Hmm. Let me first check that I've understood what you care about

    struct First(this: i8, that: i64)
    struct Second(this: i8, that: i8)
    struct Third(that: i64, this: i8)
    struct Fourth(this: i8, that: i64)
    struct Fifth(some: i8, other: i64)

You want First and Fourth as the same type, but Second and Third are different - how about Fifth?

I see that this is different from Rust's existing product types, in which First and Fourth are always different types.

Second though, can you give me some examples where I'd want this? I can't say I have ever wished I had this, but that might be a different experience.