https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114526
--- Comment #11 from Joseph S. Myers <jsm28 at gcc dot gnu.org> --- I think that simply failing to say whether a value of type X may be converted to type Y is clearly enough for it at least to be unspecified whether or when such conversions are possible in a cast at all (which is enough for rejecting the translation unit). And since no requirements are imposed relating to such conversions at either translation time or runtime, the definition of undefined behavior is met. If you try to cast from a struct to a union (for example), you violate a constraint. If you try to do a conversion between a pointer and another scalar type that's not one of the "may be converted" cases listed as allowing such a conversion, you have undefined behavior through the lack of definition (if it's an implicit conversion rather than a cast, doing it as an implicit conversion violates a constraint unless it follows the requirements for assignment expressions.)
