Argument forwarding with `...` now supports leading arguments.

```ruby
def method_missing(meth, ...)
  send(:"do_#{meth}", ...)
end
```

mrbgems/mruby-compiler/core/parse.y

@@ -2397,6 +2397,22 @@ paren_args      : '(' opt_call_args ')'
                     {
                       $$ = $2;
                     }
+                | '(' args comma tBDOT3 rparen
+                    {
+#if 1
+                      mrb_sym r = MRB_OPSYM(mul);
+                      mrb_sym b = MRB_OPSYM(and);
+                      $$ = cons(push($2, new_splat(p, new_lvar(p, r))),
+                                new_block_arg(p, new_lvar(p, b)));
+#else
+                      mrb_sym r = MRB_OPSYM(mul);
+                      mrb_sym k = MRB_OPSYM(pow);
+                      mrb_sym b = MRB_OPSYM(and);
+                      $$ = cons(list2(push($2, new_splat(p, new_lvar(p, r))),
+                                      new_kw_hash(p, list1(cons(new_kw_rest_args(p, 0), new_lvar(p, k))))),
+                                new_block_arg(p, new_lvar(p, b)));
+#endif
+                    }
                 | '(' tBDOT3 rparen
                     {
 #if 1
@@ -3493,6 +3509,24 @@ f_arglist_paren : '(' f_args rparen
                       p->lstate = EXPR_BEG;
                       p->cmd_start = TRUE;
                     }
+                | '(' f_arg ',' tBDOT3 rparen
+                    {
+#if 1
+                      /* til real keyword args implemented */
+                      mrb_sym r = MRB_OPSYM(mul);
+                      mrb_sym b = MRB_OPSYM(and);
+                      local_add_f(p, r);
+                      $$ = new_args(p, $2, 0, r, 0,
+                                    new_args_tail(p, 0, 0, b));
+#else
+                      mrb_sym r = MRB_OPSYM(mul);
+                      mrb_sym k = MRB_OPSYM(pow);
+                      mrb_sym b = MRB_OPSYM(and);
+                      local_add_f(p, r); local_add_f(p, k);
+                      $$ = new_args(p, $2, 0, r, 0,
+                                    new_args_tail(p, 0, new_kw_rest_args(p, nsym(k)), b));
+#endif
+                    }
                 | '(' tBDOT3 rparen
                     {
 #if 1

test/t/syntax.rb

@@ -686,3 +686,22 @@ assert('_0 is not numbered parameter') do
   _0 = :l
   assert_equal(:l, ->{_0}.call)
 end
+
+assert('argument forwarding') do
+  c = Class.new {
+    def a0(*a,&b)
+      assert_equal([1,2,3], a)
+      assert_not_nil(b)
+    end
+    def a(...)
+      a0(...)
+    end
+    def b(a,...)
+      assert_equal(a,1)
+      a0(1,...)
+    end
+  }
+  o = c.new
+  o.a(1,2,3){}
+  o.b(1,2,3){}
+end