error msg for #13864 (#13962)

Co-authored-by: cooldome <ariabushenko@bk.ru>

compiler/semexprs.nim

@@ -189,6 +189,11 @@ proc isCastable(conf: ConfigRef; dst, src: PType): bool =
     return false
   if skipTypes(src, abstractInst-{tyTypeDesc}).kind == tyTypeDesc:
     return false
+  if conf.selectedGC in {gcArc, gcOrc}:
+    let d = skipTypes(dst, abstractInst)
+    let s = skipTypes(src, abstractInst)
+    if d.kind == tyRef and s.kind == tyRef and s[0].isFinal != d[0].isFinal: 
+      return false
 
   var dstSize, srcSize: BiggestInt
   dstSize = computeSize(conf, dst)

tests/arc/tref_cast_error.nim

@@ -0,0 +1,15 @@
+discard """
+  cmd: "nim c --gc:arc $file"
+  errormsg: "expression cannot be cast to ref RootObj"
+  joinable: false
+"""
+
+type Variant* = object
+    refval: ref RootObj
+
+proc newVariant*[T](val: T): Variant =
+    let pt = T.new()
+    pt[] = val
+    result = Variant(refval: cast[ref RootObj](pt))
+
+var v = newVariant(@[1, 2, 3])