fix #19266; allow reassign discriminant field (#19567)

* add inUncheckedAssignSection

* add one more test

tests/objvariant/treassign.nim

@@ -34,3 +34,25 @@ proc passToVar(x: var Token) = discard
   t.curr = TokenObject(kind: t.curr.kind, foo: "abc")
 
   t.curr.kind = Token.foo
+
+
+block:
+  type
+    TokenKind = enum
+      strLit, intLit
+    Token = object
+      case kind*: TokenKind
+      of strLit:
+        s*: string
+      of intLit:
+        i*: int64
+
+  var t = Token(kind: strLit, s: "abc")
+
+  {.cast(uncheckedAssign).}:
+
+    # inside the 'cast' section it is allowed to assign to the 't.kind' field directly:
+    t.kind = intLit
+
+  {.cast(uncheckedAssign).}:
+    t.kind = strLit