Feature #18069: `instance_exec` is just ignored when the block is originally a method

• Status: Open
• Assignee: -
• Target version: -

[ttanimichi (Tsukuru Tanimichi)]

I know you can't instance_exec a proc which is generated by Method#to_proc because it has its original instance's context. But, in such a case, raising ArgumentError would be the ideal behavior.

f = -> (x) { a + x }

class A
  def a
    1
  end
end

A.new.instance_exec(1, &f) # => 2

class B
  def b(x)
    a + x
  end
end

proc = B.new.method(:b).to_proc
A.new.instance_exec(1, &proc) # => undefined local variable or method `a' for #<B:0x00007fdaf30480a0> (NameError)

[jeremyevans0 (Jeremy Evans)]

• Tracker changed from Bug to Feature
• Backport deleted (2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN)

I don't think the current behavior is a bug. Method#to_proc is currently equivalent to:

class Method
  def to_proc
    method = self
    ->(*args, **kwargs, &block) do
      method.call(*args, **kwargs, &block)
    end
  end
end

You wouldn't expect an instance_exec on that lambda to change the behavior of Method#call. So I think the current behavior is expected.

Note that it's not hard to change the behavior to raise an error in this case (and other cases like module_exec). However, changing the behavior would result in significant backwards compatibility issues. I tried a commit that raises ArgumentError in such a case: https://github.com/jeremyevans/ruby/commit/3e2db2f01281f2335c638142223f8b24531826bd. However, it broke quite a few tests: https://github.com/jeremyevans/ruby/runs/3283493124. Some of the breakage may be due to implementation choice, but I checked and at least some of the breakage is unavoidable as the tests expect to pass procs created by Method#to_proc to instance_exec (e.g. test_instance_exec_define_method_kwsplat).

As I don't think this is a bug, I'm switching this to a feature request.

[nobu (Nobuyoshi Nakada)]

A method is bound to the internal states of the receiver.
I don't think that removing the receiver from a method makes sense.