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>

1 files changed, 20 insertions, 6 deletions

diff --git a/test/ruby/test_keyword.rb b/test/ruby/test_keyword.rb
index ef594bd52e..9aca787dff 100644
--- a/test/ruby/test_keyword.rb
+++ b/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}