https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101566
--- Comment #5 from Dale Weiler <weilercdale at gmail dot com> ---
(In reply to Dale Weiler from comment #4)
> (In reply to Andrew Pinski from comment #3)
> > (In reply to Dale Weiler from comment #2)
> > > Ah, passing `T&` here instead of T does appear to avoid the issue, the
> > > question now becomes why does -fsanitize=undefined find nothing, and is
> > > the
> > > return type, i.e `declval(get<N>(t))` different in gcc compared to the
> > > other
> > > compiles. I would expect similar issues in the other compilers if this is
> > > returning a reference to the temporary given by the argument list of `get`
> >
> > So -fsanitize=undefined adds some extra code which just happens to cause GCC
> > not to optimize as much.
> >
> > Try -fsanitize=address instead which enables
> > -fsanitize-address-use-after-scope which should detect the problem but I see
> > it does not ....
>
> I'm unsure why this would be a temporary, given:
>
> auto lambda = [=](auto&& f) mutable -> decltype(auto) { return f(&ts...); };
>
> This should capture all `ts...` by copy, the `f(&ts...)` should reference
> the captures of `this` inside the lambda (of the compiler generated
> structure.)
>
> When `get` is called, the values from the lambda are returned by
> dereferencing since the function is called with pointers (synthesized with
> `&ts...` in the lambda). I verified this by printing
> `typeid(decltype(get<N>(t))).name()` passed to `__cxa_demangle`. There isn't
> any reference types, just to be sure that's not some runtime quirk, the
> compile-time result of `std::is_same_v<int, decltype(get<0>)>` is true.
>
> Where is the temporary happening?
This is curious, omitting `decltype(auto)` for get, as in just `auto` seems to
work around the issue as well.
template<Size N, typename T>
constexpr auto get(T tuple) { return *tuple(Get<N>{}); }
I'm a bit out of my element here, in that I don't understand the semantic
differences between the two and if that behavior is correct.
