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diff --git a/spec/ruby/library/set/compare_by_identity_spec.rb b/spec/ruby/library/set/compare_by_identity_spec.rb
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+++ b/spec/ruby/library/set/compare_by_identity_spec.rb
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+require File.expand_path('../../../spec_helper', __FILE__)
+require 'set'
+
+ruby_version_is '2.4' do
+  describe "Set#compare_by_identity" do
+    it "compares its members by identity" do
+      a = "a"
+      b1 = "b"
+      b2 = "b"
+
+      set = Set.new
+      set.compare_by_identity
+      set.merge([a, a, b1, b2])
+      set.to_a.sort.should == [a, b1, b2].sort
+    end
+
+    it "causes future comparisons on the receiver to be made by identity" do
+      elt = [1]
+      set = Set.new
+      set << elt
+      set.member?(elt.dup).should be_true
+      set.compare_by_identity
+      set.member?(elt.dup).should be_false
+    end
+
+    it "rehashes internally so that old members can be looked up" do
+      set = Set.new
+      (1..10).each { |k| set << k }
+      o = Object.new
+      def o.hash; 123; end
+      set << o
+      set.compare_by_identity
+      set.member?(o).should be_true
+    end
+
+    it "returns self" do
+      set = Set.new
+      result = set.compare_by_identity
+      result.should equal(set)
+    end
+
+    it "is idempotent and has no effect on an already compare_by_identity set" do
+      set = Set.new.compare_by_identity
+      set << :foo
+      set.compare_by_identity.should equal(set)
+      set.compare_by_identity?.should == true
+      set.to_a.should == [:foo]
+    end
+
+    it "uses the semantics of BasicObject#equal? to determine members identity" do
+      :a.equal?(:a).should == true
+      Set.new.compare_by_identity.merge([:a, :a]).to_a.should == [:a]
+
+      ary1 = [1]
+      ary2 = [1]
+      ary1.equal?(ary2).should == false
+      Set.new.compare_by_identity.merge([ary1, ary2]).to_a.sort.should == [ary1, ary2].sort
+    end
+
+    it "uses #equal? semantics, but doesn't actually call #equal? to determine identity" do
+      set = Set.new.compare_by_identity
+      obj = mock("equal")
+      obj.should_not_receive(:equal?)
+      set << :foo
+      set << obj
+      set.to_a.should == [:foo, obj]
+    end
+
+    it "does not call #hash on members" do
+      elt = mock("element")
+      elt.should_not_receive(:hash)
+      set = Set.new.compare_by_identity
+      set << elt
+      set.member?(elt).should be_true
+    end
+
+    it "regards #dup'd objects as having different identities" do
+      a1 = "a"
+      a2 = a1.dup
+
+      set = Set.new.compare_by_identity
+      set.merge([a1, a2])
+      set.to_a.sort.should == [a1, a2].sort
+    end
+
+    it "regards #clone'd objects as having different identities" do
+      a1 = "a"
+      a2 = a1.clone
+
+      set = Set.new.compare_by_identity
+      set.merge([a1, a2])
+      set.to_a.sort.should == [a1, a2].sort
+    end
+
+    it "raises a RuntimeError on frozen sets" do
+      set = Set.new.freeze
+      lambda {
+        set.compare_by_identity
+      }.should raise_error(RuntimeError, /frozen Hash/)
+    end
+
+    it "persists over #dups" do
+      set = Set.new.compare_by_identity
+      set << :a
+      set_dup = set.dup
+      set_dup.should == set
+      set_dup << :a
+      set_dup.to_a.should == [:a]
+    end
+
+    it "persists over #clones" do
+      set = Set.new.compare_by_identity
+      set << :a
+      set_clone = set.clone
+      set_clone.should == set
+      set_clone << :a
+      set_clone.to_a.should == [:a]
+    end
+
+    it "is not equal to set what does not compare by identity" do
+      Set.new([1, 2]).should == Set.new([1, 2])
+      Set.new([1, 2]).should_not == Set.new([1, 2]).compare_by_identity
+    end
+  end
+end
+
+ruby_version_is '2.4' do
+  describe "Set#compare_by_identity?" do
+    it "returns false by default" do
+      Set.new.compare_by_identity?.should == false
+    end
+
+    it "returns true once #compare_by_identity has been invoked on self" do
+      set = Set.new
+      set.compare_by_identity
+      set.compare_by_identity?.should == true
+    end
+
+    it "returns true when called multiple times on the same set" do
+      set = Set.new
+      set.compare_by_identity
+      set.compare_by_identity?.should == true
+      set.compare_by_identity?.should == true
+      set.compare_by_identity?.should == true
+    end
+  end
+end