https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101566
--- Comment #4 from Dale Weiler <weilercdale at gmail dot com> ---
(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?
