[Bug c++/86282] Evaluation order in if constexpr expression seems to be irrelevant for evaluating type traits

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86282

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Using std::conjunction:

template<typename T>
  using has_begin2 = std::bool_constant<has_begin<T>>;

template<typename T>
  struct has_nested_begin
  {
    // ill-formed unless has_begin<T> is true:
    static constexpr bool value =
has_begin<std::remove_reference_t<decltype(*std::begin(std::declval<T>()))>>;
  };

template<typename T>
int f(T)
{
    if (std::conjunction_v<has_begin2<T>, has_nested_begin<T>>)
      return 0;
    return 1;
}

When has_begin2<T>::value is false, has_nested_begin<T>::value is not needed,
so is not instantiated.