https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- Comment #42 from Rich Felker <bugdal at aerifal dot cx> --- > I'm not saying that such an implementation will be a good idea, but just a > remark: You could, in fact, keep restrict for the arguments in this case, > because the object pointed to by src and dest is not accessed at all when > src==dest. So this is correct code according to the standard. (The exact > semantics of restrict are a bit involved...) Nope, UB is invoked as soon as you evaluate src==dest, even with no dereferencing. The semantics of restrict are such that the behavior of the code must be unchanged if the pointer were replaced to a pointer to a relocated copy of the pointed-to object. Since this would alter the result of the == operator, that constraint is not satisfied and thereby the behavior is undefined.
