Warn suspicious flag to Regexp.new

Now second argument should be `true`, `false`, `nil` or Integer.
This flag is confused with third argument some times.

NEWS.md

@@ -118,6 +118,10 @@ Note: We're only listing outstanding class updates.
     * Proc#dup returns an instance of subclass. [[Bug #17545]]
     * Proc#parameters now accepts lambda keyword. [[Feature #15357]]
 
+* Regexp
+    * Regexp.new now warns second argument, other than `true`, `false`,
+      `nil` or Integer.  [[Feature #18788]]
+
 * Refinement
     * Refinement#refined_class has been added. [[Feature #12737]]
 
@@ -263,3 +267,4 @@ The following deprecated APIs are removed.
 [Bug #18625]:     https://bugs.ruby-lang.org/issues/18625
 [Bug #18633]:     https://bugs.ruby-lang.org/issues/18633
 [Bug #18782]:     https://bugs.ruby-lang.org/issues/18782
+[Feature #18788]: https://bugs.ruby-lang.org/issues/18788

common.mk

@@ -12057,12 +12057,15 @@ re.$(OBJEXT): $(hdrdir)/ruby.h
 re.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 re.$(OBJEXT): $(top_srcdir)/internal/array.h
 re.$(OBJEXT): $(top_srcdir)/internal/bits.h
+re.$(OBJEXT): $(top_srcdir)/internal/class.h
 re.$(OBJEXT): $(top_srcdir)/internal/compilers.h
 re.$(OBJEXT): $(top_srcdir)/internal/gc.h
 re.$(OBJEXT): $(top_srcdir)/internal/hash.h
 re.$(OBJEXT): $(top_srcdir)/internal/imemo.h
+re.$(OBJEXT): $(top_srcdir)/internal/object.h
 re.$(OBJEXT): $(top_srcdir)/internal/ractor.h
 re.$(OBJEXT): $(top_srcdir)/internal/re.h
+re.$(OBJEXT): $(top_srcdir)/internal/serial.h
 re.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 re.$(OBJEXT): $(top_srcdir)/internal/string.h
 re.$(OBJEXT): $(top_srcdir)/internal/time.h

re.c

@@ -20,6 +20,7 @@
 #include "internal/imemo.h"
 #include "internal/re.h"
 #include "internal/string.h"
+#include "internal/object.h"
 #include "internal/ractor.h"
 #include "internal/variable.h"
 #include "regint.h"
@@ -3716,7 +3717,8 @@ rb_reg_initialize_m(int argc, VALUE *argv, VALUE self)
     else {
         if (opts != Qundef) {
 	    if (FIXNUM_P(opts)) flags = FIX2INT(opts);
-	    else if (RTEST(opts)) flags = ONIG_OPTION_IGNORECASE;
+	    else if (!NIL_P(opts) && rb_bool_expected(opts, "ignorecase", FALSE))
+		flags = ONIG_OPTION_IGNORECASE;
 	}
         if (n_flag != Qundef && !NIL_P(n_flag)) {
 	    char *kcode = StringValuePtr(n_flag);

test/ruby/test_regexp.rb

@@ -628,6 +628,12 @@ class TestRegexp < Test::Unit::TestCase
     assert_raise(RegexpError) { Regexp.new("((?<v>))\\g<0>") }
   end
 
+  def test_initialize_bool_warning
+    assert_warning(/expected true or false as ignorecase/) do
+      Regexp.new("foo", :i)
+    end
+  end
+
   def test_match_control_meta_escape
     assert_equal(0, /\c\xFF/ =~ "\c\xFF")
     assert_equal(0, /\c\M-\xFF/ =~ "\c\M-\xFF")