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]

2 files changed, 5 insertions, 5 deletions

diff --git a/test/-ext-/string/test_cstr.rb b/test/-ext-/string/test_cstr.rb
index 2083016db2..d909781700 100644
--- a/test/-ext-/string/test_cstr.rb
+++ b/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)
diff --git a/test/-ext-/string/test_ellipsize.rb b/test/-ext-/string/test_ellipsize.rb
index d7947041d5..d340abd58a 100644
--- a/test/-ext-/string/test_ellipsize.rb
+++ b/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