Re: [boost] Metaprogramming: parameter pack expansion/evaluation trick

namespace detail { struct expand_impl final { template constexpr inline expand_impl(Arg&&...) noexcept {} }; }

template using expand = detail::expand_impl;

But unfortunately there's no way to force the compiler to evaluate expressions in constructor call from left to right. So it did not work on GCC (but work on Clang).

There is. Just use {} and it's guaranteed to evaluate the args from left to right.