X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/af534f9a2ed86603fb6dd245e61d4bbbc4736bd8..093d51990bbbf55e926d9a721b9b2219d1e03922:/personal/_posts/2018-01-31-sharing-for-a-lifetime.md

diff --git a/personal/_posts/2018-01-31-sharing-for-a-lifetime.md b/personal/_posts/2018-01-31-sharing-for-a-lifetime.md
index 8dd10da..0b7a454 100644
--- a/personal/_posts/2018-01-31-sharing-for-a-lifetime.md
+++ b/personal/_posts/2018-01-31-sharing-for-a-lifetime.md
@@ -135,7 +135,7 @@ This is not so say that the last word on interior mutability has been spoken; th
 
 Either way, this model has certainly helped me not only in the formal work on RustBelt, but also in thinking about the Rust type system itself.
 For example, I briefly wondered why `Cell` is not covariant.
-Indeed, if `U <: T`, then it is perfectly safe to turn a `Cell<T>` into a `Cell<U>` -- that is to say, whenever some sequence of bytes is valid at `Cell<T>`, it is also valid at `Cell<U>`.
+Indeed, if `T <: U`, then it is perfectly safe to turn a `Cell<T>` into a `Cell<U>` -- that is to say, whenever some sequence of bytes is valid at `Cell<T>`, it is also valid at `Cell<U>`.
 (In fact, the owned invariant for `Cell<T>` is *exactly the same* as the one for `T`!)
 That's precisely what we need for covariance, right?
 Well, not exactly -- because our types have two invariants, we also have to require that whenever a pointer is a shared `Cell<T>`, it is a shared `Cell<U>`, and that is decisively *not* true!