[Bug c++/67762] [C++1z] 'not a constant expression" errors only with -fsanitize=undefined

[curdeius at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67762

--- Comment #8 from Curdeius Curdeius <curdeius at gmail dot com> ---
A different (rather small) reproduce.
https://godbolt.org/z/bz9sTd34o

It fails with all the versions of gcc from at least 7 (the above code needs
`auto` in template non-type parameter) to trunk from 2021-11-10.

Hope it helps!

```
#include <date/date.h>

#include <chrono>
#include <functional>

namespace detail {

static inline void validate(date::weekday v) noexcept {
    (void)v;
    assert(v.ok());
}

template <typename Duration, auto InitValue, auto GetValue = nullptr>
struct date_interval {
    using duration_type = Duration;

    duration_type begin{InitValue};

    [[nodiscard]] constexpr auto begin_value() const noexcept {
        if constexpr (GetValue != nullptr) {
            return std::invoke(GetValue, begin);
        } else {
            return begin;
        }
    }
};

}  // namespace detail

using days_interval = detail::date_interval<date::weekday, /*date::Sunday*/ 7,
                                            &date::weekday::c_encoding>;

int main() {
    days_interval days;
    return (int)days.begin_value();
}
```