[ruby-core:91350] [Ruby trunk Feature#15575] Prohibit to pass a block singleton class

Issue #15575 has been updated by alanwu (Alan Wu).


Another option would be to make everything use lexical scope, if the strangeness is indeed because locals are inaccessible inside class << self.
So imagine the following printing 1 2 3.
``` ruby
foo = 1
class A
  bar = 2
  def hi
    baz = 3
    class << self
      p foo, bar, baz
    end
  end
end
A.new.hi
```
Is it less strange if everything used the same scoping rule?



----------------------------------------
Feature #15575: Prohibit to pass a block singleton class
https://bugs.ruby-lang.org/issues/15575#change-76602

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
The following code works now:

```
def foo
  class << Object.new
    yield
  end
end

foo{ p :ok } #=> :ok
```

but I think this feature is very strange because local variables are not active in singleton class.

How about to prohibit this feature?

plan: warning at ruby 2.7 and prohibit it in ruby 3.




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