Ruby bug causing weirdness in RSpec

old_ruby_bug.rb

RUBY_VERSION='2.1.10'
# Fails with 2.0.0 and 2.1.10
# Passes with 2.3.1 and 2.4.1
# Have not tried with any other versions

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rspec"
end

class Parent
  def method_in_parent
    'wtf'
  end
end

class Child < Parent
  def method_in_child
    method_in_parent
  end
end

describe 'an unrelated test' do
  it 'should have no effect on the next test' do
    allow_any_instance_of(Child).to receive(:method_in_parent)
  end
end

describe 'but in ruby 2.1.10' do
  before do
    allow_any_instance_of(Parent).to receive(:method_in_parent)
  end

  it 'this test fails' do
    expect(Child.new.method_in_child).not_to eq('wtf')
  end
end

# If you change both allows to Parent, both to Child,
# or swap them so Parent is stubbed first, the test
# will pass regardless of Ruby version.

RSpec::Core::Runner.invoke