fixes #20836; fixes #20833; fixes unsafeNew (#20841)

* fixes #20836; fixes `unsafeNew`

* fixes #20833

compiler/semmagic.nim

@@ -586,7 +586,10 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     else:
       result = plugin(c, n)
   of mNew:
-    result = addDefaultFieldForNew(c, n)
+    if n[0].sym.name.s == "unsafeNew": # special case for unsafeNew
+      result = n
+    else:
+      result = addDefaultFieldForNew(c, n)
   of mNewFinalize:
     result = semNewFinalize(c, n)
   of mDestroy:

tests/objects/tunsafenew.nim

@@ -0,0 +1,10 @@
+discard """
+  errormsg: "conversion from int literal(-1) to Natural is invalid"
+"""
+
+type
+  Obj = object
+    case b: bool
+    else: discard
+var o: ref Obj
+unsafeNew(o, -1)