All uses auto&& in function signatures potentially undefined behavior

[kelbon]

cppfront/include/cpp2util.h

Line 719 in 2f37277

constexpr auto operator_as( std::variant<Ts...> const& x ) -> auto&& {

Its not equal to decltype(auto).
If returns by value, then auto&& generates dangling rvalue reference, while decltype(auto) returns by value as needed

https://godbolt.org/z/7Gfr5fb1d

template< typename C, typename X >
    requires std::is_same_v<C, X>
auto as( X const& x ) -> auto&& {
    return 5;
}

int main() {
    static_assert(std::is_same_v<decltype(as<int>(5)), int&&>);
}

[JohelEGP]

This is https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#f45-dont-return-a-t.

[hsutter]

Good catch, thanks!