http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55137
--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> 2012-11-08 20:10:09 UTC --- (In reply to comment #3) > If it is valid, the series of foldings that result into the overflow are first > folding sizeof (int) - 1UL into sizeof (int) + 18446744073709551615UL and > later > on conversion of -1 + (int) (sizeof (int) + 18446744073709551615UL) to > -1 + (int) sizeof (int) + (int) 18446744073709551615UL. Under C++ semantics, this folding introduces implementation-defined behavior, not undefined behavior, so it's still a constant-expression; conversion to a signed integer is different from overflow in the language. I have been working around this difference between back end and language semantics in cxx_eval_constant_expression by unsetting TREE_OVERFLOW after folding NOP_EXPR, but that doesn't help this example. The semantics of overflow and conversion to signed integer are different in C as well; the only difference from C++ is that C allows the implementation to trap *or* produce an implementation-defined value. Perhaps we could distinguish them in the compiler as well...
