CALLER_SETUP_ARG removes an empty keyword hash from argv

...only when a "remove_empty_keyword_hash" flag is specified.

After CALLER_SETUP_ARG is called, `ci->flag & VM_CALL_KW_SPLAT` must not
be used.  Instead. use `calling->kw_splat`.  This is because
CALLER_SETUP_ARG may modify argv and update `calling->kw_splat`, and
`ci->flag & VM_CALL_KW_SPLAT` may be inconsistent with the result.

test/ruby/test_keyword.rb

@@ -464,12 +464,8 @@ class TestKeywordArguments < Test::Unit::TestCase
     def c.m(arg, **args)
       [arg, args]
     end
-    assert_warn(/The keyword argument is passed as the last hash parameter.* for `m'/m) do
-      assert_equal([kw, kw], c.send(:m, **{}))
-    end
-    assert_warn(/The keyword argument is passed as the last hash parameter.* for `m'/m) do
-      assert_equal([kw, kw], c.send(:m, **kw))
-    end
+    assert_raise(ArgumentError) { c.send(:m, **{}) }
+    assert_raise(ArgumentError) { c.send(:m, **kw) }
     assert_warn(/The keyword argument is passed as the last hash parameter.* for `m'/m) do
       assert_equal([h, kw], c.send(:m, **h))
     end
@@ -643,12 +639,8 @@ class TestKeywordArguments < Test::Unit::TestCase
     def c.method_missing(_, arg, **args)
       [arg, args]
     end
-    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
-      assert_equal([kw, kw], c.send(:m, **{}))
-    end
-    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
-      assert_equal([kw, kw], c.send(:m, **kw))
-    end
+    assert_raise(ArgumentError) { c.send(:m, **{}) }
+    assert_raise(ArgumentError) { c.send(:m, **kw) }
     assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
       assert_equal([h, kw], c.send(:m, **h))
     end
@@ -737,9 +729,7 @@ class TestKeywordArguments < Test::Unit::TestCase
       define_method(:m) {|arg, **opt| [arg, opt] }
     end
     assert_raise(ArgumentError) { c.m(**{}) }
-    assert_warn(/The keyword argument is passed as the last hash parameter/m) do
-      assert_equal([kw, kw], c.m(**kw))
-    end
+    assert_raise(ArgumentError) { c.m(**kw) }
     assert_warn(/The keyword argument is passed as the last hash parameter/m) do
       assert_equal([h, kw], c.m(**h))
     end

vm_insnhelper.c

@@ -1740,7 +1740,7 @@ rb_iseq_only_kwparam_p(const rb_iseq_t *iseq)
 static inline void
 CALLER_SETUP_ARG(struct rb_control_frame_struct *restrict cfp,
                  struct rb_calling_info *restrict calling,
-                 const struct rb_call_info *restrict ci, int cfunc)
+                 const struct rb_call_info *restrict ci, int remove_empty_keyword_hash)
 {
     if (UNLIKELY(IS_ARGS_SPLAT(ci))) {
         vm_caller_setup_arg_splat(cfp, calling);
@@ -1748,6 +1748,13 @@ CALLER_SETUP_ARG(struct rb_control_frame_struct *restrict cfp,
     if (UNLIKELY(IS_ARGS_KEYWORD(ci))) {
         vm_caller_setup_arg_kw(cfp, calling, ci);
     }
+    if (UNLIKELY(calling->kw_splat && remove_empty_keyword_hash)) {
+        if (RHASH_EMPTY_P(cfp->sp[-1])) {
+            cfp->sp--;
+            calling->argc--;
+            calling->kw_splat = 0;
+        }
+    }
 }
 
 #define USE_OPT_HIST 0
@@ -2189,12 +2196,6 @@ vm_call_cfunc_with_frame(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp
     int argc = calling->argc;
     int orig_argc = argc;
 
-    if (UNLIKELY(calling->kw_splat)) {
-        if (RHASH_EMPTY_P(*(GET_SP()-1))) {
-            argc--;
-            calling->kw_splat = 0;
-        }
-    }
     if (UNLIKELY(IS_ARGS_KW_OR_KW_SPLAT(ci))) {
         frame_type |= VM_FRAME_FLAG_CFRAME_KW;
     }
@@ -2315,12 +2316,6 @@ vm_call_opt_send(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, struct
 	ci = &ci_entry.ci;
 	ci_entry.ci = *orig_ci;
     }
-    unsigned int kw_splat = ci->flag & VM_CALL_KW_SPLAT;
-    if (!kw_splat && (ci->flag & VM_CALL_KWARG)) {
-        /* TODO: delegate kw_arg without making a Hash object */
-        ci->flag = ci->flag & ~VM_CALL_KWARG;
-        kw_splat = VM_CALL_KW_SPLAT;
-    }
 
     /* setup new cc */
     cc_entry = *orig_cc;
@@ -2350,7 +2345,7 @@ vm_call_opt_send(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, struct
     }
 
     cc->me = rb_callable_method_entry_with_refinements(CLASS_OF(calling->recv), ci->mid, NULL);
-    ci->flag = VM_CALL_FCALL | VM_CALL_OPT_SEND | kw_splat;
+    ci->flag = VM_CALL_FCALL | VM_CALL_OPT_SEND | (calling->kw_splat ? VM_CALL_KW_SPLAT : 0);
     return vm_call_method(ec, reg_cfp, calling, ci, cc);
 }
 
@@ -2924,13 +2919,6 @@ vm_callee_setup_block_arg(rb_execution_context_t *ec, struct rb_calling_info *ca
 
 	CALLER_SETUP_ARG(cfp, calling, ci, 1); /* splat arg */
 
-        if (UNLIKELY(calling->kw_splat)) {
-            if (RHASH_EMPTY_P(argv[calling->argc-1])) {
-                calling->argc--;
-                calling->kw_splat = 0;
-            }
-        }
-
 	if (arg_setup_type == arg_setup_block &&
 	    calling->argc == 1 &&
 	    iseq->body->param.flags.has_lead &&