[JacobCarlborg]

This is very common in D and D doesn't not have any macro system. It uses regular syntax (more or less). D was doing this way before Zig existed and before C++ had constexpr. Simple example of platform specific members:

  struct Socket
  {
      version (Posix)
          int handle;
      else version (Windows)
          SOCKET handle;
      else
          static assert(false, "Unsupported platform");
  }

Another example is the checked numeric type [1] in the D standard library. It takes two types as parameters. The first being the underlying type and the second being a "hook" type. The hook type allows the user to decide what should happen in various error conditions, like overflow, divide by zero and so on. The implementation is written in a Design By Introspection style and inspects the hook type and adopts its implementation depending on what hooks it provides. If the hook type implements the hook for overflow, that hook will be executed, otherwise it falls back to some default behavior.

[1] https://dlang.org/phobos/std_experimental_checkedint.html