Do not autosplat when calling proc with empty keyword splat

With the removal of the splatted argument when using an empty
keyword splat, the autosplat code considered an empty keyword
splat the same as no argument at all.  However, that results
in autosplat behavior changing dependent on the content of
the splatted hash, which is not what anyone would expect or
want.  This change always skips an autosplat if keywords were
provided.

Fixes [Bug #16560]

(cherry picked from commit c1d8829ef5)

test/ruby/test_proc.rb

@@ -1087,6 +1087,26 @@ class TestProc < Test::Unit::TestCase
     assert_equal([1,2,[3],4,5], r, "[ruby-core:19485]")
   end
 
+  def test_proc_autosplat
+    def self.a(arg, kw)
+      yield arg
+      yield arg, **kw
+      yield arg, kw
+    end
+
+    arr = []
+    a([1,2,3], {}) do |arg1, arg2=0|
+      arr << [arg1, arg2]
+    end
+    assert_equal([[1, 2], [[1, 2, 3], 0], [[1, 2, 3], {}]], arr)
+
+    arr = []
+    a([1,2,3], a: 1) do |arg1, arg2=0|
+      arr << [arg1, arg2]
+    end
+    assert_equal([[1, 2], [[1, 2, 3], {a: 1}], [[1, 2, 3], {a: 1}]], arr)
+  end
+
   def test_parameters
     assert_equal([], proc {}.parameters)
     assert_equal([], proc {||}.parameters)

vm_args.c

@@ -725,10 +725,10 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
 {
     const int min_argc = iseq->body->param.lead_num + iseq->body->param.post_num;
     const int max_argc = (iseq->body->param.flags.has_rest == FALSE) ? min_argc + iseq->body->param.opt_num : UNLIMITED_ARGUMENTS;
-    int opt_pc = 0;
     int given_argc;
     int kw_splat = FALSE;
     unsigned int kw_flag = ci->flag & (VM_CALL_KWARG | VM_CALL_KW_SPLAT);
+    int opt_pc = 0, allow_autosplat = !kw_flag;
     struct args_info args_body, *args;
     VALUE keyword_hash = Qnil;
     VALUE * const orig_sp = ec->cfp->sp;
@@ -879,6 +879,7 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
 	break; /* do nothing special */
       case arg_setup_block:
 	if (given_argc == 1 &&
+            allow_autosplat &&
 	    (min_argc > 0 || iseq->body->param.opt_num > 1 ||
 	     iseq->body->param.flags.has_kw || iseq->body->param.flags.has_kwrest) &&
 	    !iseq->body->param.flags.ambiguous_param0 &&