[anderskaseorg]

We could choose to design the language as I wrote where the constraint is implicitly present and variance consequently ought to preserve it, or as you wrote where it is not present at all and variance is unsound. But we want Rust to be sound, so we should choose the former design. We would need a very compelling reason to prefer the latter—for example, some evidence that “it would break too much code” or “it’s impossible to implement”—but I don’t see anyone claiming to actually have such evidence. The only reason the implementation has not been updated to align with the sound design is that it takes work and that work is still ongoing.