Warn local variables which conflict with new numbered parameters

defs/id.def

@@ -57,6 +57,19 @@ firstline, predefined = __LINE__+1, %[\
   nil
 
   _                                                     UScore
+
+  # MUST be successive
+  _0  NUMPARAM_0
+  _1  NUMPARAM_1
+  _2  NUMPARAM_2
+  _3  NUMPARAM_3
+  _4  NUMPARAM_4
+  _5  NUMPARAM_5
+  _6  NUMPARAM_6
+  _7  NUMPARAM_7
+  _8  NUMPARAM_8
+  _9  NUMPARAM_9
+
   "/*NULL*/"                                            NULL
   empty?
   eql?

parse.y

@@ -11652,6 +11652,10 @@ arg_var(struct parser_params *p, ID id)
 static void
 local_var(struct parser_params *p, ID id)
 {
+    if (id >= idNUMPARAM_0 && id <= idNUMPARAM_9) {
+	rb_warn1("`_%d' is used as numbered parameter",
+		 WARN_I((int)(id - idNUMPARAM_0)));
+    }
     vtable_add(p->lvtbl->vars, id);
     if (p->lvtbl->used) {
 	vtable_add(p->lvtbl->used, (ID)p->ruby_sourceline);