Fix assertion failure with anonymous splats

When calling a method that accepts an anonymous splat and literal
keywords without any arguments, an assertion failure was previously
raised. Set rest_index to 0 when setting rest to the frozen hash,
so the args_argc calculation is accurate.

While here, add more tests for methods with anonymous splats with
and without keywords and keyword splats to confirm behavior is
correct.

Also add a basic bootstrap test that would hit the previous assertion
failure.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

bootstraptest/test_method.rb

@@ -1419,3 +1419,11 @@ assert_equal 'ok', %q{
     "ok"
   end
 }
+
+assert_equal 'ok', <<~RUBY
+  def test(*, kw: false)
+    "ok"
+  end
+
+  test
+RUBY

test/ruby/test_call.rb

@@ -374,6 +374,55 @@ class TestCall < Test::Unit::TestCase
     assert_equal({splat_modified: false}, b)
   end
 
+  def test_anon_splat
+    r2kh = Hash.ruby2_keywords_hash(kw: 2)
+    r2kea = [r2kh]
+    r2ka = [1, r2kh]
+
+    def self.s(*) ->(*a){a}.call(*) end
+    assert_equal([], s)
+    assert_equal([1], s(1))
+    assert_equal([{kw: 2}], s(kw: 2))
+    assert_equal([{kw: 2}], s(**{kw: 2}))
+    assert_equal([1, {kw: 2}], s(1, kw: 2))
+    assert_equal([1, {kw: 2}], s(1, **{kw: 2}))
+    assert_equal([{kw: 2}], s(*r2kea))
+    assert_equal([1, {kw: 2}], s(*r2ka))
+
+    singleton_class.remove_method(:s)
+    def self.s(*, kw: 0) [*->(*a){a}.call(*), kw] end
+    assert_equal([0], s)
+    assert_equal([1, 0], s(1))
+    assert_equal([2], s(kw: 2))
+    assert_equal([2], s(**{kw: 2}))
+    assert_equal([1, 2], s(1, kw: 2))
+    assert_equal([1, 2], s(1, **{kw: 2}))
+    assert_equal([2], s(*r2kea))
+    assert_equal([1, 2], s(*r2ka))
+
+    singleton_class.remove_method(:s)
+    def self.s(*, **kw) [*->(*a){a}.call(*), kw] end
+    assert_equal([{}], s)
+    assert_equal([1, {}], s(1))
+    assert_equal([{kw: 2}], s(kw: 2))
+    assert_equal([{kw: 2}], s(**{kw: 2}))
+    assert_equal([1, {kw: 2}], s(1, kw: 2))
+    assert_equal([1, {kw: 2}], s(1, **{kw: 2}))
+    assert_equal([{kw: 2}], s(*r2kea))
+    assert_equal([1, {kw: 2}], s(*r2ka))
+
+    singleton_class.remove_method(:s)
+    def self.s(*, kw: 0, **kws) [*->(*a){a}.call(*), kw, kws] end
+    assert_equal([0, {}], s)
+    assert_equal([1, 0, {}], s(1))
+    assert_equal([2, {}], s(kw: 2))
+    assert_equal([2, {}], s(**{kw: 2}))
+    assert_equal([1, 2, {}], s(1, kw: 2))
+    assert_equal([1, 2, {}], s(1, **{kw: 2}))
+    assert_equal([2, {}], s(*r2kea))
+    assert_equal([1, 2, {}], s(*r2ka))
+  end
+
   def test_kwsplat_block_eval_order
     def self.t(**kw, &b) [kw, b] end
 

vm_args.c

@@ -896,6 +896,7 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
     if (ISEQ_BODY(iseq)->param.flags.has_rest) {
         if (UNLIKELY(ISEQ_BODY(iseq)->param.flags.anon_rest && args->argc == 0 && !args->rest && !ISEQ_BODY(iseq)->param.flags.has_post)) {
            *(locals + ISEQ_BODY(iseq)->param.rest_start) = args->rest = rb_cArray_empty_frozen;
+           args->rest_index = 0;
         }
         else {
             args_setup_rest_parameter(args, locals + ISEQ_BODY(iseq)->param.rest_start);