Having a `Test` class or model causes RSpec to fail

Moved from rspec/rspec-core#1859 opened by @zedtux

RSpec doesn't allow us to have a class named Test while it should:

app/models/test.rb:6:in `<top (required)>': Test is not a class (TypeError)
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:274:in `require'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:274:in `block in require'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:240:in `load_dependency'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:274:in `require'
  from /cursus-scholar/application/spec/models/test_spec.rb:2:in `<top (required)>'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:268:in `load'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:268:in `block in load'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:240:in `load_dependency'
  from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/dependencies.rb:268:in `load'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `block in load_spec_files'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `each'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `load_spec_files'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:96:in `setup'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:84:in `run'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:69:in `run'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:37:in `invoke'
  from /usr/local/lib/ruby/gems/2.2.0/gems/rspec-core-3.1.7/exe/rspec:4:in `<top (required)>'
  from /usr/local/bin/rspec:23:in `load'
  from /usr/local/bin/rspec:23:in `<main>'

Steps to Reproduce

Here's a simple working spec with only RSpec:

class Test; end

RSpec.describe Test do
  it "works" do
    expect(described_class).to be Test
  end
end

Now if I put that in a Rails project and load require 'rails_helper' at the top you get the stack trace:

/example_app/spec/test_spec.rb:3:in `<top (required)>': Test is not a class (TypeError)
    from /rspec-core/lib/rspec/core/configuration.rb:1200:in `load'
    from /rspec-core/lib/rspec/core/configuration.rb:1200:in `block in load_spec_files'
    from /rspec-core/lib/rspec/core/configuration.rb:1198:in `each'
    from /rspec-core/lib/rspec/core/configuration.rb:1198:in `load_spec_files'
    from /rspec-core/lib/rspec/core/runner.rb:97:in `setup'
    from /rspec-core/lib/rspec/core/runner.rb:85:in `run'
    from /rspec-core/lib/rspec/core/runner.rb:70:in `run'
    from /rspec-core/lib/rspec/core/runner.rb:38:in `invoke'
    from /rspec-core/exe/rspec:4:in `<top (required)>'
    from bin/rspec:16:in `load'
    from bin/rspec:16:in `<main>'

This is failing before it even parses the spec. It fails on the line class Test; end.

Cause

This is happening because Test is already defined and it's a module. It's coming from Test::Unit.

Flipping the loading for Rails and rspec-rails with the Test class definition it seems rspec-rails raises the same error (since Test is already defined as a class) in lib/rspec/rails/matchers.rb:

begin
  require 'test/unit/assertionfailederror'
rescue LoadError
  module Test
    module Unit
      class AssertionFailedError < StandardError
      end
    end
  end
end