Bug #21341: `Namespace is not a module (TypeError)` without enabling the namespace

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

[yahonda (Yasuo Honda)]

Ruby on Rails test case has the module Namespace.

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

module Namespace
  class EmailValidator < ::EmailValidator
  end
end

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

Steps to reproduce

$ ruby -ve 'module Namespace; end'

Expected behavior

It should run without errors.

Actual behavior

It raises the TypeError.

$ 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_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

[yahonda (Yasuo Honda)]

This code comes from a Rails test case, so we can change it if needed. However, there might be other cases to consider.

[yahonda (Yasuo Honda)]

This only triggers the TypeError for the top-level module Namespace, not when it’s nested inside another module.

$ ruby -ve 'module Foo; module Namespace; end; end'
ruby 3.5.0dev (2025-05-15T08:25:56Z master 427ede2dde) +PRISM [x86_64-linux]

$ RUBY_NAMESPACE=1 ruby -ve 'module Foo; module Namespace; end; 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.
$

[Earlopain (Earlopain _)]

It looks intentional to me:

Namespace.new
(irb):1:in 'Namespace#initialize': Namespace is disabled. Set RUBY_NAMESPACE=1 environment variable to use Namespace. (RuntimeError)

But the incompatibility may not have been considered since toplevel Ruby used to warn in 3.4 for Ruby::VERSION and others.

[byroot (Jean Boussier)]

But the incompatibility may not have been considered since toplevel Ruby used to warn in 3.4 for Ruby::VERSION and others.

Yes, we likely need to emit a deprecation warning when a top-level Namespace constant is defined.

[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 rename our namespace to Ruby::Namespace or something?

[tagomoris (Satoshi Tagomori)]

I want to let Matz determine how to handle this problem :D

[hsbt (Hiroshi SHIBATA)]

• Related to Misc #21385: Namespace: Suggesting a rename added

[hsbt (Hiroshi SHIBATA)]

• Tags set to namespace