From: "matsuda (Akira Matsuda) via ruby-core" <ruby-core@...>
Date: 2025-05-20T13:07:20+00:00
Subject: [ruby-core:122205] [Ruby Bug#21341] `Namespace is not a module (TypeError)` without enabling the namespace

Issue #21341 has been updated by matsuda (Akira Matsuda).


I guess I found a widely used real-world app that defines a toplevel `Namespace` class, and it seems very difficult for them to resolve this conflict...
https://github.com/gitlabhq/gitlabhq/blob/master/app/models/namespace.rb

Perhaps we should better our namespace to `Ruby::Namespace` or something?

----------------------------------------
Bug #21341: `Namespace is not a module (TypeError)` without enabling the namespace
https://bugs.ruby-lang.org/issues/21341#change-113354

* Author: yahonda (Yasuo Honda)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-05-15T08:25:56Z master 427ede2dde) +PRISM [x86_64-linux]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Ruby on Rails test case has the `module Namespace`.

https://github.com/rails/rails/blob/main/activemodel/test/validators/namespace/email_validator.rb#L5

```ruby
module Namespace
  class EmailValidator < ::EmailValidator
  end
end
```

This code fails against Ruby 3.5.0dev without enabling the namespace feature.

### Steps to reproduce

```ruby
$ ruby -ve 'module Namespace; end'
```

### Expected behavior
It should run without errors.


### Actual behavior
It raises the TypeError.

```ruby
$ ruby -ve 'module Namespace; end'
ruby 3.5.0dev (2025-05-15T08:25:56Z master 427ede2dde) +PRISM [x86_64-linux]
-e:1:in '<main>': Namespace is not a module (TypeError)
ruby:0: previous definition of Namespace was here
```

Raising this TypeError with RUBY_NAMESPACE=1 looks expected to me.

```ruby
$ RUBY_NAMESPACE=1 ruby -ve 'module Namespace; end'
ruby 3.5.0dev (2025-05-15T08:25:56Z master 427ede2dde) +PRISM [x86_64-linux]
ruby: warning: Namespace is experimental, and the behavior may change in the future!
See doc/namespace.md for known issues, etc.
-e:1:in '<main>': Namespace is not a module (TypeError)
ruby:0: previous definition of Namespace was here
```





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