[Bug #21983] Make &nil distinguishable from nonexistent block arg

Author: nobu

ast.c

@@ -698,7 +698,7 @@ node_children(VALUE ast_value, const NODE *node)
                                             : var_name(ainfo->rest_arg)),
                                         (ainfo->no_kwarg ? Qfalse : NEW_CHILD(ast_value, (NODE *)ainfo->kw_args)),
                                         (ainfo->no_kwarg ? Qfalse : NEW_CHILD(ast_value, ainfo->kw_rest_arg)),
-                                        var_name(ainfo->block_arg));
+                                        (ainfo->no_blockarg ? Qfalse : var_name(ainfo->block_arg)));
         }
       case NODE_SCOPE:
         {

test/ruby/test_ast.rb

@@ -720,6 +720,7 @@ def test_block_arg
     assert_equal(nil, block_arg.call(''))
     assert_equal(:block, block_arg.call('&block'))
     assert_equal(:&, block_arg.call('&'))
+    assert_equal(false, block_arg.call('&nil'))
   end
 
   def test_keyword_rest