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diff --git a/spec/ruby/core/string/downcase_spec.rb b/spec/ruby/core/string/downcase_spec.rb
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+++ b/spec/ruby/core/string/downcase_spec.rb
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+# -*- encoding: utf-8 -*-
+# frozen_string_literal: false
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+describe "String#downcase" do
+  it "returns a copy of self with all uppercase letters downcased" do
+    "hELLO".downcase.should == "hello"
+    "hello".downcase.should == "hello"
+  end
+
+  it "returns a String in the same encoding as self" do
+    "hELLO".encode("US-ASCII").downcase.encoding.should == Encoding::US_ASCII
+  end
+
+  describe "full Unicode case mapping" do
+    it "works for all of Unicode with no option" do
+      "ÄÖÜ".downcase.should == "äöü"
+    end
+
+    it "updates string metadata" do
+      downcased = "\u{212A}ING".downcase
+
+      downcased.should == "king"
+      downcased.size.should == 4
+      downcased.bytesize.should == 4
+      downcased.ascii_only?.should be_true
+    end
+  end
+
+  describe "ASCII-only case mapping" do
+    it "does not downcase non-ASCII characters" do
+      "CÅR".downcase(:ascii).should == "cÅr"
+    end
+
+    it "works with substrings" do
+      "prefix TÉ"[-2..-1].downcase(:ascii).should == "tÉ"
+    end
+  end
+
+  describe "full Unicode case mapping adapted for Turkic languages" do
+    it "downcases characters according to Turkic semantics" do
+      "İ".downcase(:turkic).should == "i"
+    end
+
+    it "allows Lithuanian as an extra option" do
+      "İ".downcase(:turkic, :lithuanian).should == "i"
+    end
+
+    it "does not allow any other additional option" do
+      -> { "İ".downcase(:turkic, :ascii) }.should raise_error(ArgumentError)
+    end
+  end
+
+  describe "full Unicode case mapping adapted for Lithuanian" do
+    it "currently works the same as full Unicode case mapping" do
+      "İS".downcase(:lithuanian).should == "i\u{307}s"
+    end
+
+    it "allows Turkic as an extra option (and applies Turkic semantics)" do
+      "İS".downcase(:lithuanian, :turkic).should == "is"
+    end
+
+    it "does not allow any other additional option" do
+      -> { "İS".downcase(:lithuanian, :ascii) }.should raise_error(ArgumentError)
+    end
+  end
+
+  describe "case folding" do
+    it "case folds special characters" do
+      "ß".downcase.should == "ß"
+      "ß".downcase(:fold).should == "ss"
+    end
+  end
+
+  it "does not allow invalid options" do
+    -> { "ABC".downcase(:invalid_option) }.should raise_error(ArgumentError)
+  end
+
+  it "returns a String instance for subclasses" do
+    StringSpecs::MyString.new("FOObar").downcase.should be_an_instance_of(String)
+  end
+end
+
+describe "String#downcase!" do
+  it "modifies self in place" do
+    a = "HeLlO"
+    a.downcase!.should equal(a)
+    a.should == "hello"
+  end
+
+  it "modifies self in place for non-ascii-compatible encodings" do
+    a = "HeLlO".encode("utf-16le")
+    a.downcase!
+    a.should == "hello".encode("utf-16le")
+  end
+
+  describe "full Unicode case mapping" do
+    it "modifies self in place for all of Unicode with no option" do
+      a = "ÄÖÜ"
+      a.downcase!
+      a.should == "äöü"
+    end
+
+    it "updates string metadata" do
+      downcased = "\u{212A}ING"
+      downcased.downcase!
+
+      downcased.should == "king"
+      downcased.size.should == 4
+      downcased.bytesize.should == 4
+      downcased.ascii_only?.should be_true
+    end
+  end
+
+  describe "ASCII-only case mapping" do
+    it "does not downcase non-ASCII characters" do
+      a = "CÅR"
+      a.downcase!(:ascii)
+      a.should == "cÅr"
+    end
+
+    it "works for non-ascii-compatible encodings" do
+      a = "ABC".encode("utf-16le")
+      a.downcase!(:ascii)
+      a.should == "abc".encode("utf-16le")
+    end
+  end
+
+  describe "full Unicode case mapping adapted for Turkic languages" do
+    it "downcases characters according to Turkic semantics" do
+      a = "İ"
+      a.downcase!(:turkic)
+      a.should == "i"
+    end
+
+    it "allows Lithuanian as an extra option" do
+      a = "İ"
+      a.downcase!(:turkic, :lithuanian)
+      a.should == "i"
+    end
+
+    it "does not allow any other additional option" do
+      -> { a = "İ"; a.downcase!(:turkic, :ascii) }.should raise_error(ArgumentError)
+    end
+  end
+
+  describe "full Unicode case mapping adapted for Lithuanian" do
+    it "currently works the same as full Unicode case mapping" do
+      a = "İS"
+      a.downcase!(:lithuanian)
+      a.should == "i\u{307}s"
+    end
+
+    it "allows Turkic as an extra option (and applies Turkic semantics)" do
+      a = "İS"
+      a.downcase!(:lithuanian, :turkic)
+      a.should == "is"
+    end
+
+    it "does not allow any other additional option" do
+      -> { a = "İS"; a.downcase!(:lithuanian, :ascii) }.should raise_error(ArgumentError)
+    end
+  end
+
+  describe "case folding" do
+    it "case folds special characters" do
+      a = "ß"
+      a.downcase!
+      a.should == "ß"
+
+      a.downcase!(:fold)
+      a.should == "ss"
+    end
+  end
+
+  it "does not allow invalid options" do
+    -> { a = "ABC"; a.downcase!(:invalid_option) }.should raise_error(ArgumentError)
+  end
+
+  it "returns nil if no modifications were made" do
+    a = "hello"
+    a.downcase!.should == nil
+    a.should == "hello"
+  end
+
+  it "raises a FrozenError when self is frozen" do
+    -> { "HeLlo".freeze.downcase! }.should raise_error(FrozenError)
+    -> { "hello".freeze.downcase! }.should raise_error(FrozenError)
+  end
+
+  it "sets the result String encoding to the source String encoding" do
+    "ABC".downcase.encoding.should equal(Encoding::UTF_8)
+  end
+end