Fix keyword splat passing as regular argument

Since Ruby 3.0, Ruby has passed a keyword splat as a regular
argument in the case of a call to a Ruby method where the
method does not accept keyword arguments, if the method
call does not contain an argument splat:

```ruby
def self.f(obj) obj end
def self.fs(*obj) obj[0] end
h = {a: 1}
f(**h).equal?(h)  # Before: true; After: false
fs(**h).equal?(h) # Before: true; After: false

a = []
f(*a, **h).equal?(h)  # Before and After: false
fs(*a, **h).equal?(h) # Before and After: false
```

The fact that the behavior differs when passing an empty
argument splat makes it obvious that something is not
working the way it is intended.  Ruby 2 always copied
the keyword splat hash, and that is the expected behavior
in Ruby 3.

This bug is because of a missed check in setup_parameters_complex.
If the keyword splat passed is not mutable, then it points to
an existing object and not a new object, and therefore it must
be copied.

Now, there are 3 specs for the broken behavior of directly
using the keyword splatted hash.  Fix two specs and add a
new version guard. Do not keep the specs for the broken
behavior for earlier Ruby versions, in case this fix is
backported. For the ruby2_keywords spec, just remove the
related line, since that line is unrelated to what the
spec is testing.

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

spec/ruby/core/module/ruby2_keywords_spec.rb

@@ -22,7 +22,6 @@ describe "Module#ruby2_keywords" do
     end
 
     h = {a: 1}
-    obj.regular(**h).should.equal?(h)
 
     last = mark(**h).last
     Hash.ruby2_keywords_hash?(last).should == true

spec/ruby/language/hash_spec.rb

@@ -220,15 +220,17 @@ describe "The ** operator" do
     h.should == { one: 1, two: 2 }
   end
 
-  it "does not copy when calling a method taking a positional Hash" do
-    def m(h)
-      h.delete(:one); h
-    end
+  ruby_bug "#20012", ""..."3.3" do
+    it "makes a copy when calling a method taking a positional Hash" do
+      def m(h)
+        h.delete(:one); h
+      end
 
-    h = { one: 1, two: 2 }
-    m(**h).should == { two: 2 }
-    m(**h).should.equal?(h)
-    h.should == { two: 2 }
+      h = { one: 1, two: 2 }
+      m(**h).should == { two: 2 }
+      m(**h).should_not.equal?(h)
+      h.should == { one: 1, two: 2 }
+    end
   end
 
   ruby_version_is "3.1" do

spec/ruby/language/keyword_arguments_spec.rb

@@ -87,13 +87,16 @@ describe "Keyword arguments" do
   end
 
   context "**" do
-    it "does not copy a non-empty Hash for a method taking (*args)" do
-      def m(*args)
-        args[0]
-      end
+    ruby_version_is "3.3" do
+      it "copies a non-empty Hash for a method taking (*args)" do
+        def m(*args)
+          args[0]
+        end
 
-      h = {a: 1}
-      m(**h).should.equal?(h)
+        h = {a: 1}
+        m(**h).should_not.equal?(h)
+        h.should == {a: 1}
+      end
     end
 
     it "copies the given Hash for a method taking (**kwargs)" do

test/ruby/test_keyword.rb

@@ -237,16 +237,16 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal(true, Hash.ruby2_keywords_hash?(marked))
   end
 
+  def assert_equal_not_same(kw, res)
+    assert_instance_of(Hash, res)
+    assert_equal(kw, res)
+    assert_not_same(kw, res)
+  end
+
   def test_keyword_splat_new
     kw = {}
     h = {a: 1}
 
-    def self.assert_equal_not_same(kw, res)
-      assert_instance_of(Hash, res)
-      assert_equal(kw, res)
-      assert_not_same(kw, res)
-    end
-
     def self.yo(**kw) kw end
     m = method(:yo)
     assert_equal(false, yo(**{}).frozen?)
@@ -459,6 +459,20 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal_not_same h, yo(*a, **h)
   end
 
+  def test_keyword_splat_to_non_keyword_method
+    h = {a: 1}.freeze
+
+    def self.yo(kw) kw end
+    assert_equal_not_same(h, yo(**h))
+    assert_equal_not_same(h, method(:yo).(**h))
+    assert_equal_not_same(h, :yo.to_proc.(self, **h))
+
+    def self.yoa(*kw) kw[0] end
+    assert_equal_not_same(h, yoa(**h))
+    assert_equal_not_same(h, method(:yoa).(**h))
+    assert_equal_not_same(h, :yoa.to_proc.(self, **h))
+  end
+
   def test_regular_kwsplat
     kw = {}
     h = {:a=>1}

vm_args.c

@@ -609,6 +609,10 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                 kw_flag &= ~(VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT);
             }
             else {
+                if (!(kw_flag & VM_CALL_KW_SPLAT_MUT)) {
+                    converted_keyword_hash = rb_hash_dup(converted_keyword_hash);
+                }
+
                 if (last_arg != converted_keyword_hash) {
                     last_arg = converted_keyword_hash;
                     args->argv[args->argc-1] = last_arg;