Make String methods return String instances when called on a subclass instance

This modifies the following String methods to return String instances
instead of subclass instances:

* String#*
* String#capitalize
* String#center
* String#chomp
* String#chop
* String#delete
* String#delete_prefix
* String#delete_suffix
* String#downcase
* String#dump
* String#each/#each_line
* String#gsub
* String#ljust
* String#lstrip
* String#partition
* String#reverse
* String#rjust
* String#rpartition
* String#rstrip
* String#scrub
* String#slice!
* String#slice/#[]
* String#split
* String#squeeze
* String#strip
* String#sub
* String#succ/#next
* String#swapcase
* String#tr
* String#tr_s
* String#upcase

This also fixes a bug in String#swapcase where it would return the
receiver instead of a copy of the receiver if the receiver was the
empty string.

Some string methods were left to return subclass instances:

* String#+@
* String#-@

Both of these methods will return the receiver (subclass instance)
in some cases, so it is best to keep the returned class consistent.

Fixes [#10845]

spec/ruby/core/string/capitalize_spec.rb

@@ -80,12 +80,21 @@ describe "String#capitalize" do
     -> { "abc".capitalize(:invalid_option) }.should raise_error(ArgumentError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("hello").capitalize.should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("Hello").capitalize.should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("hello").capitalize.should be_an_instance_of(String)
+      StringSpecs::MyString.new("Hello").capitalize.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#capitalize!" do
   it "capitalizes self in place" do
     a = "hello"

spec/ruby/core/string/center_spec.rb

@@ -91,6 +91,7 @@ describe "String#center with length, padding" do
     -> { "hello".center(0, "")  }.should raise_error(ArgumentError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on subclasses" do
       StringSpecs::MyString.new("").center(10).should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").center(10).should be_an_instance_of(StringSpecs::MyString)
@@ -99,6 +100,18 @@ describe "String#center with length, padding" do
       "".center(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
       "foo".center(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on subclasses" do
+      StringSpecs::MyString.new("").center(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").center(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").center(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+
+      "".center(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+      "foo".center(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "when padding is tainted and self is untainted returns a tainted string if and only if length is longer than self" do

spec/ruby/core/string/chomp_spec.rb

@@ -46,10 +46,19 @@ describe "String#chomp" do
       end
     end
 
+    ruby_version_is ''...'3.0' do
       it "returns subclass instances when called on a subclass" do
         str = StringSpecs::MyString.new("hello\n").chomp
         str.should be_an_instance_of(StringSpecs::MyString)
       end
+    end
+
+    ruby_version_is '3.0' do
+      it "returns String instances when called on a subclass" do
+        str = StringSpecs::MyString.new("hello\n").chomp
+        str.should be_an_instance_of(String)
+      end
+    end
 
     it "removes trailing characters that match $/ when it has been assigned a value" do
       $/ = "cdef"

spec/ruby/core/string/chop_spec.rb

@@ -61,11 +61,19 @@ describe "String#chop" do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("hello\n").chop.should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("hello\n").chop.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#chop!" do
   it "removes the final character" do
     "abc".chop!.should == "ab"

spec/ruby/core/string/delete_prefix_spec.rb

@@ -41,12 +41,21 @@ describe "String#delete_prefix" do
     'hello'.delete_prefix(o).should == 'o'
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance when called on a subclass instance" do
       s = StringSpecs::MyString.new('hello')
       s.delete_prefix('hell').should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns a String instance when called on a subclass instance" do
+      s = StringSpecs::MyString.new('hello')
+      s.delete_prefix('hell').should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#delete_prefix!" do
   it "removes the found prefix" do
     s = 'hello'

spec/ruby/core/string/delete_spec.rb

@@ -93,11 +93,19 @@ describe "String#delete" do
     -> { "hello world".delete(mock('x')) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("oh no!!!").delete("!").should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("oh no!!!").delete("!").should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#delete!" do
   it "modifies self in place and returns self" do
     a = "hello"

spec/ruby/core/string/delete_suffix_spec.rb

@@ -41,12 +41,21 @@ describe "String#delete_suffix" do
     'hello'.delete_suffix(o).should == 'h'
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance when called on a subclass instance" do
       s = StringSpecs::MyString.new('hello')
       s.delete_suffix('ello').should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns a String instance when called on a subclass instance" do
+      s = StringSpecs::MyString.new('hello')
+      s.delete_suffix('ello').should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#delete_suffix!" do
   it "removes the found prefix" do
     s = 'hello'

spec/ruby/core/string/downcase_spec.rb

@@ -76,11 +76,19 @@ describe "String#downcase" do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance for subclasses" do
       StringSpecs::MyString.new("FOObar").downcase.should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns a String instance for subclasses" do
+      StringSpecs::MyString.new("FOObar").downcase.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#downcase!" do
   it "modifies self in place" do
     a = "HeLlO"

spec/ruby/core/string/dump_spec.rb

@@ -19,9 +19,17 @@ describe "String#dump" do
     "foo".freeze.dump.should_not.frozen?
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance" do
       StringSpecs::MyString.new.dump.should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns a String instance" do
+      StringSpecs::MyString.new.dump.should be_an_instance_of(String)
+    end
+  end
 
   it "wraps string with \"" do
     "foo".dump.should == '"foo"'

spec/ruby/core/string/gsub_spec.rb

@@ -236,12 +236,23 @@ describe "String#gsub with pattern and replacement" do
     -> { "hello".gsub(/[aeiou]/, nil)           }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("").gsub(//, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("").gsub(/foo/, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").gsub(/foo/, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").gsub("foo", "").should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("").gsub(//, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("").gsub(/foo/, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").gsub(/foo/, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").gsub("foo", "").should be_an_instance_of(String)
+    end
+  end
 
   # Note: $~ cannot be tested because mspec messes with it
 

spec/ruby/core/string/ljust_spec.rb

@@ -74,6 +74,7 @@ describe "String#ljust with length, padding" do
     -> { "hello".ljust(10, '') }.should raise_error(ArgumentError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on subclasses" do
       StringSpecs::MyString.new("").ljust(10).should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").ljust(10).should be_an_instance_of(StringSpecs::MyString)
@@ -82,6 +83,18 @@ describe "String#ljust with length, padding" do
       "".ljust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
       "foo".ljust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on subclasses" do
+      StringSpecs::MyString.new("").ljust(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").ljust(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").ljust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+
+      "".ljust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+      "foo".ljust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "when padding is tainted and self is untainted returns a tainted string if and only if length is longer than self" do

spec/ruby/core/string/rjust_spec.rb

@@ -74,6 +74,7 @@ describe "String#rjust with length, padding" do
     -> { "hello".rjust(10, '') }.should raise_error(ArgumentError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on subclasses" do
       StringSpecs::MyString.new("").rjust(10).should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").rjust(10).should be_an_instance_of(StringSpecs::MyString)
@@ -82,6 +83,18 @@ describe "String#rjust with length, padding" do
       "".rjust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
       "foo".rjust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on subclasses" do
+      StringSpecs::MyString.new("").rjust(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").rjust(10).should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").rjust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+
+      "".rjust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+      "foo".rjust(10, StringSpecs::MyString.new("x")).should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "when padding is tainted and self is untainted returns a tainted string if and only if length is longer than self" do

spec/ruby/core/string/shared/each_line.rb

@@ -93,11 +93,21 @@ describe :string_each_line, shared: true do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "yields subclass instances for subclasses" do
       a = []
       StringSpecs::MyString.new("hello\nworld").send(@method) { |s| a << s.class }
       a.should == [StringSpecs::MyString, StringSpecs::MyString]
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "yields String instances for subclasses" do
+      a = []
+      StringSpecs::MyString.new("hello\nworld").send(@method) { |s| a << s.class }
+      a.should == [String, String]
+    end
+  end
 
   it "returns self" do
     s = "hello\nworld"

spec/ruby/core/string/shared/slice.rb

@@ -163,12 +163,23 @@ describe :string_slice_index_length, shared: true do
     -> { "hello".send(@method, 0, bignum_value) }.should raise_error(RangeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.send(@method, 0,0).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, 0,4).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, 1,4).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.send(@method, 0,0).should be_an_instance_of(String)
+      s.send(@method, 0,4).should be_an_instance_of(String)
+      s.send(@method, 1,4).should be_an_instance_of(String)
+    end
+  end
 
   it "handles repeated application" do
     "hello world".send(@method, 6, 5).send(@method, 0, 1).should == 'w'
@@ -252,12 +263,23 @@ describe :string_slice_range, shared: true do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.send(@method, 0...0).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, 0..4).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, 1..4).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.send(@method, 0...0).should be_an_instance_of(String)
+      s.send(@method, 0..4).should be_an_instance_of(String)
+      s.send(@method, 1..4).should be_an_instance_of(String)
+    end
+  end
 
   it "calls to_int on range arguments" do
     from = mock('from')
@@ -348,11 +370,21 @@ describe :string_slice_regexp, shared: true do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.send(@method, //).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, /../).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.send(@method, //).should be_an_instance_of(String)
+      s.send(@method, /../).should be_an_instance_of(String)
+    end
+  end
 
   it "sets $~ to MatchData when there is a match and nil when there's none" do
     'hello'.send(@method, /./)
@@ -436,11 +468,21 @@ describe :string_slice_regexp_index, shared: true do
     -> { "hello".send(@method, /(.)(.)(.)/, nil) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.send(@method, /(.)(.)/, 0).should be_an_instance_of(StringSpecs::MyString)
       s.send(@method, /(.)(.)/, 1).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.send(@method, /(.)(.)/, 0).should be_an_instance_of(String)
+      s.send(@method, /(.)(.)/, 1).should be_an_instance_of(String)
+    end
+  end
 
   it "sets $~ to MatchData when there is a match and nil when there's none" do
     'hello'.send(@method, /.(.)/, 0)
@@ -493,6 +535,7 @@ describe :string_slice_string, shared: true do
     -> { "hello".send(@method, o) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance when given a subclass instance" do
       s = StringSpecs::MyString.new("el")
       r = "hello".send(@method, s)
@@ -501,6 +544,16 @@ describe :string_slice_string, shared: true do
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns a String instance when given a subclass instance" do
+      s = StringSpecs::MyString.new("el")
+      r = "hello".send(@method, s)
+      r.should == "el"
+      r.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe :string_slice_regexp_group, shared: true do
   not_supported_on :opal do
     it "returns the capture for the given name" do
@@ -567,10 +620,19 @@ describe :string_slice_regexp_group, shared: true do
       -> { "hello".send(@method, /(?<q>)/, '') }.should raise_error(IndexError)
     end
 
+    ruby_version_is ''...'3.0' do
       it "returns subclass instances" do
         s = StringSpecs::MyString.new("hello")
         s.send(@method, /(?<q>.)/, 'q').should be_an_instance_of(StringSpecs::MyString)
       end
+    end
+
+    ruby_version_is '3.0' do
+      it "returns String instances" do
+        s = StringSpecs::MyString.new("hello")
+        s.send(@method, /(?<q>.)/, 'q').should be_an_instance_of(String)
+      end
+    end
 
     it "sets $~ to MatchData when there is a match and nil when there's none" do
       'hello'.send(@method, /(?<hi>.(.))/, 'hi')

spec/ruby/core/string/shared/succ.rb

@@ -59,11 +59,21 @@ describe :string_succ, shared: true do
     "\xFF\xFF".send(@method).should == "\x01\x00\x00"
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("").send(@method).should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("a").send(@method).should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("z").send(@method).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("").send(@method).should be_an_instance_of(String)
+      StringSpecs::MyString.new("a").send(@method).should be_an_instance_of(String)
+      StringSpecs::MyString.new("z").send(@method).should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "taints the result if self is tainted" do

spec/ruby/core/string/slice_spec.rb

@@ -142,12 +142,21 @@ describe "String#slice! with index, length" do
     "hello".slice!(obj, obj).should == "ll"
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.slice!(0, 0).should be_an_instance_of(StringSpecs::MyString)
       s.slice!(0, 4).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
 
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0, 0).should be_an_instance_of(String)
+      s.slice!(0, 4).should be_an_instance_of(String)
+    end
+  end
 
   it "returns the substring given by the character offsets" do
     "hellö there".slice!(1,0).should == ""
@@ -196,11 +205,21 @@ describe "String#slice! Range" do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.slice!(0...0).should be_an_instance_of(StringSpecs::MyString)
       s.slice!(0..4).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(0...0).should be_an_instance_of(String)
+      s.slice!(0..4).should be_an_instance_of(String)
+    end
+  end
 
   it "calls to_int on range arguments" do
     from = mock('from')
@@ -299,11 +318,21 @@ describe "String#slice! with Regexp" do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.slice!(//).should be_an_instance_of(StringSpecs::MyString)
       s.slice!(/../).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(//).should be_an_instance_of(String)
+      s.slice!(/../).should be_an_instance_of(String)
+    end
+  end
 
   it "returns the matching portion of self with a multi byte character" do
     "hëllo there".slice!(/[ë](.)\1/).should == "ëll"
@@ -383,11 +412,21 @@ describe "String#slice! with Regexp, index" do
     "har".slice!(/(.)(.)(.)/, obj).should == "a"
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       s = StringSpecs::MyString.new("hello")
       s.slice!(/(.)(.)/, 0).should be_an_instance_of(StringSpecs::MyString)
       s.slice!(/(.)(.)/, 1).should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      s = StringSpecs::MyString.new("hello")
+      s.slice!(/(.)(.)/, 0).should be_an_instance_of(String)
+      s.slice!(/(.)(.)/, 1).should be_an_instance_of(String)
+    end
+  end
 
   it "returns the encoding aware capture for the given index" do
     "hår".slice!(/(.)(.)(.)/, 0).should == "hår"
@@ -461,12 +500,23 @@ describe "String#slice! with String" do
     -> { "hello".slice!(o) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance when given a subclass instance" do
       s = StringSpecs::MyString.new("el")
       r = "hello".slice!(s)
       r.should == "el"
       r.should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns a subclass instance when given a subclass instance" do
+      s = StringSpecs::MyString.new("el")
+      r = "hello".slice!(s)
+      r.should == "el"
+      r.should be_an_instance_of(String)
+    end
+  end
 
   it "raises a FrozenError if self is frozen" do
     -> { "hello hello".freeze.slice!('llo')     }.should raise_error(FrozenError)

spec/ruby/core/string/split_spec.rb

@@ -159,6 +159,7 @@ describe "String#split with String" do
     "foo".split("bar", 3).should == ["foo"]
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances based on self" do
       ["", "x.y.z.", "  x  y  "].each do |str|
         ["", ".", " "].each do |pat|
@@ -182,6 +183,25 @@ describe "String#split with String" do
       s = StringSpecs::StringWithRaisingConstructor.new('silly:string')
       s.split(':').first.should == 'silly'
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances based on self" do
+      ["", "x.y.z.", "  x  y  "].each do |str|
+        ["", ".", " "].each do |pat|
+          [-1, 0, 1, 2].each do |limit|
+            StringSpecs::MyString.new(str).split(pat, limit).each do |x|
+              x.should be_an_instance_of(String)
+            end
+
+            str.split(StringSpecs::MyString.new(pat), limit).each do |x|
+              x.should be_an_instance_of(String)
+            end
+          end
+        end
+      end
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "taints the resulting strings if self is tainted" do
@@ -355,6 +375,7 @@ describe "String#split with Regexp" do
     "foo".split(/bar/, 3).should == ["foo"]
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances based on self" do
       ["", "x:y:z:", "  x  y  "].each do |str|
         [//, /:/, /\s+/].each do |pat|
@@ -374,6 +395,21 @@ describe "String#split with Regexp" do
       s = StringSpecs::StringWithRaisingConstructor.new('silly:string')
       s.split(/:/).first.should == 'silly'
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances based on self" do
+      ["", "x:y:z:", "  x  y  "].each do |str|
+        [//, /:/, /\s+/].each do |pat|
+          [-1, 0, 1, 2].each do |limit|
+            StringSpecs::MyString.new(str).split(pat, limit).each do |x|
+              x.should be_an_instance_of(String)
+            end
+          end
+        end
+      end
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "taints the resulting strings if self is tainted" do
@@ -493,6 +529,7 @@ describe "String#split with Regexp" do
     end
 
     describe "for a String subclass" do
+      ruby_version_is ''...'3.0' do
         it "yields instances of the same subclass" do
           a = []
           StringSpecs::MyString.new("a|b").split("|") { |str| a << str }
@@ -505,5 +542,20 @@ describe "String#split with Regexp" do
           last.should == "b"
         end
       end
+
+      ruby_version_is '3.0' do
+        it "yields instances of String" do
+          a = []
+          StringSpecs::MyString.new("a|b").split("|") { |str| a << str }
+          first, last = a
+
+          first.should be_an_instance_of(String)
+          first.should == "a"
+
+          last.should be_an_instance_of(String)
+          last.should == "b"
+        end
+      end
+    end
   end
 end

spec/ruby/core/string/squeeze_spec.rb

@@ -80,11 +80,19 @@ describe "String#squeeze" do
     -> { "hello world".squeeze(mock('x')) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("oh no!!!").squeeze("!").should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("oh no!!!").squeeze("!").should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#squeeze!" do
   it "modifies self in place and returns self" do
     a = "yellow moon"

spec/ruby/core/string/sub_spec.rb

@@ -192,12 +192,23 @@ describe "String#sub with pattern, replacement" do
     -> { "hello".sub(/[aeiou]/, 99) }.should raise_error(TypeError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("").sub(//, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("").sub(/foo/, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").sub(/foo/, "").should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("foo").sub("foo", "").should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("").sub(//, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("").sub(/foo/, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").sub(/foo/, "").should be_an_instance_of(String)
+      StringSpecs::MyString.new("foo").sub("foo", "").should be_an_instance_of(String)
+    end
+  end
 
   it "sets $~ to MatchData of match and nil when there's none" do
     'hello.'.sub('hello', 'x')

spec/ruby/core/string/swapcase_spec.rb

@@ -73,12 +73,21 @@ describe "String#swapcase" do
     -> { "abc".swapcase(:invalid_option) }.should raise_error(ArgumentError)
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("").swapcase.should be_an_instance_of(StringSpecs::MyString)
       StringSpecs::MyString.new("hello").swapcase.should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("").swapcase.should be_an_instance_of(String)
+      StringSpecs::MyString.new("hello").swapcase.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#swapcase!" do
   it "modifies self in place" do
     a = "cYbEr_PuNk11"

spec/ruby/core/string/tr_s_spec.rb

@@ -45,9 +45,17 @@ describe "String#tr_s" do
     "bla".tr_s(from_str, to_str).should == "BlA"
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("hello").tr_s("e", "a").should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances when called on a subclass" do
+      StringSpecs::MyString.new("hello").tr_s("e", "a").should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "taints the result when self is tainted" do

spec/ruby/core/string/tr_spec.rb

@@ -57,9 +57,17 @@ describe "String#tr" do
     "bla".tr(from_str, to_str).should == "BlA"
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances when called on a subclass" do
       StringSpecs::MyString.new("hello").tr("e", "a").should be_an_instance_of(StringSpecs::MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns Stringinstances when called on a subclass" do
+      StringSpecs::MyString.new("hello").tr("e", "a").should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "taints the result when self is tainted" do

spec/ruby/core/string/upcase_spec.rb

@@ -73,11 +73,19 @@ describe "String#upcase" do
     end
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns a subclass instance for subclasses" do
       StringSpecs::MyString.new("fooBAR").upcase.should be_an_instance_of(StringSpecs::MyString)
     end
   end
 
+  ruby_version_is '3.0' do
+    it "returns a String instance for subclasses" do
+      StringSpecs::MyString.new("fooBAR").upcase.should be_an_instance_of(String)
+    end
+  end
+end
+
 describe "String#upcase!" do
   it "modifies self in place" do
     a = "HeLlO"

spec/ruby/shared/string/times.rb

@@ -32,11 +32,21 @@ describe :string_times, shared: true do
     @object.call("", max_long).should == ""
   end
 
+  ruby_version_is ''...'3.0' do
     it "returns subclass instances" do
       @object.call(MyString.new("cool"), 0).should be_an_instance_of(MyString)
       @object.call(MyString.new("cool"), 1).should be_an_instance_of(MyString)
       @object.call(MyString.new("cool"), 2).should be_an_instance_of(MyString)
     end
+  end
+
+  ruby_version_is '3.0' do
+    it "returns String instances" do
+      @object.call(MyString.new("cool"), 0).should be_an_instance_of(String)
+      @object.call(MyString.new("cool"), 1).should be_an_instance_of(String)
+      @object.call(MyString.new("cool"), 2).should be_an_instance_of(String)
+    end
+  end
 
   ruby_version_is ''...'2.7' do
     it "always taints the result when self is tainted" do

string.c

@@ -1228,6 +1228,13 @@ rb_str_new_frozen(VALUE orig)
     return str_new_frozen(rb_obj_class(orig), orig);
 }
 
+static VALUE
+rb_str_new_frozen_String(VALUE orig)
+{
+    if (OBJ_FROZEN(orig) && rb_obj_class(orig) == rb_cString) return orig;
+    return str_new_frozen(rb_cString, orig);
+}
+
 VALUE
 rb_str_tmp_frozen_acquire(VALUE orig)
 {
@@ -1336,6 +1343,14 @@ str_new_empty(VALUE str)
     return v;
 }
 
+static VALUE
+str_new_empty_String(VALUE str)
+{
+    VALUE v = rb_str_new(0, 0);
+    rb_enc_copy(v, str);
+    return v;
+}
+
 #define STR_BUF_MIN_SIZE 63
 STATIC_ASSERT(STR_BUF_MIN_SIZE, STR_BUF_MIN_SIZE > RSTRING_EMBED_LEN_MAX);
 
@@ -2036,10 +2051,10 @@ rb_str_times(VALUE str, VALUE times)
     int termlen;
 
     if (times == INT2FIX(1)) {
-	return rb_str_dup(str);
+        return str_duplicate(rb_cString, str);
     }
     if (times == INT2FIX(0)) {
-	str2 = str_alloc(rb_obj_class(str));
+        str2 = str_alloc(rb_cString);
 	rb_enc_copy(str2, str);
 	return str2;
     }
@@ -2048,7 +2063,7 @@ rb_str_times(VALUE str, VALUE times)
 	rb_raise(rb_eArgError, "negative argument");
     }
     if (RSTRING_LEN(str) == 1 && RSTRING_PTR(str)[0] == 0) {
-       str2 = str_alloc(rb_obj_class(str));
+       str2 = str_alloc(rb_cString);
        if (!STR_EMBEDDABLE_P(len, 1)) {
            RSTRING(str2)->as.heap.aux.capa = len;
            RSTRING(str2)->as.heap.ptr = ZALLOC_N(char, (size_t)len + 1);
@@ -2064,7 +2079,7 @@ rb_str_times(VALUE str, VALUE times)
 
     len *= RSTRING_LEN(str);
     termlen = TERM_LEN(str);
-    str2 = str_new0(rb_obj_class(str), 0, len, termlen);
+    str2 = str_new0(rb_cString, 0, len, termlen);
     ptr2 = RSTRING_PTR(str2);
     if (len) {
         n = RSTRING_LEN(str);
@@ -2545,13 +2560,13 @@ rb_str_subseq(VALUE str, long beg, long len)
     if (!STR_EMBEDDABLE_P(len, TERM_LEN(str)) &&
 	SHARABLE_SUBSTRING_P(beg, len, RSTRING_LEN(str))) {
 	long olen;
-	str2 = rb_str_new_shared(rb_str_new_frozen(str));
+        str2 = rb_str_new_shared(rb_str_new_frozen_String(str));
 	RSTRING(str2)->as.heap.ptr += beg;
 	olen = RSTRING(str2)->as.heap.len;
 	if (olen > len) RSTRING(str2)->as.heap.len = len;
     }
     else {
-        str2 = rb_str_new_with_class(str, RSTRING_PTR(str)+beg, len);
+        str2 = rb_str_new(RSTRING_PTR(str)+beg, len);
 	RB_GC_GUARD(str);
     }
 
@@ -2664,14 +2679,14 @@ str_substr(VALUE str, long beg, long len, int empty)
 	SHARABLE_SUBSTRING_P(p, len, RSTRING_END(str))) {
 	long ofs = p - RSTRING_PTR(str);
 	str2 = rb_str_new_frozen(str);
-	str2 = str_new_shared(rb_obj_class(str2), str2);
+        str2 = str_new_shared(rb_cString, str2);
 	RSTRING(str2)->as.heap.ptr += ofs;
 	RSTRING(str2)->as.heap.len = len;
 	ENC_CODERANGE_CLEAR(str2);
     }
     else {
 	if (!len && !empty) return Qnil;
-	str2 = rb_str_new_with_class(str, p, len);
+        str2 = rb_str_new(p, len);
 	RB_GC_GUARD(str);
     }
     rb_enc_cr_str_copy_for_substr(str2, str);
@@ -4215,7 +4230,7 @@ VALUE
 rb_str_succ(VALUE orig)
 {
     VALUE str;
-    str = rb_str_new_with_class(orig, RSTRING_PTR(orig), RSTRING_LEN(orig));
+    str = rb_str_new(RSTRING_PTR(orig), RSTRING_LEN(orig));
     rb_enc_cr_str_copy_for_substr(str, orig);
     return str_succ(str);
 }
@@ -4444,7 +4459,7 @@ rb_str_upto_each(VALUE beg, VALUE end, int excl, int (*each)(VALUE, VALUE), VALU
     if (n > 0 || (excl && n == 0)) return beg;
 
     after_end = rb_funcallv(end, succ, 0, 0);
-    current = rb_str_dup(beg);
+    current = str_duplicate(rb_cString, beg);
     while (!rb_str_equal(current, after_end)) {
 	VALUE next = Qnil;
 	if (excl || !rb_str_equal(current, end))
@@ -4492,7 +4507,7 @@ rb_str_upto_endless_each(VALUE beg, int (*each)(VALUE, VALUE), VALUE arg)
 	}
     }
     /* normal case */
-    current = rb_str_dup(beg);
+    current = str_duplicate(rb_cString, beg);
     while (1) {
 	VALUE next = rb_funcallv(current, succ, 0, 0);
 	if ((*each)(current, arg)) break;
@@ -4582,7 +4597,7 @@ rb_str_aref(VALUE str, VALUE indx)
     }
     else if (RB_TYPE_P(indx, T_STRING)) {
 	if (rb_str_index(str, indx, 0) != -1)
-	    return rb_str_dup(indx);
+            return str_duplicate(rb_cString, indx);
 	return Qnil;
     }
     else {
@@ -5005,7 +5020,7 @@ rb_str_slice_bang(int argc, VALUE *argv, VALUE str)
 	beg = rb_str_index(str, indx, 0);
 	if (beg == -1) return Qnil;
 	len = RSTRING_LEN(indx);
-	result = rb_str_dup(indx);
+        result = str_duplicate(rb_cString, indx);
         goto squash;
     }
     else {
@@ -5028,7 +5043,7 @@ rb_str_slice_bang(int argc, VALUE *argv, VALUE str)
     beg = p - RSTRING_PTR(str);
 
   subseq:
-    result = rb_str_new_with_class(str, RSTRING_PTR(str)+beg, len);
+    result = rb_str_new(RSTRING_PTR(str)+beg, len);
     rb_enc_cr_str_copy_for_substr(result, str);
 
   squash:
@@ -5107,7 +5122,7 @@ rb_pat_search(VALUE pat, VALUE str, long pos, int set_backref_str)
 	pos = rb_strseq_index(str, pat, pos, 1);
 	if (set_backref_str) {
 	    if (pos >= 0) {
-		str = rb_str_new_frozen(str);
+                str = rb_str_new_frozen_String(str);
 		rb_backref_set_string(str, pos, RSTRING_LEN(pat));
 	    }
 	    else {
@@ -5301,7 +5316,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 static VALUE
 rb_str_sub(int argc, VALUE *argv, VALUE str)
 {
-    str = rb_str_dup(str);
+    str = str_duplicate(rb_cString, str);
     rb_str_sub_bang(argc, argv, str);
     return str;
 }
@@ -5341,7 +5356,7 @@ str_gsub(int argc, VALUE *argv, VALUE str, int bang)
     beg = rb_pat_search(pat, str, 0, need_backref);
     if (beg < 0) {
 	if (bang) return Qnil;	/* no match, no substitution */
-	return rb_str_dup(str);
+        return str_duplicate(rb_cString, str);
     }
 
     offset = 0;
@@ -5422,7 +5437,6 @@ str_gsub(int argc, VALUE *argv, VALUE str, int bang)
         str_shared_replace(str, dest);
     }
     else {
-	RBASIC_SET_CLASS(dest, rb_obj_class(str));
 	str = dest;
     }
 
@@ -5685,12 +5699,12 @@ str_byte_substr(VALUE str, long beg, long len, int empty)
 
     if (!STR_EMBEDDABLE_P(len, TERM_LEN(str)) && SHARABLE_SUBSTRING_P(beg, len, n)) {
 	str2 = rb_str_new_frozen(str);
-	str2 = str_new_shared(rb_obj_class(str2), str2);
+        str2 = str_new_shared(rb_cString, str2);
 	RSTRING(str2)->as.heap.ptr += beg;
 	RSTRING(str2)->as.heap.len = len;
     }
     else {
-	str2 = rb_str_new_with_class(str, p, len);
+        str2 = rb_str_new(p, len);
     }
 
     str_enc_copy(str2, str);
@@ -5792,9 +5806,9 @@ rb_str_reverse(VALUE str)
     char *s, *e, *p;
     int cr;
 
-    if (RSTRING_LEN(str) <= 1) return rb_str_dup(str);
+    if (RSTRING_LEN(str) <= 1) return str_duplicate(rb_cString, str);
     enc = STR_ENC_GET(str);
-    rev = rb_str_new_with_class(str, 0, RSTRING_LEN(str));
+    rev = rb_str_new(0, RSTRING_LEN(str));
     s = RSTRING_PTR(str); e = RSTRING_END(str);
     p = RSTRING_END(rev);
     cr = ENC_CODERANGE(str);
@@ -6273,7 +6287,7 @@ rb_str_dump(VALUE str)
 	len += clen;
     }
 
-    result = rb_str_new_with_class(str, 0, len);
+    result = rb_str_new(0, len);
     p = RSTRING_PTR(str); pend = p + RSTRING_LEN(str);
     q = RSTRING_PTR(result); qend = q + len + 1;
 
@@ -6696,7 +6710,7 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
     size_t buffer_count = 0;
     int buffer_length_or_invalid;
 
-    if (RSTRING_LEN(source) == 0) return rb_str_dup(source);
+    if (RSTRING_LEN(source) == 0) return str_duplicate(rb_cString, source);
 
     source_current = (OnigUChar*)RSTRING_PTR(source);
     source_end = (OnigUChar*)RSTRING_END(source);
@@ -6732,12 +6746,12 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
     }
 
     if (buffer_count==1) {
-	target = rb_str_new_with_class(source, (const char*)current_buffer->space, target_length);
+        target = rb_str_new((const char*)current_buffer->space, target_length);
     }
     else {
 	char *target_current;
 
-	target = rb_str_new_with_class(source, 0, target_length);
+        target = rb_str_new(0, target_length);
 	target_current = RSTRING_PTR(target);
         current_buffer = DATA_PTR(buffer_anchor);
 	while (current_buffer) {
@@ -6870,12 +6884,12 @@ rb_str_upcase(int argc, VALUE *argv, VALUE str)
     flags = check_case_options(argc, argv, flags);
     enc = str_true_enc(str);
     if (case_option_single_p(flags, enc, str)) {
-        ret = rb_str_new_with_class(str, RSTRING_PTR(str), RSTRING_LEN(str));
+        ret = rb_str_new(RSTRING_PTR(str), RSTRING_LEN(str));
         str_enc_copy(ret, str);
         upcase_single(ret);
     }
     else if (flags&ONIGENC_CASE_ASCII_ONLY) {
-        ret = rb_str_new_with_class(str, 0, RSTRING_LEN(str));
+        ret = rb_str_new(0, RSTRING_LEN(str));
         rb_str_ascii_casemap(str, ret, &flags, enc);
     }
     else {
@@ -6998,12 +7012,12 @@ rb_str_downcase(int argc, VALUE *argv, VALUE str)
     flags = check_case_options(argc, argv, flags);
     enc = str_true_enc(str);
     if (case_option_single_p(flags, enc, str)) {
-        ret = rb_str_new_with_class(str, RSTRING_PTR(str), RSTRING_LEN(str));
+        ret = rb_str_new(RSTRING_PTR(str), RSTRING_LEN(str));
         str_enc_copy(ret, str);
         downcase_single(ret);
     }
     else if (flags&ONIGENC_CASE_ASCII_ONLY) {
-        ret = rb_str_new_with_class(str, 0, RSTRING_LEN(str));
+        ret = rb_str_new(0, RSTRING_LEN(str));
         rb_str_ascii_casemap(str, ret, &flags, enc);
     }
     else {
@@ -7078,7 +7092,7 @@ rb_str_capitalize(int argc, VALUE *argv, VALUE str)
     enc = str_true_enc(str);
     if (RSTRING_LEN(str) == 0 || !RSTRING_PTR(str)) return str;
     if (flags&ONIGENC_CASE_ASCII_ONLY) {
-        ret = rb_str_new_with_class(str, 0, RSTRING_LEN(str));
+        ret = rb_str_new(0, RSTRING_LEN(str));
         rb_str_ascii_casemap(str, ret, &flags, enc);
     }
     else {
@@ -7142,9 +7156,9 @@ rb_str_swapcase(int argc, VALUE *argv, VALUE str)
 
     flags = check_case_options(argc, argv, flags);
     enc = str_true_enc(str);
-    if (RSTRING_LEN(str) == 0 || !RSTRING_PTR(str)) return str;
+    if (RSTRING_LEN(str) == 0 || !RSTRING_PTR(str)) return str_duplicate(rb_cString, str);
     if (flags&ONIGENC_CASE_ASCII_ONLY) {
-        ret = rb_str_new_with_class(str, 0, RSTRING_LEN(str));
+        ret = rb_str_new(0, RSTRING_LEN(str));
         rb_str_ascii_casemap(str, ret, &flags, enc);
     }
     else {
@@ -7518,7 +7532,7 @@ rb_str_tr_bang(VALUE str, VALUE src, VALUE repl)
 static VALUE
 rb_str_tr(VALUE str, VALUE src, VALUE repl)
 {
-    str = rb_str_dup(str);
+    str = str_duplicate(rb_cString, str);
     tr_trans(str, src, repl, 0);
     return str;
 }
@@ -7697,7 +7711,7 @@ rb_str_delete_bang(int argc, VALUE *argv, VALUE str)
 static VALUE
 rb_str_delete(int argc, VALUE *argv, VALUE str)
 {
-    str = rb_str_dup(str);
+    str = str_duplicate(rb_cString, str);
     rb_str_delete_bang(argc, argv, str);
     return str;
 }
@@ -7805,7 +7819,7 @@ rb_str_squeeze_bang(int argc, VALUE *argv, VALUE str)
 static VALUE
 rb_str_squeeze(int argc, VALUE *argv, VALUE str)
 {
-    str = rb_str_dup(str);
+    str = str_duplicate(rb_cString, str);
     rb_str_squeeze_bang(argc, argv, str);
     return str;
 }
@@ -7842,7 +7856,7 @@ rb_str_tr_s_bang(VALUE str, VALUE src, VALUE repl)
 static VALUE
 rb_str_tr_s(VALUE str, VALUE src, VALUE repl)
 {
-    str = rb_str_dup(str);
+    str = str_duplicate(rb_cString, str);
     tr_trans(str, src, repl, 1);
     return str;
 }
@@ -7986,12 +8000,12 @@ split_string(VALUE result, VALUE str, long beg, long len, long empty_count)
 	/* make different substrings */
 	if (result) {
 	    do {
-		rb_ary_push(result, str_new_empty(str));
+                rb_ary_push(result, str_new_empty_String(str));
 	    } while (--empty_count > 0);
 	}
 	else {
 	    do {
-		rb_yield(str_new_empty(str));
+                rb_yield(str_new_empty_String(str));
 	    } while (--empty_count > 0);
 	}
     }
@@ -8107,7 +8121,7 @@ rb_str_split_m(int argc, VALUE *argv, VALUE str)
 	else if (lim == 1) {
 	    if (RSTRING_LEN(str) == 0)
                 return result ? rb_ary_new2(0) : str;
-	    tmp = rb_str_dup(str);
+            tmp = str_duplicate(rb_cString, str);
 	    if (!result) {
 		rb_yield(tmp);
                 return str;
@@ -9194,7 +9208,7 @@ static VALUE
 rb_str_chomp(int argc, VALUE *argv, VALUE str)
 {
     VALUE rs = chomp_rs(argc, argv);
-    if (NIL_P(rs)) return rb_str_dup(str);
+    if (NIL_P(rs)) return str_duplicate(rb_cString, str);
     return rb_str_subseq(str, 0, chompped_length(str, rs));
 }
 
@@ -9281,7 +9295,7 @@ rb_str_lstrip(VALUE str)
     long len, loffset;
     RSTRING_GETMEM(str, start, len);
     loffset = lstrip_offset(str, start, start+len, STR_ENC_GET(str));
-    if (loffset <= 0) return rb_str_dup(str);
+    if (loffset <= 0) return str_duplicate(rb_cString, str);
     return rb_str_subseq(str, loffset, len - loffset);
 }
 
@@ -9374,7 +9388,7 @@ rb_str_rstrip(VALUE str)
     RSTRING_GETMEM(str, start, olen);
     roffset = rstrip_offset(str, start, start+olen, enc);
 
-    if (roffset <= 0) return rb_str_dup(str);
+    if (roffset <= 0) return str_duplicate(rb_cString, str);
     return rb_str_subseq(str, 0, olen-roffset);
 }
 
@@ -9447,7 +9461,7 @@ rb_str_strip(VALUE str)
     loffset = lstrip_offset(str, start, start+olen, enc);
     roffset = rstrip_offset(str, start+loffset, start+olen, enc);
 
-    if (loffset <= 0 && roffset <= 0) return rb_str_dup(str);
+    if (loffset <= 0 && roffset <= 0) return str_duplicate(rb_cString, str);
     return rb_str_subseq(str, loffset, olen-loffset-roffset);
 }
 
@@ -9843,7 +9857,7 @@ rb_str_justify(int argc, VALUE *argv, VALUE str, char jflag)
 	}
     }
     len = str_strlen(str, enc); /* rb_enc_check */
-    if (width < 0 || len >= width) return rb_str_dup(str);
+    if (width < 0 || len >= width) return str_duplicate(rb_cString, str);
     n = width - len;
     llen = (jflag == 'l') ? 0 : ((jflag == 'r') ? n : n/2);
     rlen = n - llen;
@@ -9859,7 +9873,7 @@ rb_str_justify(int argc, VALUE *argv, VALUE str, char jflag)
        rb_raise(rb_eArgError, "argument too big");
     }
     len += size;
-    res = str_new0(rb_obj_class(str), 0, len, termlen);
+    res = str_new0(rb_cString, 0, len, termlen);
     p = RSTRING_PTR(res);
     if (flen <= 1) {
        memset(p, *f, llen);
@@ -10006,7 +10020,7 @@ rb_str_partition(VALUE str, VALUE sep)
 					     RSTRING_LEN(str)-pos-RSTRING_LEN(sep)));
 
   failed:
-    return rb_ary_new3(3, rb_str_dup(str), str_new_empty(str), str_new_empty(str));
+    return rb_ary_new3(3, str_duplicate(rb_cString, str), str_new_empty_String(str), str_new_empty_String(str));
 }
 
 /*
@@ -10054,7 +10068,7 @@ rb_str_rpartition(VALUE str, VALUE sep)
 		          rb_str_subseq(str, pos+RSTRING_LEN(sep),
 					RSTRING_LEN(str)-pos-RSTRING_LEN(sep)));
   failed:
-    return rb_ary_new3(3, str_new_empty(str), str_new_empty(str), rb_str_dup(str));
+    return rb_ary_new3(3, str_new_empty_String(str), str_new_empty_String(str), str_duplicate(rb_cString, str));
 }
 
 /*
@@ -10200,7 +10214,7 @@ rb_str_delete_prefix(VALUE str, VALUE prefix)
     long prefixlen;
 
     prefixlen = deleted_prefix_length(str, prefix);
-    if (prefixlen <= 0) return rb_str_dup(str);
+    if (prefixlen <= 0) return str_duplicate(rb_cString, str);
 
     return rb_str_subseq(str, prefixlen, RSTRING_LEN(str) - prefixlen);
 }
@@ -10286,7 +10300,7 @@ rb_str_delete_suffix(VALUE str, VALUE suffix)
     long suffixlen;
 
     suffixlen = deleted_suffix_length(str, suffix);
-    if (suffixlen <= 0) return rb_str_dup(str);
+    if (suffixlen <= 0) return str_duplicate(rb_cString, str);
 
     return rb_str_subseq(str, 0, RSTRING_LEN(str) - suffixlen);
 }
@@ -10417,7 +10431,7 @@ rb_str_ellipsize(VALUE str, long len)
     else if (len <= ellipsislen ||
 	     !(e = rb_enc_step_back(p, e, e, len = ellipsislen, enc))) {
 	if (rb_enc_asciicompat(enc)) {
-	    ret = rb_str_new_with_class(str, ellipsis, len);
+            ret = rb_str_new(ellipsis, len);
 	    rb_enc_associate(ret, enc);
 	}
 	else {
@@ -10742,7 +10756,7 @@ str_scrub(int argc, VALUE *argv, VALUE str)
 {
     VALUE repl = argc ? (rb_check_arity(argc, 0, 1), argv[0]) : Qnil;
     VALUE new = rb_str_scrub(str, repl);
-    return NIL_P(new) ? rb_str_dup(str): new;
+    return NIL_P(new) ? str_duplicate(rb_cString, str): new;
 }
 
 /*

test/-ext-/string/test_cstr.rb

@@ -13,13 +13,13 @@ class Test_StringCStr < Test::Unit::TestCase
   end
 
   def test_long
-    s = Bug::String.new("abcdef")*100000
+    s = Bug::String.new(Bug::String.new("abcdef")*100000)
     s.cstr_unterm('x')
     assert_equal(0, s.cstr_term, Bug4319)
   end
 
   def test_shared
-    s = Bug::String.new("abcdef")*5
+    s = Bug::String.new(Bug::String.new("abcdef")*5)
     s = s.unterminated_substring(0, 29)
     assert_equal(0, s.cstr_term, Bug4319)
   end
@@ -28,7 +28,7 @@ class Test_StringCStr < Test::Unit::TestCase
     s0 = Bug::String.new("abcdefgh"*8)
 
     [4, 4*3-1, 8*3-1, 64].each do |n|
-      s = s0[0, n]
+      s = Bug::String.new(s0[0, n])
       s.cstr_unterm('x')
       s.freeze
       assert_equal(0, s.cstr_term)
@@ -67,7 +67,7 @@ class Test_StringCStr < Test::Unit::TestCase
     n = 100
     len = str.size * n
     WCHARS.each do |enc|
-      s = Bug::String.new(str.encode(enc))*n
+      s = Bug::String.new(Bug::String.new(str.encode(enc))*n)
       s.cstr_unterm('x')
       assert_nothing_raised(ArgumentError, enc.name) {s.cstr_term}
       s.set_len(s.bytesize / 2)

test/-ext-/string/test_ellipsize.rb

@@ -10,7 +10,7 @@ class Test_StringEllipsize < Test::Unit::TestCase
   def assert_equal_with_class(expected, result, *rest)
     assert_equal(expected.encoding, result.encoding, *rest)
     assert_equal(expected, result, result.encoding.name)
-    assert_instance_of(Bug::String, result, *rest)
+    assert_instance_of(String, result, *rest)
   end
 
   def test_longer

test/ruby/test_string.rb

@@ -2089,6 +2089,8 @@ CODE
 
   def test_swapcase
     assert_equal(S("hi&LOW"), S("HI&low").swapcase)
+    s = S("")
+    assert_not_same(s, s.swapcase)
   end
 
   def test_swapcase!