a.rb

#!/usr/bin/env ruby -I.

class A
end

class A::B
end

class B
end

class C
end

class D
  autoload :B, "b"
end

class E
  autoload :B, "f"
end

require "minitest/autorun"

class TestConstants < Minitest::Test
  def test_toplevel_nested_reopen
    # Defining nested constant with same name as
    # toplevel constant doesn't reopen constant.
    refute_equal B, A::B
  end

  def test_class_walk_toplevel
    # Referencing nested constants which don't exist inside the nested context
    # but do exist in the toplevel context will return the toplevel constant
    # with a warning.
    out, error = capture_io do
      assert_equal B, C::B
    end

    assert_match /warning: toplevel constant B referenced by C::B\Z/, error
  end

  def test_constant_autoloaded
    # Autoloaded constants don't output warnings, and don't reopen similarly
    # named toplevel constants.
    assert_equal "b", D.autoload?(:B)

    out, error = capture_io do
      assert D::B.is_a? Class
      refute_equal B, D::B
    end

    assert_nil D.autoload?(:B)
    assert_empty error
  end

  def test_constant_autoload_fails
    # When autoloading a file fails to defined a declared autoload constant, we
    # get a TypeError.
    assert_raises TypeError do
      E::B
    end
  end
end

b.rb

class D::B
end

f.rb

class E::F
end

$ ./a.rb
Run options: --seed 6615

# Running:

....

Finished in 0.002155s, 1855.9272 runs/s, 5103.7997 assertions/s.

4 runs, 11 assertions, 0 failures, 0 errors, 0 skips