Handle non-String $LOAD_PATH values more carefully

In addition to String values, $LOAD_PATH can also take objects that
respond_to the `to_path` method, like Pathname objects. So `irb` should
be able to handle those objects too.

And if $LOAD_PATH contains objects that can't be converted into String,
`irb` should simply ignore it.

lib/irb/completion.rb

@@ -60,7 +60,14 @@ def self.retrieve_gem_and_system_load_path
           end
         }
       }.flatten if defined?(Gem::Specification)
-      (gem_paths.to_a | $LOAD_PATH).sort
+      candidates = (gem_paths.to_a | $LOAD_PATH)
+      candidates.map do |p|
+        if p.respond_to?(:to_path)
+          p.to_path
+        else
+          String(p) rescue nil
+        end
+      end.compact.sort
     end
 
     def self.retrieve_files_to_require_from_load_path

test/irb/test_completion.rb

@@ -4,6 +4,11 @@
 
 module TestIRB
   class TestCompletion < Test::Unit::TestCase
+    def setup
+      # make sure require completion candidates are not cached
+      IRB::InputCompletor.class_variable_set(:@@files_from_load_path, nil)
+    end
+
     def test_nonstring_module_name
       begin
         require "irb/completion"
@@ -84,6 +89,29 @@ def test_complete_require
       end
     end
 
+    def test_complete_require_with_pathname_in_load_path
+      temp_dir = Dir.mktmpdir
+      File.write(File.join(temp_dir, "foo.rb"), "test")
+      test_path = Pathname.new(temp_dir.to_s)
+      $LOAD_PATH << test_path
+
+      candidates = IRB::InputCompletor::CompletionProc.("'foo", "require ", "")
+      assert_equal ["'foo"], candidates
+    ensure
+      $LOAD_PATH.pop if test_path
+      FileUtils.remove_entry(temp_dir) if temp_dir
+    end
+
+    def test_complete_require_with_malformed_object_in_load_path
+      object = Object.new
+      def object.to_s; raise; end
+      $LOAD_PATH << object
+
+      assert_empty IRB::InputCompletor::CompletionProc.("'foo", "require ", "")
+    ensure
+      $LOAD_PATH.pop if object
+    end
+
     def test_complete_require_library_name_first
       pend 'Need to use virtual library paths'
       candidates = IRB::InputCompletor::CompletionProc.("'csv", "require ", "")