Support **nil syntax for specifying a method does not accept keyword arguments

This syntax means the method should be treated as a method that
uses keyword arguments, but no specific keyword arguments are
supported, and therefore calling the method with keyword arguments
will raise an ArgumentError.  It is still allowed to double splat
an empty hash when calling the method, as that does not pass
any keyword arguments.

compile.c

@@ -1700,6 +1700,9 @@ iseq_set_arguments(rb_iseq_t *iseq, LINK_ANCHOR *const optargs, const NODE *cons
 	    body->param.keyword = keyword;
 	    body->param.flags.has_kwrest = TRUE;
 	}
+	else if (args->no_kwarg) {
+	    body->param.flags.accepts_no_kwarg = TRUE;
+	}
 
 	if (block_id) {
 	    body->param.block_start = arg_size++;

node.h

@@ -450,6 +450,7 @@ struct rb_args_info {
     NODE *kw_rest_arg;
 
     NODE *opt_args;
+    int no_kwarg;
 };
 
 struct rb_ary_pattern_info {

parse.y

@@ -3245,6 +3245,10 @@ block_args_tail	: f_block_kwarg ',' f_kwrest opt_f_block_arg
 		    {
 			$$ = new_args_tail(p, Qnone, $1, $2, &@1);
 		    }
+		| f_no_kwarg opt_f_block_arg
+		    {
+			$$ = new_args_tail(p, Qnone, rb_intern("nil"), $2, &@1);
+		    }
 		| f_block_arg
 		    {
 			$$ = new_args_tail(p, Qnone, Qnone, $1, &@1);
@@ -4712,6 +4716,10 @@ args_tail	: f_kwarg ',' f_kwrest opt_f_block_arg
 		    {
 			$$ = new_args_tail(p, Qnone, $1, $2, &@1);
 		    }
+		| f_no_kwarg opt_f_block_arg
+		    {
+			$$ = new_args_tail(p, Qnone, rb_intern("nil"), $2, &@1);
+		    }
 		| f_block_arg
 		    {
 			$$ = new_args_tail(p, Qnone, Qnone, $1, &@1);
@@ -4968,6 +4976,9 @@ kwrest_mark	: tPOW
 		| tDSTAR
 		;
 
+f_no_kwarg	: kwrest_mark keyword_nil
+		;
+
 f_kwrest	: kwrest_mark tIDENTIFIER
 		    {
 			arg_var(p, shadowing_lvar(p, get_id($2)));
@@ -11125,6 +11136,9 @@ new_args_tail(struct parser_params *p, NODE *kw_args, ID kw_rest_arg, ID block,
 	args->kw_rest_arg = NEW_DVAR(kw_rest_arg, loc);
 	args->kw_rest_arg->nd_cflag = kw_bits;
     }
+    else if (kw_rest_arg == rb_intern("nil")) {
+	args->no_kwarg = 1;
+    }
     else if (kw_rest_arg) {
 	args->kw_rest_arg = NEW_DVAR(kw_rest_arg, loc);
     }

test/ruby/test_keyword.rb

@@ -126,6 +126,34 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal(1, f10(b: 42))
   end
 
+  def f11(**nil)
+    local_variables
+  end
+
+  def test_f11
+    h = {}
+
+    assert_equal([], f11)
+    assert_equal([], f11(**{}))
+    assert_equal([], f11(**h))
+  end
+
+  def f12(**nil, &b)
+    [b, local_variables]
+  end
+
+  def test_f12
+    h = {}
+    b = proc{}
+
+    assert_equal([nil, [:b]], f12)
+    assert_equal([nil, [:b]], f12(**{}))
+    assert_equal([nil, [:b]], f12(**h))
+    assert_equal([b, [:b]], f12(&b))
+    assert_equal([b, [:b]], f12(**{}, &b))
+    assert_equal([b, [:b]], f12(**h, &b))
+  end
+
   def test_method_parameters
     assert_equal([[:key, :str], [:key, :num]], method(:f1).parameters);
     assert_equal([[:req, :x], [:key, :str], [:key, :num]], method(:f2).parameters);

test/ruby/test_syntax.rb

@@ -249,6 +249,22 @@ class TestSyntax < Test::Unit::TestCase
     assert_syntax_error('def o.foo(@@foo: a) end', /class variable/)
   end
 
+  def test_keywords_specified_and_not_accepted
+    assert_syntax_error('def o.foo(a:, **nil) end', /unexpected/)
+    assert_syntax_error('def o.foo(a:, **nil, &b) end', /unexpected/)
+    assert_syntax_error('def o.foo(**a, **nil) end', /unexpected/)
+    assert_syntax_error('def o.foo(**a, **nil, &b) end', /unexpected/)
+    assert_syntax_error('def o.foo(**nil, **a) end', /unexpected/)
+    assert_syntax_error('def o.foo(**nil, **a, &b) end', /unexpected/)
+
+    assert_syntax_error('proc do |a:, **nil| end', /unexpected/)
+    assert_syntax_error('proc do |a:, **nil, &b| end', /unexpected/)
+    assert_syntax_error('proc do |**a, **nil| end', /unexpected/)
+    assert_syntax_error('proc do |**a, **nil, &b| end', /unexpected/)
+    assert_syntax_error('proc do |**nil, **a| end', /unexpected/)
+    assert_syntax_error('proc do |**nil, **a, &b| end', /unexpected/)
+  end
+
   def test_optional_self_reference
     bug9593 = '[ruby-core:61299] [Bug #9593]'
     o = Object.new

vm_args.c

@@ -702,6 +702,10 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
 	args->rest = Qfalse;
     }
 
+    if (kw_flag && iseq->body->param.flags.accepts_no_kwarg) {
+	rb_raise(rb_eArgError, "no keywords accepted");
+    }
+
     switch (arg_setup_type) {
       case arg_setup_method:
 	break; /* do nothing special */

vm_core.h

@@ -385,6 +385,7 @@ struct rb_iseq_constant_body {
 	    unsigned int has_block  : 1;
 
 	    unsigned int ambiguous_param0 : 1; /* {|a|} */
+	    unsigned int accepts_no_kwarg : 1;
 	} flags;
 
 	unsigned int size;