Bug #2788: nil.instance_eval pushes nil onto cref

• Status: Closed
• Assignee: nobu (Nobuyoshi Nakada)
• Target version: 2.0.0
• ruby -v: ruby 1.9.2dev (2010-02-18 trunk 26704) [i686-linux]

[shugo (Shugo Maeda)]

=begin
A singleton class definition of nil pushes NilClass onto cref.
It is reasonable because NilClass has nil as the only instance.
However, nil.instance_eval pushes nil onto cref, which means that method definitions are not permitted in that context.

defiant:ruby$ ruby-trunk -ve 'class <<nil; def foo; puts "foo" end; end; nil.foo'
ruby 1.9.2dev (2010-02-18 trunk 26704) [i686-linux]
foo
defiant:ruby$ ruby-trunk -ve 'nil.instance_eval {|i| def foo; puts "foo" end }; nil.foo'
ruby 1.9.2dev (2010-02-18 trunk 26704) [i686-linux]
-e:1:in block in <main>': no class/module to add method (TypeError) from -e:1:in instance_eval'
from -e:1:in `'

The behavior is the same in Ruby 1.8.7.
Is it intended or a bug?
=end

[akr (Akira Tanaka)]

• Project changed from 8 to Ruby
• Category changed from core to core

[marcandre (Marc-Andre Lafortune)]

• Assignee changed from matz (Yukihiro Matsumoto) to ko1 (Koichi Sasada)
• Target version set to 2.0.0

Behavior is intended, as per tests in bootstraptest/test_eval.rb and [ruby-core:16808], but I'm don't think it's the right way to go, since nil, true and false do have singleton classes.

Koichi, do you agree the following patch would be good?

diff --git a/vm_eval.c b/vm_eval.c
index aa32621..dfd4588 100644
--- a/vm_eval.c
+++ b/vm_eval.c
@@ -1346,7 +1346,7 @@ rb_obj_instance_eval(int argc, VALUE *argv, VALUE self)
{
VALUE klass;

• if (SPECIAL_CONST_P(self)) {

• if (FIXNUM_P(self) || SYMBOL_P(self)) {
  klass = Qnil;
  }
  else {
  @@ -1378,7 +1378,7 @@ rb_obj_instance_exec(int argc, VALUE *argv, VALUE self)
  {
  VALUE klass;

• if (SPECIAL_CONST_P(self)) {

• if (FIXNUM_P(self) || SYMBOL_P(self)) {
  klass = Qnil;
  }
  else {
  diff --git a/bootstraptest/test_eval.rb b/bootstraptest/test_eval.rb
  index c347d50..e25cc260 100644
  --- a/bootstraptest/test_eval.rb
  +++ b/bootstraptest/test_eval.rb
  @@ -264,24 +264,18 @@ assert_equal 'ok', %q{
  }, '[ruby-core:16794]'

assert_equal 'ok', %q{

• begin
• nil.instance_eval {

•  def a() :a end
  

• }
• rescue TypeError
• :ok
• end
  -}, '[ruby-core:16796]'

• nil.instance_eval {
• def defd_using_instance_eval() :ok end
• }
• nil.defd_using_instance_eval
  +}, '[ruby-core:28324]'

assert_equal 'ok', %q{

• begin
• nil.instance_exec {

•  def a() :a end
  

• }
• rescue TypeError
• :ok
• end
  -}, '[ruby-core:16796]'

• nil.instance_exec {
• def defd_using_instance_exec() :ok end
• }
• nil.defd_using_instance_exec
  +}, '[ruby-core:28324]'

assert_normal_exit %q{
eval("", method(:proc).call {}.binding)

[marcandre (Marc-Andre Lafortune)]

test-all also points out that @@class_level_variable would then refer to NilClass/FalseClass/TrueClass's class variables, contrary to other singleton classes. This would be consistent with class << nil vs class << my_string.

[ko1 (Koichi Sasada)]

• Assignee changed from ko1 (Koichi Sasada) to matz (Yukihiro Matsumoto)

I'm so sorry to miss your ticket for long time.

Always I get confusing about instance eval with special variables (nil, false, etc).
Matz (shugo-san is specialist?), could you determine the specification?

[ko1 (Koichi Sasada)]

• Assignee changed from matz (Yukihiro Matsumoto) to nobu (Nobuyoshi Nakada)

nobu has patach.

[shugo (Shugo Maeda)]

• Priority changed from 3 to Normal

ko1 (Koichi Sasada) wrote:

nobu has patach.

Does he have a Hebrew niqqud vowel sign?
Aside from the joke, why not apply the patch, Nobu?

[usa (Usaku NAKAMURA)]

Hello,

In message "[ruby-core:47015] [ruby-trunk - Bug #2788] nil.instance_eval pushes nil onto cref"
on Aug.06,2012 17:15:16, [e-mail] wrote:

Aside from the joke, why not apply the patch, Nobu?

Wait until he drives his time machine.

Regards,

U.Nakamura [e-mail]

[nobu (Nobuyoshi Nakada)]

• Status changed from Assigned to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r36647.
Shugo, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

method in instance_eval

• class.c (rb_special_singleton_class_of): utility function.
• vm_eval.c (eval_under): special deal for class variable scope with
  instance_eval.
• vm_eval.c (rb_obj_instance_eval, rb_obj_instance_exec): allow method
  definition in instance_eval of special constants. [ruby-core:28324]
  [Bug #2788]