Function instantiate 

pub fn instantiate(
    namespace: &[SmolStr],
    base: &str,
    args: &[DataType],
    span: Span,
    ctx: &mut TypeCtx<'_>,
) -> DataType

Resolves base<args> to an ordinary struct or enum, registering it the first time it is asked for.

Past this point nothing generic is left: the instantiation is a concrete type with concrete field types, and every later stage (field access, method dispatch, the artifact codec, the VM) treats it like any other.