[ruby-core:91339] [Ruby trunk Feature#15575] Prohibit to pass a block singleton class

Issue #15575 has been updated by Eregon (Benoit Daloze).


Agreed.
IMHO `class << expr` should be no different than the normal `class Name`, but currently various things are allowed in that context such as `return`, `yield`, etc.

Constant resolution also becomes somewhat dynamically-scoped with `class << expr` which is also a weird exception.
So longer term maybe it would be good to deprecate `class <<` entirely and use `singleton_class.class_exec do` instead.

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Feature #15575: Prohibit to pass a block singleton class
https://bugs.ruby-lang.org/issues/15575#change-76588

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
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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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