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diff --git a/spec/ruby/core/array/bsearch_index_spec.rb b/spec/ruby/core/array/bsearch_index_spec.rb
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+++ b/spec/ruby/core/array/bsearch_index_spec.rb
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+require File.expand_path('../../../spec_helper', __FILE__)
+require File.expand_path('../../enumerable/shared/enumeratorized', __FILE__)
+
+ruby_version_is "2.3" do
+  describe "Array#bsearch_index" do
+    context "when not passed a block" do
+      before :each do
+        @enum = [1, 2, 42, 100, 666].bsearch_index
+      end
+
+      it "returns an Enumerator" do
+        @enum.should be_an_instance_of(Enumerator)
+      end
+
+      it "returns an Enumerator with unknown size" do
+        @enum.size.should be_nil
+      end
+
+      it "returns index of element when block condition is satisfied" do
+        @enum.each { |x| x >= 33 }.should == 2
+      end
+    end
+
+    it "raises a TypeError when block returns a String" do
+      lambda { [1, 2, 3].bsearch_index { "not ok" } }.should raise_error(TypeError)
+    end
+
+    it "returns nil when block is empty" do
+      [1, 2, 3].bsearch_index {}.should be_nil
+    end
+
+    context "minimum mode" do
+      before :each do
+        @array = [0, 4, 7, 10, 12]
+      end
+
+      it "returns index of first element which satisfies the block" do
+        @array.bsearch_index { |x| x >= 4 }.should == 1
+        @array.bsearch_index { |x| x >= 6 }.should == 2
+        @array.bsearch_index { |x| x >= -1 }.should == 0
+      end
+
+      it "returns nil when block condition is never satisfied" do
+        @array.bsearch_index { false }.should be_nil
+        @array.bsearch_index { |x| x >= 100 }.should be_nil
+      end
+    end
+
+    context "find any mode" do
+      before :each do
+        @array = [0, 4, 7, 10, 12]
+      end
+
+      it "returns the index of any matched elements where element is between 4 <= x < 8" do
+        [1, 2].should include(@array.bsearch_index { |x| 1 - x / 4 })
+      end
+
+      it "returns the index of any matched elements where element is between 8 <= x < 10" do
+        @array.bsearch_index { |x| 4 - x / 2 }.should be_nil
+      end
+
+      it "returns nil when block never returns 0" do
+        @array.bsearch_index { |x| 1 }.should be_nil
+        @array.bsearch_index { |x| -1 }.should be_nil
+      end
+
+      it "returns the middle element when block always returns zero" do
+        @array.bsearch_index { |x| 0 }.should == 2
+      end
+
+      context "magnitude does not effect the result" do
+        it "returns the index of any matched elements where element is between 4n <= xn < 8n" do
+          [1, 2].should include(@array.bsearch_index { |x| (1 - x / 4) * (2**100) })
+        end
+
+        it "returns nil when block never returns 0" do
+          @array.bsearch_index { |x| 1 * (2**100) }.should be_nil
+          @array.bsearch_index { |x| (-1) * (2**100) }.should be_nil
+        end
+
+        it "handles values from Bignum#coerce" do
+          [1, 2].should include(@array.bsearch_index { |x| (2**100).coerce((1 - x / 4) * (2**100)).first })
+        end
+      end
+    end
+  end
+end