cannot call template/macros with varargs[typed] to varargs[untyped]

[slangmgh]

The following code works with develop version of v0.18.0, but doesn't works for v 0.19.0

import macros

proc convert_hidden_stdconv(args: NimNode): NimNode =
   var n = args
   while n.len == 1 and n[0].kind == nnkHiddenStdConv:
      n = n[0][1]
   return n

macro t2(s: int, v: varargs[untyped]): untyped =
   let v = convert_hidden_stdconv(v)
   echo v.treeRepr
   let (v1, v2) = (v[0], v[1])
   quote do:
      echo `v1`, " ", `v2`

template t1(s: int, v: varargs[typed]) =
   #static:
   #   dumpTree v
   echo s
   t2(s, v)

when isMainModule:
   t1(10, "hello", 18.0)

The compiler error is:

Error: unhandled exception: D:\Tool\Nim\compiler\sigmatch.nim(2236, 18) `n.sons[a].sons[0].kind == nkEmpty and n.sons[a].sons[1].kind == nkArgList and
    len(n.sons[a].sons[1]) == 0`  [AssertionError]

If we change the argument of the t1, t2 to both varargs[typed] or varargs[untyped], it works. Even if I change the t1 as varargs[untyped] and t2 as varargs[typed], it still works. Only varargs[untyped] call varargs[typed] doesn't work.

[slangmgh]

I comment the assertion lines in the sigmatch.nim, then the code compiles and works.

[slangmgh]

The code is added to fix #8715

[mratsim]

cc @narimiran @Araq backport to 0.19.4?