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diff --git a/spec/rubyspec/core/hash/compare_by_identity_spec.rb b/spec/rubyspec/core/hash/compare_by_identity_spec.rb
new file mode 100644
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+++ b/spec/rubyspec/core/hash/compare_by_identity_spec.rb
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+require File.expand_path('../../../spec_helper', __FILE__)
+require File.expand_path('../fixtures/classes', __FILE__)
+
+describe "Hash#compare_by_identity" do
+  before :each do
+    @h = {}
+    @idh = {}.compare_by_identity
+  end
+
+  it "causes future comparisons on the receiver to be made by identity" do
+    @h[[1]] = :a
+    @h[[1]].should == :a
+    @h.compare_by_identity
+    @h[[1].dup].should be_nil
+  end
+
+  it "rehashes internally so that old keys can be looked up" do
+    h = {}
+    (1..10).each { |k| h[k] = k }
+    o = Object.new
+    def o.hash; 123; end
+    h[o] = 1
+    h.compare_by_identity
+    h[o].should == 1
+  end
+
+  it "returns self" do
+    h = {}
+    h[:foo] = :bar
+    h.compare_by_identity.should equal h
+  end
+
+  it "has no effect on an already compare_by_identity hash" do
+    @idh[:foo] = :bar
+    @idh.compare_by_identity.should equal @idh
+    @idh.compare_by_identity?.should == true
+    @idh[:foo].should == :bar
+  end
+
+  it "uses the semantics of BasicObject#equal? to determine key identity" do
+    [1].should_not equal([1])
+    @idh[[1]] = :c
+    @idh[[1]] = :d
+    :bar.should equal(:bar)
+    @idh[:bar] = :e
+    @idh[:bar] = :f
+    @idh.values.should == [:c, :d, :f]
+  end
+
+  it "uses #equal? semantics, but doesn't actually call #equal? to determine identity" do
+    obj = mock('equal')
+    obj.should_not_receive(:equal?)
+    @idh[:foo] = :glark
+    @idh[obj] = :a
+    @idh[obj].should == :a
+  end
+
+  it "does not call #hash on keys" do
+    key = HashSpecs::ByIdentityKey.new
+    @idh[key] = 1
+    @idh[key].should == 1
+  end
+
+  it "regards #dup'd objects as having different identities" do
+    key = ['foo']
+    @idh[key.dup] = :str
+    @idh[key].should be_nil
+  end
+
+  it "regards #clone'd objects as having different identities" do
+    key = ['foo']
+    @idh[key.clone] = :str
+    @idh[key].should be_nil
+  end
+
+  it "regards references to the same object as having the same identity" do
+    o = Object.new
+    @h[o] = :o
+    @h[:a] = :a
+    @h[o].should == :o
+  end
+
+  it "raises a RuntimeError on frozen hashes" do
+    @h = @h.freeze
+    lambda { @h.compare_by_identity }.should raise_error(RuntimeError)
+  end
+
+  # Behaviour confirmed in bug #1871
+  it "persists over #dups" do
+    @idh['foo'] = :bar
+    @idh['foo'] = :glark
+    @idh.dup.should == @idh
+    @idh.dup.size.should == @idh.size
+  end
+
+  it "persists over #clones" do
+    @idh['foo'] = :bar
+    @idh['foo'] = :glark
+    @idh.clone.should == @idh
+    @idh.clone.size.should == @idh.size
+  end
+
+  it "does not copy string keys" do
+    foo = 'foo'
+    @idh[foo] = true
+    @idh[foo] = true
+    @idh.size.should == 1
+    @idh.keys.first.object_id.should == foo.object_id
+  end
+
+  ruby_bug "#12855", "2.2.0"..."2.4.1" do
+    it "gives different identity for string literals" do
+      @idh['foo'] = 1
+      @idh['foo'] = 2
+      @idh.values.should == [1, 2]
+      @idh.size.should == 2
+    end
+  end
+end
+
+describe "Hash#compare_by_identity?" do
+  it "returns false by default" do
+    h = {}
+    h.compare_by_identity?.should be_false
+  end
+
+  it "returns true once #compare_by_identity has been invoked on self" do
+    h = {}
+    h.compare_by_identity
+    h.compare_by_identity?.should be_true
+  end
+
+  it "returns true when called multiple times on the same ident hash" do
+    h = {}
+    h.compare_by_identity
+    h.compare_by_identity?.should be_true
+    h.compare_by_identity?.should be_true
+    h.compare_by_identity?.should be_true
+  end
+end