C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)

[Jason Merrill]

On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@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