Don't pass an empty keyword hash when double splatting empty hash when calling cfunc

This mirrors earlier changes in keyword argument separation for
calling Ruby methods and calling procs/lambdas, so that behavior
is kept the same.

test/ruby/test_keyword.rb

@@ -227,6 +227,144 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal([1, h3], f[**h3])
   end
 
+  def test_cfunc_kwsplat_call
+    kw = {}
+    h = {'a'=>1}
+    h2 = {'a'=>1}
+    h3 = {'a'=>1, :a=>1}
+
+    sc = Class.new do
+      attr_reader :args
+      class << self
+        alias [] new
+      end
+    end
+
+    c = Class.new(sc) do
+      def initialize(*args)
+        @args = args
+      end
+    end
+    assert_equal([], c[**{}].args)
+    assert_equal([], c[**kw].args)
+    assert_equal([h], c[**h].args)
+    assert_equal([h2], c[**h2].args)
+    assert_equal([h3], c[**h3].args)
+
+    c = Class.new(sc) do
+      def initialize; end
+    end
+    assert_nil(c[**{}].args)
+    assert_nil(c[**kw].args)
+    assert_raise(ArgumentError) { c[**h] }
+    assert_raise(ArgumentError) { c[**h2] }
+    assert_raise(ArgumentError) { c[**h3] }
+
+    c = Class.new(sc) do
+      def initialize(args)
+        @args = args
+      end
+    end
+    assert_raise(ArgumentError) { c[**{}] }
+    assert_raise(ArgumentError) { c[**kw] }
+    assert_equal(h, c[**h].args)
+    assert_equal(h2, c[**h2].args)
+    assert_equal(h3, c[**h3].args)
+
+    c = Class.new(sc) do
+      def initialize(**args)
+        @args = args
+      end
+    end
+    assert_equal(kw, c[**{}].args)
+    assert_equal(kw, c[**kw].args)
+    assert_equal(h, c[**h].args)
+    assert_equal(h2, c[**h2].args)
+    assert_equal(h3, c[**h3].args)
+
+    c = Class.new(sc) do
+      def initialize(arg, **args)
+        @args = [arg, args]
+      end
+    end
+    assert_raise(ArgumentError) { c[**{}] }
+    assert_raise(ArgumentError) { c[**kw] }
+    assert_equal([h, kw], c[**h].args)
+    assert_equal([h2, kw], c[**h2].args)
+    assert_equal([h3, kw], c[**h3].args)
+
+    c = Class.new(sc) do
+      def initialize(arg=1, **args)
+        @args = [arg=1, args]
+      end
+    end
+    assert_equal([1, kw], c[**{}].args)
+    assert_equal([1, kw], c[**kw].args)
+    assert_equal([1, h], c[**h].args)
+    assert_equal([1, h2], c[**h2].args)
+    assert_equal([1, h3], c[**h3].args)
+  end
+
+  def test_method_kwsplat_call
+    kw = {}
+    h = {'a'=>1}
+    h2 = {'a'=>1}
+    h3 = {'a'=>1, :a=>1}
+
+    c = Object.new
+    def c.m(*args)
+      args
+    end
+    assert_equal([], c.method(:m)[**{}])
+    assert_equal([], c.method(:m)[**kw])
+    assert_equal([h], c.method(:m)[**h])
+    assert_equal([h2], c.method(:m)[**h2])
+    assert_equal([h3], c.method(:m)[**h3])
+
+    def c.m; end
+    assert_nil(c.method(:m)[**{}])
+    assert_nil(c.method(:m)[**kw])
+    assert_raise(ArgumentError) { c.method(:m)[**h] }
+    assert_raise(ArgumentError) { c.method(:m)[**h2] }
+    assert_raise(ArgumentError) { c.method(:m)[**h3] }
+
+    def c.m(args)
+      args
+    end
+    assert_raise(ArgumentError) { c.method(:m)[**{}] }
+    assert_raise(ArgumentError) { c.method(:m)[**kw] }
+    assert_equal(h, c.method(:m)[**h])
+    assert_equal(h2, c.method(:m)[**h2])
+    assert_equal(h3, c.method(:m)[**h3])
+
+    def c.m(**args)
+      args
+    end
+    assert_equal(kw, c.method(:m)[**{}])
+    assert_equal(kw, c.method(:m)[**kw])
+    assert_equal(h, c.method(:m)[**h])
+    assert_equal(h2, c.method(:m)[**h2])
+    assert_equal(h3, c.method(:m)[**h3])
+
+    def c.m(arg, **args)
+      [arg, args]
+    end
+    assert_raise(ArgumentError) { c.method(:m)[**{}] }
+    assert_raise(ArgumentError) { c.method(:m)[**kw] }
+    assert_equal([h, kw], c.method(:m)[**h])
+    assert_equal([h2, kw], c.method(:m)[**h2])
+    assert_equal([h3, kw], c.method(:m)[**h3])
+
+    def c.m(arg=1, **args)
+      [arg=1, args]
+    end
+    assert_equal([1, kw], c.method(:m)[**{}])
+    assert_equal([1, kw], c.method(:m)[**kw])
+    assert_equal([1, h], c.method(:m)[**h])
+    assert_equal([1, h2], c.method(:m)[**h2])
+    assert_equal([1, h3], c.method(:m)[**h3])
+  end
+
   def p1
     Proc.new do |str: "foo", num: 424242|
       [str, num]

vm_insnhelper.c

@@ -2186,6 +2186,13 @@ vm_call_cfunc_with_frame(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp
     VALUE recv = calling->recv;
     VALUE block_handler = calling->block_handler;
     int argc = calling->argc;
+    int orig_argc = argc;
+
+    if (UNLIKELY(IS_ARGS_KW_SPLAT(ci))) {
+        if (RHASH_EMPTY_P(*(GET_SP()-1))) {
+            argc--;
+        }
+    }
 
     RUBY_DTRACE_CMETHOD_ENTRY_HOOK(ec, me->owner, me->def->original_id);
     EXEC_EVENT_HOOK(ec, RUBY_EVENT_C_CALL, recv, me->def->original_id, ci->mid, me->owner, Qundef);
@@ -2196,7 +2203,7 @@ vm_call_cfunc_with_frame(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp
 
     if (len >= 0) rb_check_arity(argc, len, len);
 
-    reg_cfp->sp -= argc + 1;
+    reg_cfp->sp -= orig_argc + 1;
     val = (*cfunc->invoker)(recv, argc, reg_cfp->sp + 1, cfunc->func);
 
     CHECK_CFP_CONSISTENCY("vm_call_cfunc");