Change UNDEF Node structure

Change UNDEF Node to hold their items to keep the original grammar
structure.

For example:

```
undef a, b
```

Before:

```
@ NODE_BLOCK (id: 4, line: 1, location: (1,6)-(1,10))*
+- nd_head (1):
|   @ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,7))
|   +- nd_undef:
|       @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
|       +- string: :a
+- nd_head (2):
    @ NODE_UNDEF (id: 3, line: 1, location: (1,9)-(1,10))
    +- nd_undef:
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```

After:

```
@ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,10))*
+- nd_undefs:
    +- length: 2
    +- element (0):
    |   @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
    |   +- string: :a
    +- element (1):
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```

ast.c

@@ -333,6 +333,24 @@ dump_array(VALUE ast_value, const struct RNode_LIST *node)
     return ary;
 }
 
+static VALUE
+dump_parser_array(VALUE ast_value, rb_parser_ary_t *p_ary)
+{
+    VALUE ary;
+
+    if (p_ary->data_type != PARSER_ARY_DATA_NODE) {
+        rb_bug("unexpected rb_parser_ary_data_type: %d", p_ary->data_type);
+    }
+
+    ary = rb_ary_new();
+
+    for (long i = 0; i < p_ary->len; i++) {
+        rb_ary_push(ary, NEW_CHILD(ast_value, p_ary->data[i]));
+    }
+
+    return ary;
+}
+
 static VALUE
 var_name(ID id)
 {
@@ -577,7 +595,7 @@ node_children(VALUE ast_value, const NODE *node)
       case NODE_VALIAS:
         return rb_ary_new_from_args(2, ID2SYM(RNODE_VALIAS(node)->nd_alias), ID2SYM(RNODE_VALIAS(node)->nd_orig));
       case NODE_UNDEF:
-        return rb_ary_new_from_node_args(ast_value, 1, RNODE_UNDEF(node)->nd_undef);
+        return rb_ary_new_from_args(1, dump_parser_array(ast_value, RNODE_UNDEF(node)->nd_undefs));
       case NODE_CLASS:
         return rb_ary_new_from_node_args(ast_value, 3, RNODE_CLASS(node)->nd_cpath, RNODE_CLASS(node)->nd_super, RNODE_CLASS(node)->nd_body);
       case NODE_MODULE:

compile.c

@@ -10954,10 +10954,18 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const no
         break;
       }
       case NODE_UNDEF:{
-        ADD_INSN1(ret, node, putspecialobject, INT2FIX(VM_SPECIAL_OBJECT_VMCORE));
-        ADD_INSN1(ret, node, putspecialobject, INT2FIX(VM_SPECIAL_OBJECT_CBASE));
-        CHECK(COMPILE(ret, "undef arg", RNODE_UNDEF(node)->nd_undef));
-        ADD_SEND(ret, node, id_core_undef_method, INT2FIX(2));
+        const rb_parser_ary_t *ary = RNODE_UNDEF(node)->nd_undefs;
+
+        for (long i = 0; i < ary->len; i++) {
+            ADD_INSN1(ret, node, putspecialobject, INT2FIX(VM_SPECIAL_OBJECT_VMCORE));
+            ADD_INSN1(ret, node, putspecialobject, INT2FIX(VM_SPECIAL_OBJECT_CBASE));
+            CHECK(COMPILE(ret, "undef arg", ary->data[i]));
+            ADD_SEND(ret, node, id_core_undef_method, INT2FIX(2));
+
+            if (i < ary->len - 1) {
+                ADD_INSN(ret, node, pop);
+            }
+        }
 
         if (popped) {
             ADD_INSN(ret, node, pop);

node.c

@@ -162,6 +162,14 @@ parser_tokens_free(rb_ast_t *ast, rb_parser_ary_t *tokens)
     xfree(tokens);
 }
 
+static void
+parser_nodes_free(rb_ast_t *ast, rb_parser_ary_t *nodes)
+{
+    /* Do nothing for nodes because nodes are freed when rb_ast_t is freed */
+    xfree(nodes->data);
+    xfree(nodes);
+}
+
 static void
 free_ast_value(rb_ast_t *ast, void *ctx, NODE *node)
 {
@@ -206,6 +214,9 @@ free_ast_value(rb_ast_t *ast, void *ctx, NODE *node)
       case NODE_IMAGINARY:
         xfree(RNODE_IMAGINARY(node)->val);
         break;
+      case NODE_UNDEF:
+        parser_nodes_free(ast, RNODE_UNDEF(node)->nd_undefs);
+        break;
       default:
         break;
     }

node_dump.c

@@ -92,6 +92,9 @@
 #define F_NODE2(name, n, ann) \
     COMPOUND_FIELD1(#name, ann) {dump_node(buf, indent, comment, n);}
 
+#define F_ARRAY(name, type, ann) \
+    COMPOUND_FIELD1(#name, ann) {dump_parser_array(buf, indent, comment, type(node)->name);}
+
 #define ANN(ann) \
     if (comment) { \
         A_INDENT; A("| # " ann "\n"); \
@@ -164,6 +167,28 @@ dump_array(VALUE buf, VALUE indent, int comment, const NODE *node)
     F_NODE(nd_next, RNODE_LIST, "next element");
 }
 
+static void
+dump_parser_array(VALUE buf, VALUE indent, int comment, const rb_parser_ary_t *ary)
+{
+    int field_flag;
+    const char *next_indent = default_indent;
+
+    if (ary->data_type != PARSER_ARY_DATA_NODE) {
+        rb_bug("unexpected rb_parser_ary_data_type: %d", ary->data_type);
+    }
+
+    F_CUSTOM1(length, "length") { A_LONG(ary->len); }
+    for (long i = 0; i < ary->len; i++) {
+        if (i == ary->len - 1) LAST_NODE;
+        A_INDENT;
+        rb_str_catf(buf, "+- element (%s%ld):\n",
+                    comment ? "statement #" : "", i);
+        D_INDENT;
+        dump_node(buf, indent, comment, ary->data[i]);
+        D_DEDENT;
+    }
+}
+
 static void
 dump_node(VALUE buf, VALUE indent, int comment, const NODE * node)
 {
@@ -896,10 +921,10 @@ dump_node(VALUE buf, VALUE indent, int comment, const NODE * node)
 
       case NODE_UNDEF:
         ANN("method undef statement");
-        ANN("format: undef [nd_undef]");
+        ANN("format: undef [nd_undefs]");
         ANN("example: undef foo");
         LAST_NODE;
-        F_NODE(nd_undef, RNODE_UNDEF, "old name");
+        F_ARRAY(nd_undefs, RNODE_UNDEF, "nd_undefs");
         return;
 
       case NODE_CLASS:

parse.y

@@ -2487,7 +2487,6 @@ rb_parser_string_hash_cmp(rb_parser_string_t *str1, rb_parser_string_t *str2)
             memcmp(ptr1, ptr2, len1) != 0);
 }
 
-#ifndef RIPPER
 static void
 rb_parser_ary_extend(rb_parser_t *p, rb_parser_ary_t *ary, long len)
 {
@@ -2503,7 +2502,7 @@ rb_parser_ary_extend(rb_parser_t *p, rb_parser_ary_t *ary, long len)
 
 /*
  * Do not call this directly.
- * Use rb_parser_ary_new_capa_for_script_line() or rb_parser_ary_new_capa_for_ast_token() instead.
+ * Use rb_parser_ary_new_capa_for_XXX() instead.
  */
 static rb_parser_ary_t *
 parser_ary_new_capa(rb_parser_t *p, long len)
@@ -2524,6 +2523,7 @@ parser_ary_new_capa(rb_parser_t *p, long len)
     return ary;
 }
 
+#ifndef RIPPER
 static rb_parser_ary_t *
 rb_parser_ary_new_capa_for_script_line(rb_parser_t *p, long len)
 {
@@ -2539,10 +2539,19 @@ rb_parser_ary_new_capa_for_ast_token(rb_parser_t *p, long len)
     ary->data_type = PARSER_ARY_DATA_AST_TOKEN;
     return ary;
 }
+#endif
+
+static rb_parser_ary_t *
+rb_parser_ary_new_capa_for_node(rb_parser_t *p, long len)
+{
+    rb_parser_ary_t *ary = parser_ary_new_capa(p, len);
+    ary->data_type = PARSER_ARY_DATA_NODE;
+    return ary;
+}
 
 /*
  * Do not call this directly.
- * Use rb_parser_ary_push_script_line() or rb_parser_ary_push_ast_token() instead.
+ * Use rb_parser_ary_push_XXX() instead.
  */
 static rb_parser_ary_t *
 parser_ary_push(rb_parser_t *p, rb_parser_ary_t *ary, rb_parser_ary_data val)
@@ -2554,6 +2563,7 @@ parser_ary_push(rb_parser_t *p, rb_parser_ary_t *ary, rb_parser_ary_data val)
     return ary;
 }
 
+#ifndef RIPPER
 static rb_parser_ary_t *
 rb_parser_ary_push_ast_token(rb_parser_t *p, rb_parser_ary_t *ary, rb_parser_ast_token_t *val)
 {
@@ -2571,7 +2581,18 @@ rb_parser_ary_push_script_line(rb_parser_t *p, rb_parser_ary_t *ary, rb_parser_s
     }
     return parser_ary_push(p, ary, val);
 }
+#endif
 
+static rb_parser_ary_t *
+rb_parser_ary_push_node(rb_parser_t *p, rb_parser_ary_t *ary, NODE *val)
+{
+    if (ary->data_type != PARSER_ARY_DATA_NODE) {
+        rb_bug("unexpected rb_parser_ary_data_type: %d", ary->data_type);
+    }
+    return parser_ary_push(p, ary, val);
+}
+
+#ifndef RIPPER
 static void
 rb_parser_ast_token_free(rb_parser_t *p, rb_parser_ast_token_t *token)
 {
@@ -2593,6 +2614,9 @@ rb_parser_ary_free(rb_parser_t *p, rb_parser_ary_t *ary)
       case PARSER_ARY_DATA_SCRIPT_LINE:
         foreach_ary(data) {rb_parser_string_free(p, *data);}
         break;
+      case PARSER_ARY_DATA_NODE:
+        /* Do nothing because nodes are freed when rb_ast_t is freed */
+        break;
       default:
         rb_bug("unexpected rb_parser_ary_data_type: %d", ary->data_type);
         break;
@@ -3774,8 +3798,8 @@ undef_list	: fitem
                     }
                 | undef_list ',' {SET_LEX_STATE(EXPR_FNAME|EXPR_FITEM);} fitem
                     {
-                        NODE *undef = NEW_UNDEF($4, &@4);
-                        $$ = block_append(p, $1, undef);
+                        nd_set_last_loc($1, @4.end_pos);
+                        rb_parser_ary_push_node(p, RNODE_UNDEF($1)->nd_undefs, $4);
                     /*% ripper: rb_ary_push($:1, $:4) %*/
                     }
                 ;
@@ -12286,7 +12310,8 @@ static rb_node_undef_t *
 rb_node_undef_new(struct parser_params *p, NODE *nd_undef, const YYLTYPE *loc)
 {
     rb_node_undef_t *n = NODE_NEWNODE(NODE_UNDEF, rb_node_undef_t, loc);
-    n->nd_undef = nd_undef;
+    n->nd_undefs = rb_parser_ary_new_capa_for_node(p, 1);
+    rb_parser_ary_push_node(p, n->nd_undefs, nd_undef);
 
     return n;
 }

rubyparser.h

@@ -225,7 +225,8 @@ typedef void* rb_parser_ary_data;
 
 enum rb_parser_ary_data_type {
     PARSER_ARY_DATA_AST_TOKEN = 1,
-    PARSER_ARY_DATA_SCRIPT_LINE
+    PARSER_ARY_DATA_SCRIPT_LINE,
+    PARSER_ARY_DATA_NODE
 };
 
 typedef struct rb_parser_ary {
@@ -885,7 +886,7 @@ typedef struct RNode_VALIAS {
 typedef struct RNode_UNDEF {
     NODE node;
 
-    struct RNode *nd_undef;
+    rb_parser_ary_t *nd_undefs;
 } rb_node_undef_t;
 
 typedef struct RNode_CLASS {