[Bug c++/101872] static_cast succeeds in CRTP with incorrect type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101872

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Original testcase (since we won't know if godbolt will be around in 10-20 years
...):

#include <tuple>

template <template <typename> typename ContainerImpl, typename Policy>
struct BaseContainer {
    using Impl = ContainerImpl<Policy>; // this isn't our actual Impl (missing
Handlers template args...)...

    Impl & impl() { return static_cast<Impl&>(*this); } // this shouldn't
compile

    int start() {
        return impl().on_start(); // ... so impl() casts this to incorrect
value
    }
};

template <typename Policy, typename ...Handlers>
struct BaseUiContainer : BaseContainer<BaseUiContainer, Policy> {

    int on_start() {
        return m_x * 2; // ... and because of the wrong cast this->m_x is
pointing elsewhere
    }

private:
    std::tuple<Handlers...> m_handlers; // comment this out to get the correct
output of 4
    int m_x = 2;
};

struct Handler {
    char a[128];
};

template <typename Policy>
struct MyUiContainer : BaseUiContainer<Policy, Handler> {
};

int main() {
    struct P {};
    MyUiContainer<P> container;
    return container.start();
}