How to declare a variable using std::meta::info of a non static data member inside a template for loop?

In this loop:

template for (constexpr std::meta::info MemberInfo : / non-static data members /) {
    using MemberType = [:MemberInfo:];

MemberInfo is a reflection which represents a non-static data member - it can only be spliced in certain contexts. Like obj.[:MemberInfo:] or &[:MemberInfo:]. It does not represent a type, which is what you need on the right-hand side of an alias declaration.

Hopefully a production implementation will provide a clear error message in this context, since it certainly can, but the one you're using is an experimental one. That will all get better over time.

What you need is a type — specifically a reflection representing a type. That is:

    using MemberType = [: type_of(MemberInfo) :];
    //                    ^~~~~~~

With that change, your code compiles.

Conceptually, you can think of this as the category error of writing this:

using MemberType = Position::X; // error: not a type

instead of this:

using MemberType = decltype(Position::X); // ok