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diff --git a/spec/ruby/core/string/insert_spec.rb b/spec/ruby/core/string/insert_spec.rb
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+++ b/spec/ruby/core/string/insert_spec.rb
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+# -*- encoding: utf-8 -*-
+# frozen_string_literal: false
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+describe "String#insert with index, other" do
+  it "inserts other before the character at the given index" do
+    "abcd".insert(0, 'X').should == "Xabcd"
+    "abcd".insert(3, 'X').should == "abcXd"
+    "abcd".insert(4, 'X').should == "abcdX"
+  end
+
+  it "modifies self in place" do
+    a = "abcd"
+    a.insert(4, 'X').should == "abcdX"
+    a.should == "abcdX"
+  end
+
+  it "inserts after the given character on an negative count" do
+    "abcd".insert(-5, 'X').should == "Xabcd"
+    "abcd".insert(-3, 'X').should == "abXcd"
+    "abcd".insert(-1, 'X').should == "abcdX"
+  end
+
+  it "raises an IndexError if the index is beyond string" do
+    -> { "abcd".insert(5, 'X')  }.should raise_error(IndexError)
+    -> { "abcd".insert(-6, 'X') }.should raise_error(IndexError)
+  end
+
+  it "converts index to an integer using to_int" do
+    other = mock('-3')
+    other.should_receive(:to_int).and_return(-3)
+
+    "abcd".insert(other, "XYZ").should == "abXYZcd"
+  end
+
+  it "converts other to a string using to_str" do
+    other = mock('XYZ')
+    other.should_receive(:to_str).and_return("XYZ")
+
+    "abcd".insert(-3, other).should == "abXYZcd"
+  end
+
+  it "raises a TypeError if other can't be converted to string" do
+    -> { "abcd".insert(-6, Object.new)}.should raise_error(TypeError)
+    -> { "abcd".insert(-6, [])        }.should raise_error(TypeError)
+    -> { "abcd".insert(-6, mock('x')) }.should raise_error(TypeError)
+  end
+
+  it "raises a FrozenError if self is frozen" do
+    str = "abcd".freeze
+    -> { str.insert(4, '')  }.should raise_error(FrozenError)
+    -> { str.insert(4, 'X') }.should raise_error(FrozenError)
+  end
+
+  it "inserts a character into a multibyte encoded string" do
+    "ありがとう".insert(1, 'ü').should == "あüりがとう"
+  end
+
+  it "returns a String in the compatible encoding" do
+    str = "".force_encoding(Encoding::US_ASCII)
+    str.insert(0, "ありがとう")
+    str.encoding.should == Encoding::UTF_8
+  end
+
+  it "raises an Encoding::CompatibilityError if the encodings are incompatible" do
+    pat = "ア".encode Encoding::EUC_JP
+    -> do
+      "あれ".insert 0, pat
+    end.should raise_error(Encoding::CompatibilityError)
+  end
+
+  it "should not call subclassed string methods" do
+    cls = Class.new(String) do
+      def replace(arg)
+        raise "should not call replace"
+      end
+    end
+    cls.new("abcd").insert(0, 'X').should == "Xabcd"
+  end
+end