[ruby/irb] Use gem paths to complete "require"

https://github.com/ruby/irb/commit/c870b23455

lib/irb/completion.rb

@@ -38,8 +38,21 @@ module IRB
 
     BASIC_WORD_BREAK_CHARACTERS = " \t\n`><=;|&{("
 
+    def self.retrieve_gem_and_system_load_path
+      gem_paths = Gem::Specification.latest_specs(true).map { |s|
+        s.require_paths.map { |p|
+          if File.absolute_path?(p)
+            p
+          else
+            File.join(s.full_gem_path, p)
+          end
+        }
+      }.flatten
+      (gem_paths + $LOAD_PATH).uniq.sort
+    end
+
     def self.retrieve_files_to_require_from_load_path
-      @@files_from_load_path ||= $LOAD_PATH.flat_map { |path|
+      @@files_from_load_path ||= retrieve_gem_and_system_load_path.flat_map { |path|
         begin
           Dir.glob("**/*.{rb,#{RbConfig::CONFIG['DLEXT']}}", base: path)
         rescue Errno::ENOENT