On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <pola...@redhat.com> wrote: > This is a similar problem to 83116: we'd cached a constexpr call, but after a > store the result had become invalid, yet we used the wrong result again when > encountering the same call later. This resulted in evaluating a THROW_EXPR > which doesn't work. Details in > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5 > > The fix for 83116 didn't work here, because when evaluating the body of the > ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do > cache.
> It seems that we may no longer rely on the constexpr call table when we > do cxx_eval_store_expression, because that just rewrites *valp, i.e. the > value of an object. Might be too big a hammer again, but I couldn't think > of how I could guard the caching of a constexpr call. > This doesn't manifest in C++14 because build_special_member_call in C++17 is > more aggressive with copy elisions (as required by P0135 which changed how we > view prvalues). In C++14 build_special_member_call produces a CALL_EXPR, so > expand_default_init calls maybe_constant_init, for which STRICT is false, so > we avoid caching as per 83116. So it sounds like the problem is using cxx_constant_value for the diagnostic when it has different semantics from the maybe_constant_init that follows right after. I guess we want a cxx_constant_init function that is a hybrid of the two. Jason
