Issue #21341 has been updated by 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. ---------------------------------------- Bug #21341: `Namespace is not a module (TypeError)` without enabling the namespace https://bugs.ruby-lang.org/issues/21341#change-113353 * 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/namespa... ```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 ``` -- https://bugs.ruby-lang.org/