Make ruby2_keywords methods correctly handle **{} optimization

Previously, this code:

  ruby2_keywords def foo(*a) a.last end
  foo(**{})

Returned an empty frozen hash.  However, the final hash should
not be frozen in this case, as it wouldn't be if foo accepted
a keyword splat.

Use a new unfrozen empty hash instead of reusing the frozen empty
hash in this case.

Fixes [Bug #16642]

2 files changed, 22 insertions, 0 deletions

diff --git a/test/ruby/test_keyword.rb b/test/ruby/test_keyword.rb
index bbf3953c17..a0459553ac 100644
--- a/test/ruby/test_keyword.rb
+++ b/test/ruby/test_keyword.rb
@@ -5051,4 +5051,18 @@ class TestKeywordArgumentsSymProcRefinements < Test::Unit::TestCase
       mock.new.foo
     end
   end
+
+  def test_ruby2_keywords_hash_empty_kw_splat
+    def self.foo(*a) a.last end
+    singleton_class.send(:ruby2_keywords, :foo)
+    bug16642 = '[ruby-core:97203] [Bug #16642]'
+
+    res = foo(**{})
+    assert_equal({}, res, bug16642)
+    assert_equal(false, res.frozen?, bug16642)
+
+    res = foo(*[], **{})
+    assert_equal({}, res, bug16642)
+    assert_equal(false, res.frozen?, bug16642)
+  end
 end
diff --git a/vm_args.c b/vm_args.c
index 7bf61cefe7..1efb84a0a3 100644
--- a/vm_args.c
+++ b/vm_args.c
@@ -821,6 +821,10 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                         kw_flag &= ~VM_CALL_KW_SPLAT;
                     }
                     else {
+                        if (RB_TYPE_P(rest_last, T_HASH) && rb_obj_frozen_p(rest_last)) {
+                            rest_last = rb_hash_new();
+                            RARRAY_ASET(args->rest, len - 1, rest_last);
+                        }
                         flag_keyword_hash = rest_last;
                     }
                 }
@@ -844,6 +848,10 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                         kw_flag &= ~VM_CALL_KW_SPLAT;
                     }
                     else {
+                        if (RB_TYPE_P(last_arg, T_HASH) && rb_obj_frozen_p(last_arg)) {
+                            last_arg = rb_hash_new();
+                            args->argv[args->argc-1] = last_arg;
+                        }
                         flag_keyword_hash = last_arg;
                     }
                 }