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diff --git a/spec/ruby/core/array/flatten_spec.rb b/spec/ruby/core/array/flatten_spec.rb
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+++ b/spec/ruby/core/array/flatten_spec.rb
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+require File.expand_path('../../../spec_helper', __FILE__)
+require File.expand_path('../fixtures/classes', __FILE__)
+
+describe "Array#flatten" do
+  it "returns a one-dimensional flattening recursively" do
+    [[[1, [2, 3]],[2, 3, [4, [4, [5, 5]], [1, 2, 3]]], [4]], []].flatten.should == [1, 2, 3, 2, 3, 4, 4, 5, 5, 1, 2, 3, 4]
+  end
+
+  it "takes an optional argument that determines the level of recursion" do
+    [ 1, 2, [3, [4, 5] ] ].flatten(1).should == [1, 2, 3, [4, 5]]
+  end
+
+  it "returns dup when the level of recursion is 0" do
+    a = [ 1, 2, [3, [4, 5] ] ]
+    a.flatten(0).should == a
+    a.flatten(0).should_not equal(a)
+  end
+
+  it "ignores negative levels" do
+    [ 1, 2, [ 3, 4, [5, 6] ] ].flatten(-1).should == [1, 2, 3, 4, 5, 6]
+    [ 1, 2, [ 3, 4, [5, 6] ] ].flatten(-10).should == [1, 2, 3, 4, 5, 6]
+  end
+
+  it "tries to convert passed Objects to Integers using #to_int" do
+    obj = mock("Converted to Integer")
+    obj.should_receive(:to_int).and_return(1)
+
+    [ 1, 2, [3, [4, 5] ] ].flatten(obj).should == [1, 2, 3, [4, 5]]
+  end
+
+  it "raises a TypeError when the passed Object can't be converted to an Integer" do
+    obj = mock("Not converted")
+    lambda { [ 1, 2, [3, [4, 5] ] ].flatten(obj) }.should raise_error(TypeError)
+  end
+
+  it "does not call flatten on elements" do
+    obj = mock('[1,2]')
+    obj.should_not_receive(:flatten)
+    [obj, obj].flatten.should == [obj, obj]
+
+    obj = [5, 4]
+    obj.should_not_receive(:flatten)
+    [obj, obj].flatten.should == [5, 4, 5, 4]
+  end
+
+  it "raises an ArgumentError on recursive arrays" do
+    x = []
+    x << x
+    lambda { x.flatten }.should raise_error(ArgumentError)
+
+    x = []
+    y = []
+    x << y
+    y << x
+    lambda { x.flatten }.should raise_error(ArgumentError)
+  end
+
+  it "flattens any element which responds to #to_ary, using the return value of said method" do
+    x = mock("[3,4]")
+    x.should_receive(:to_ary).at_least(:once).and_return([3, 4])
+    [1, 2, x, 5].flatten.should == [1, 2, 3, 4, 5]
+
+    y = mock("MyArray[]")
+    y.should_receive(:to_ary).at_least(:once).and_return(ArraySpecs::MyArray[])
+    [y].flatten.should == []
+
+    z = mock("[2,x,y,5]")
+    z.should_receive(:to_ary).and_return([2, x, y, 5])
+    [1, z, 6].flatten.should == [1, 2, 3, 4, 5, 6]
+  end
+
+  ruby_version_is "2.3" do
+    it "does not call #to_ary on elements beyond the given level" do
+      obj = mock("1")
+      obj.should_not_receive(:to_ary)
+      [[obj]].flatten(1)
+    end
+  end
+
+  it "returns subclass instance for Array subclasses" do
+    ArraySpecs::MyArray[].flatten.should be_an_instance_of(ArraySpecs::MyArray)
+    ArraySpecs::MyArray[1, 2, 3].flatten.should be_an_instance_of(ArraySpecs::MyArray)
+    ArraySpecs::MyArray[1, [2], 3].flatten.should be_an_instance_of(ArraySpecs::MyArray)
+    ArraySpecs::MyArray[1, [2, 3], 4].flatten.should == ArraySpecs::MyArray[1, 2, 3, 4]
+    [ArraySpecs::MyArray[1, 2, 3]].flatten.should be_an_instance_of(Array)
+  end
+
+  it "is not destructive" do
+    ary = [1, [2, 3]]
+    ary.flatten
+    ary.should == [1, [2, 3]]
+  end
+
+  describe "with a non-Array object in the Array" do
+    before :each do
+      @obj = mock("Array#flatten")
+      ScratchPad.record []
+    end
+
+    it "does not call #to_ary if the method is not defined" do
+      [@obj].flatten.should == [@obj]
+    end
+
+    it "does not raise an exception if #to_ary returns nil" do
+      @obj.should_receive(:to_ary).and_return(nil)
+      [@obj].flatten.should == [@obj]
+    end
+
+    it "raises a TypeError if #to_ary does not return an Array" do
+      @obj.should_receive(:to_ary).and_return(1)
+      lambda { [@obj].flatten }.should raise_error(TypeError)
+    end
+
+    it "does not call #to_ary if not defined when #respond_to_missing? returns false" do
+      def @obj.respond_to_missing?(*args) ScratchPad << args; false end
+
+      [@obj].flatten.should == [@obj]
+      ScratchPad.recorded.should == [[:to_ary, false]]
+    end
+
+    it "calls #to_ary if not defined when #respond_to_missing? returns true" do
+      def @obj.respond_to_missing?(*args) ScratchPad << args; true end
+
+      lambda { [@obj].flatten }.should raise_error(NoMethodError)
+      ScratchPad.recorded.should == [[:to_ary, false]]
+    end
+
+    it "calls #method_missing if defined" do
+      @obj.should_receive(:method_missing).with(:to_ary).and_return([1, 2, 3])
+      [@obj].flatten.should == [1, 2, 3]
+    end
+  end
+
+  it "returns a tainted array if self is tainted" do
+    [].taint.flatten.tainted?.should be_true
+  end
+
+  it "returns an untrusted array if self is untrusted" do
+    [].untrust.flatten.untrusted?.should be_true
+  end
+
+  it "performs respond_to? and method_missing-aware checks when coercing elements to array" do
+    bo = BasicObject.new
+    [bo].flatten.should == [bo]
+
+    def bo.method_missing(name, *)
+      [1,2]
+    end
+
+    [bo].flatten.should == [1,2]
+
+    def bo.respond_to?(name, *)
+      false
+    end
+
+    [bo].flatten.should == [bo]
+
+    def bo.respond_to?(name, *)
+      true
+    end
+
+    [bo].flatten.should == [1,2]
+  end
+end
+
+describe "Array#flatten!" do
+  it "modifies array to produce a one-dimensional flattening recursively" do
+    a = [[[1, [2, 3]],[2, 3, [4, [4, [5, 5]], [1, 2, 3]]], [4]], []]
+    a.flatten!
+    a.should == [1, 2, 3, 2, 3, 4, 4, 5, 5, 1, 2, 3, 4]
+  end
+
+  it "returns self if made some modifications" do
+    a = [[[1, [2, 3]],[2, 3, [4, [4, [5, 5]], [1, 2, 3]]], [4]], []]
+    a.flatten!.should equal(a)
+  end
+
+  it "returns nil if no modifications took place" do
+    a = [1, 2, 3]
+    a.flatten!.should == nil
+    a = [1, [2, 3]]
+    a.flatten!.should_not == nil
+  end
+
+  it "should not check modification by size" do
+    a = [1, 2, [3]]
+    a.flatten!.should_not == nil
+    a.should == [1, 2, 3]
+  end
+
+  it "takes an optional argument that determines the level of recursion" do
+    [ 1, 2, [3, [4, 5] ] ].flatten!(1).should == [1, 2, 3, [4, 5]]
+  end
+
+  # redmine #1440
+  it "returns nil when the level of recursion is 0" do
+    a = [ 1, 2, [3, [4, 5] ] ]
+    a.flatten!(0).should == nil
+  end
+
+  it "treats negative levels as no arguments" do
+    [ 1, 2, [ 3, 4, [5, 6] ] ].flatten!(-1).should == [1, 2, 3, 4, 5, 6]
+    [ 1, 2, [ 3, 4, [5, 6] ] ].flatten!(-10).should == [1, 2, 3, 4, 5, 6]
+  end
+
+  it "tries to convert passed Objects to Integers using #to_int" do
+    obj = mock("Converted to Integer")
+    obj.should_receive(:to_int).and_return(1)
+
+    [ 1, 2, [3, [4, 5] ] ].flatten!(obj).should == [1, 2, 3, [4, 5]]
+  end
+
+  it "raises a TypeError when the passed Object can't be converted to an Integer" do
+    obj = mock("Not converted")
+    lambda { [ 1, 2, [3, [4, 5] ] ].flatten!(obj) }.should raise_error(TypeError)
+  end
+
+  it "does not call flatten! on elements" do
+    obj = mock('[1,2]')
+    obj.should_not_receive(:flatten!)
+    [obj, obj].flatten!.should == nil
+
+    obj = [5, 4]
+    obj.should_not_receive(:flatten!)
+    [obj, obj].flatten!.should == [5, 4, 5, 4]
+  end
+
+  it "raises an ArgumentError on recursive arrays" do
+    x = []
+    x << x
+    lambda { x.flatten! }.should raise_error(ArgumentError)
+
+    x = []
+    y = []
+    x << y
+    y << x
+    lambda { x.flatten! }.should raise_error(ArgumentError)
+  end
+
+  it "flattens any elements which responds to #to_ary, using the return value of said method" do
+    x = mock("[3,4]")
+    x.should_receive(:to_ary).at_least(:once).and_return([3, 4])
+    [1, 2, x, 5].flatten!.should == [1, 2, 3, 4, 5]
+
+    y = mock("MyArray[]")
+    y.should_receive(:to_ary).at_least(:once).and_return(ArraySpecs::MyArray[])
+    [y].flatten!.should == []
+
+    z = mock("[2,x,y,5]")
+    z.should_receive(:to_ary).and_return([2, x, y, 5])
+    [1, z, 6].flatten!.should == [1, 2, 3, 4, 5, 6]
+
+    ary = [ArraySpecs::MyArray[1, 2, 3]]
+    ary.flatten!
+    ary.should be_an_instance_of(Array)
+    ary.should == [1, 2, 3]
+  end
+
+  it "raises a RuntimeError on frozen arrays when the array is modified" do
+    nested_ary = [1, 2, []]
+    nested_ary.freeze
+    lambda { nested_ary.flatten! }.should raise_error(RuntimeError)
+  end
+
+  # see [ruby-core:23663]
+  it "raises a RuntimeError on frozen arrays when the array would not be modified" do
+    lambda { ArraySpecs.frozen_array.flatten! }.should raise_error(RuntimeError)
+    lambda { ArraySpecs.empty_frozen_array.flatten! }.should raise_error(RuntimeError)
+  end
+end