+
+      out, err = execute_lines(
+        "show_source '#{self.class.name}::PrivateInstanceTest#show_source_test_method'\n",
+      )
+
+      assert_empty err
+      assert_include(out, code.lines[1..-2].join)
+    end
+
+
+    def test_show_source_private
+      pend if RUBY_ENGINE == 'truffleruby'
+      eval(code = <<-EOS, binding, __FILE__, __LINE__ + 1)
+        class PrivateTest
+          private def show_source_test_method
+            unless true
+            end
+          end unless private_method_defined?(:show_source_test_method)
+        end
+
+        Instance = PrivateTest.new unless defined?(Instance)
+      EOS
+
+      out, err = execute_lines(
+        "show_source '#{self.class.name}::Instance.show_source_test_method'\n",
+      )
+
+      assert_empty err
+      assert_include(out, code.lines[1..-4].join)
+    end
   end
 
   class WorkspaceCommandTestCase < CommandTestCase