[ruby-core:120291] [Ruby master Bug#20943] Constant defined in `Data.define` block

["matz (Yukihiro Matsumoto) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20943 has been updated by matz (Yukihiro Matsumoto).


Blocks do not introduce new scope even with `instance_eval` nor `class_eval` (along with `Data.define` etc. with blocks). Changing this behavior might cause serious compatibility problems.
If someone is willing to survey and estimate how much influence it could cause, we'd like to discuss. Otherwise, it's safe to keep the behavior as it is.

Matz.


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Bug #20943: Constant defined in `Data.define` block
https://bugs.ruby-lang.org/issues/20943#change-111056

* Author: nobu (Nobuyoshi Nakada)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
>From https://github.com/ruby/ruby/pull/12274:
> A couple times in code review I've seen constants inadvertently leak to top level from within a `Struct` or `Data` do block. I think it would be nice to show reopening the `Data` class when a constant is defined, so the constant is defined within the namespace. In this case, `Measure::NONE` instead of top level `Object::NONE`. It would also show readers that it's okay to reopen a `Data` class, which seems nice since some folk might not realize. Thanks for considering!

However, I think that `NONE` probably might be intended to be defined under `Measure`.

Current:
```ruby
Measure = Data.define(:amount, :unit) do
  NONE = Data.define
end
p NONE #=> NONE
```

Another:
```ruby
Measure = Data.define(:amount, :unit) do
  NONE = Data.define
  p NONE #=> Measure::NONE
end
p NONE # uninitialized constant NONE (NameError)
```

@zverok How do think?



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