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]

1 files changed, 10 insertions, 2 deletions

diff --git a/spec/ruby/core/string/downcase_spec.rb b/spec/ruby/core/string/downcase_spec.rb
index 8c840bdcc4..4427c9df10 100644
--- a/spec/ruby/core/string/downcase_spec.rb
+++ b/spec/ruby/core/string/downcase_spec.rb
@@ -76,8 +76,16 @@ describe "String#downcase" do
     end
   end
 
-  it "returns a subclass instance for subclasses" do
-    StringSpecs::MyString.new("FOObar").downcase.should be_an_instance_of(StringSpecs::MyString)
+  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