Fix Regexp.new - show warning "flags ignored" when passed regexp and options

Author: andrykonchin

spec/ruby/core/regexp/new_spec.rb

@@ -11,7 +11,7 @@
 
 describe "Regexp.new given a Regexp" do
   it_behaves_like :regexp_new_regexp, :new
-  it_behaves_like :regexp_new_string_binary, :compile
+  it_behaves_like :regexp_new_string_binary, :new
 end
 
 describe "Regexp.new given a non-String/Regexp" do

spec/tags/core/regexp/compile_tags.txt

@@ -1,5 +1,4 @@
 fails:Regexp.compile given a String ignores the third argument if it is 'e' or 'euc' (case-insensitive)
 fails:Regexp.compile given a String ignores the third argument if it is 's' or 'sjis' (case-insensitive)
 fails:Regexp.compile given a String ignores the third argument if it is 'u' or 'utf8' (case-insensitive)
-fails:Regexp.compile given a Regexp does not honour options given as additional arguments
 fails(immutable regexp):Regexp.compile works by default for subclasses with overridden #initialize

spec/tags/core/regexp/new_tags.txt

@@ -1,5 +1,4 @@
 fails:Regexp.new given a String ignores the third argument if it is 'e' or 'euc' (case-insensitive)
 fails:Regexp.new given a String ignores the third argument if it is 's' or 'sjis' (case-insensitive)
 fails:Regexp.new given a String ignores the third argument if it is 'u' or 'utf8' (case-insensitive)
-fails:Regexp.new given a Regexp does not honour options given as additional arguments
 fails(immutable regexp):Regexp.new works by default for subclasses with overridden #initialize

src/main/ruby/truffleruby/core/regexp.rb

@@ -130,8 +130,9 @@ def self.union(*patterns)
   end
   Truffle::Graal.always_split(method(:union))
 
-  def self.new(pattern, opts=nil, lang=nil)
+  def self.new(pattern, opts=undefined, lang=nil)
     if Primitive.object_kind_of?(pattern, Regexp)
+      warn 'flags ignored' unless Primitive.undefined?(opts)
       opts = pattern.options
       pattern = pattern.source
     elsif Primitive.object_kind_of?(pattern, String)
@@ -141,7 +142,7 @@ def self.new(pattern, opts=nil, lang=nil)
 
     if Primitive.object_kind_of?(opts, Integer)
       opts = opts & (OPTION_MASK | KCODE_MASK) if opts > 0
-    elsif opts
+    elsif !Primitive.undefined?(opts) && opts
       opts = IGNORECASE
     else
       opts = 0