Copy encoding flags when copying a regex [Bug #20039]

* 🐛 Fixes [Bug #20039](https://bugs.ruby-lang.org/issues/20039)

When a Regexp is initialized with another Regexp, we simply copy the
properties from the original. However, the flags on the original were
not being copied correctly. This caused an issue when the original had
multibyte characters and was being compared with an ASCII string.
Without the forced encoding flag (`KCODE_FIXED`) transferred on to the
new Regexp, the comparison would fail. See the included test for an
example.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

re.c

@@ -3853,6 +3853,8 @@ reg_copy(VALUE copy, VALUE orig)
     RB_OBJ_WRITE(copy, &RREGEXP(copy)->src, RREGEXP(orig)->src);
     RREGEXP_PTR(copy)->timelimit = RREGEXP_PTR(orig)->timelimit;
     rb_enc_copy(copy, orig);
+    FL_SET_RAW(copy, FL_TEST_RAW(orig, KCODE_FIXED|REG_ENCODING_NONE));
+
     return copy;
 }
 

test/ruby/test_regexp.rb

@@ -1936,6 +1936,16 @@ def test_bug_19476 # [Bug #19476]
     assert_equal("123456789".match(/(?:x?\dx?){2,}/)[0], "123456789")
   end
 
+  def test_encoding_flags_are_preserved_when_initialized_with_another_regexp
+    re = Regexp.new("\u2018hello\u2019".encode("UTF-8"))
+    str = "".encode("US-ASCII")
+
+    assert_nothing_raised do
+      str.match?(re)
+      str.match?(Regexp.new(re))
+    end
+  end
+
   def test_bug_19537 # [Bug #19537]
     str = 'aac'
     re = '^([ab]{1,3})(a?)*$'