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>

3 files changed, 19 insertions, 15 deletions

diff --git a/spec/ruby/core/module/ruby2_keywords_spec.rb b/spec/ruby/core/module/ruby2_keywords_spec.rb
index dc16d712c7..a72612a670 100644
--- a/spec/ruby/core/module/ruby2_keywords_spec.rb
+++ b/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
diff --git a/spec/ruby/language/hash_spec.rb b/spec/ruby/language/hash_spec.rb
index 9e2b9bd4c5..6ac382c42c 100644
--- a/spec/ruby/language/hash_spec.rb
+++ b/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
diff --git a/spec/ruby/language/keyword_arguments_spec.rb b/spec/ruby/language/keyword_arguments_spec.rb
index e2c816f622..ffb5b1fab0 100644
--- a/spec/ruby/language/keyword_arguments_spec.rb
+++ b/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