UBSAN-safe way to convert from unsigned int to int

[Florian Weimer via Gcc-help <gcc-help@xxxxxxxxxxx>]

Is there a concise way to convert X to a value Y of type int, so that

  X == (unsigned int) Y

(assuming that X is of type unsigned int to start with)?  GCC supports
this as an extension, using Y = (int) X, but this extension is
apparently unavailable if UBSAN is active.

Thanks,
Florian