Bug #10929: NilClass#to_proc and & don't mix?

• Status: Rejected
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -
• ruby -v: ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
• Tags: core, patch, maybenotbug

[trans (Thomas Sawyer)]

This is sort of like a "who would ever care" kind of bug. Nonetheless technically it seems like it is one. So I thought it best that I report it just the same.

  class NilClass
    def to_proc
      Proc.new{ |*x| nil }
    end
  end

  def f(&b)
    b.call(1)
  end  

  f(&nil)
  => NoMethodError: undefined method `call' for nil:NilClass

(Maybe it was fixed already. Filing out this issue reminded me I need to update my version of Ruby.)

[hanachin (Seiei Miyagi)]

• File block_from_nil.patch block_from_nil.patch added

I wrote a patch for this issue.

diff --git bootstraptest/test_block.rb bootstraptest/test_block.rb
index cdc5960..4347fdd 100644
--- bootstraptest/test_block.rb
+++ bootstraptest/test_block.rb
@@ -611,3 +611,17 @@ assert_equal 'ok', %q{
   end
   'ok'
 }
+
+assert_equal 'ok', %q{
+  class NilClass
+    def to_proc
+      Proc.new {|x| x }
+    end
+  end
+
+  def foo(&blk)
+    blk.call('ok')
+  end
+
+  foo(&nil)
+}, '[ruby-core:68384] [Bug #10929]'
diff --git vm_args.c vm_args.c
index cf7256b..e1cb8ac 100644
--- vm_args.c
+++ vm_args.c
@@ -755,7 +755,7 @@ vm_caller_setup_arg_block(const rb_thread_t *th, rb_control_frame_t *reg_cfp, rb

 	proc = *(--reg_cfp->sp);

-	if (proc != Qnil) {
+	if (proc != Qnil || rb_respond_to(Qnil, idTo_proc)) {
 	    if (!rb_obj_is_proc(proc)) {
 		VALUE b;

[ko1 (Koichi Sasada)]

• Assignee set to matz (Yukihiro Matsumoto)

I'm not sure we support it or not.
For me, it is easy to understand that passing nil is special case, passing no block.

[mame (Yusuke Endoh)]

• Status changed from Open to Feedback

The suggested change is harmful. It will break delegation: def foo(&blk); bar(&blk); end; foo().

Ruby allows redefinition of Integer#/ which is incredibly harmful, so it is possible to allow NilClass#to_proc. But do you really want to allow that?

[ko1 (Koichi Sasada)]

• Status changed from Feedback to Rejected

it can break delegation methods.