`Hash#dup` for kwsplat arguments

On `f(*a, **kw)` method calls, a rest keyword parameter is identically
same Hash object is passed and it should make `#dup`ed Hahs.

fix https://bugs.ruby-lang.org/issues/19526

compile.c

@@ -5789,6 +5789,16 @@ check_keyword(const NODE *node)
 }
 #endif
 
+static bool
+keyword_node_single_splat_p(NODE *kwnode)
+{
+    RUBY_ASSERT(keyword_node_p(kwnode));
+
+    NODE *node = kwnode->nd_head;
+    return node->nd_head == NULL &&
+           node->nd_next->nd_next == NULL;
+}
+
 static int
 setup_args_core(rb_iseq_t *iseq, LINK_ANCHOR *const args, const NODE *argn,
                 int dup_rest, unsigned int *flag_ptr, struct rb_callinfo_kwarg **kwarg_ptr)
@@ -5881,7 +5891,9 @@ setup_args_core(rb_iseq_t *iseq, LINK_ANCHOR *const args, const NODE *argn,
           if (kwnode) {
               // f(*a, k:1)
               *flag_ptr |= VM_CALL_KW_SPLAT;
-              *flag_ptr |= VM_CALL_KW_SPLAT_MUT;
+              if (!keyword_node_single_splat_p(kwnode)) {
+                  *flag_ptr |= VM_CALL_KW_SPLAT_MUT;
+              }
               compile_hash(iseq, args, kwnode, TRUE, FALSE);
               argc += 1;
           }

test/ruby/test_keyword.rb

@@ -447,6 +447,16 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal(false, public_send(:yo, **{}).frozen?)
     assert_equal_not_same(kw, public_send(:yo, **kw))
     assert_equal_not_same(h, public_send(:yo, **h))
+
+    def self.yo(*a, **kw) = kw
+    assert_equal_not_same kw, yo(**kw)
+    assert_equal_not_same kw, yo(**kw, **kw)
+
+    singleton_class.send(:remove_method, :yo)
+    def self.yo(opts) = opts
+    assert_equal_not_same h, yo(*[], **h)
+    a = []
+    assert_equal_not_same h, yo(*a, **h)
   end
 
   def test_regular_kwsplat

vm_args.c

@@ -450,6 +450,18 @@ ignore_keyword_hash_p(VALUE keyword_hash, const rb_iseq_t * const iseq, unsigned
            RHASH_EMPTY_P(keyword_hash);
 }
 
+static VALUE
+check_kwrestarg(VALUE keyword_hash, unsigned int *kw_flag)
+{
+    if (!(*kw_flag & VM_CALL_KW_SPLAT_MUT)) {
+        *kw_flag |= VM_CALL_KW_SPLAT_MUT;
+        return rb_hash_dup(keyword_hash);
+    }
+    else {
+        return keyword_hash;
+    }
+}
+
 static int
 setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * const iseq,
                          struct rb_calling_info *const calling,
@@ -528,12 +540,14 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
             keyword_hash = Qnil;
         }
         else if (UNLIKELY(ISEQ_BODY(iseq)->param.flags.ruby2_keywords)) {
-            flag_keyword_hash = keyword_hash;
-            rb_ary_push(args->rest, keyword_hash);
+            converted_keyword_hash = check_kwrestarg(converted_keyword_hash, &kw_flag);
+            flag_keyword_hash = converted_keyword_hash;
+            rb_ary_push(args->rest, converted_keyword_hash);
             keyword_hash = Qnil;
         }
         else if (!ISEQ_BODY(iseq)->param.flags.has_kwrest && !ISEQ_BODY(iseq)->param.flags.has_kw) {
-            rb_ary_push(args->rest, keyword_hash);
+            converted_keyword_hash = check_kwrestarg(converted_keyword_hash, &kw_flag);
+            rb_ary_push(args->rest, converted_keyword_hash);
             keyword_hash = Qnil;
         }