update trait_solver_5

Author: BoxyUwU

code/examples/stderr/trait_solver_5.stderr

@@ -1,12 +0,0 @@
-error[E0119]: conflicting implementations of trait `Overlap<_>` for type `()`
- --> examples/trait_solver_5.rs:6:1
-  |
-5 | impl<T, U: Sub<T>> Overlap<T> for U {}
-  | ----------------------------------- first implementation here
-6 | impl<T> Overlap<T> for () {}
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
-  |
-  = note: downstream crates may implement trait `Sub<_>` for type `()`
-
-For more information about this error, try `rustc --explain E0119`.
-error: could not compile `code` (example "trait_solver_5") due to 1 previous error

code/examples/trait_solver_5.rs

@@ -1,8 +1,8 @@
-trait Super {}
-trait Sub<T>: Super {}
+#![crate_type = "lib"]
 
-trait Overlap<T> {}
+pub trait Super {}
+pub trait Sub<T>: Super {}
+
+pub trait Overlap<T> {}
 impl<T, U: Sub<T>> Overlap<T> for U {}
 impl<T> Overlap<T> for () {}
-
-fn main() {}

src/trait_solver/5.md

@@ -1,4 +1,4 @@
-# Trait Solver 5 @BoxyUwU @WaffleLapkin
+# Trait Solver 5 @BoxyUwU @WaffleLapkin @lcnr
 
 {{#include ../include/quiz-is-wip.md}}
 
@@ -13,6 +13,8 @@
 {{#include ../../code/examples/stderr/trait_solver_5.stderr}}
 ```
 
-<!-- FIXME: explanation (also: doesn't error on newer versions)-->
+In order for these impls to overlap the type `()` must implement `Sub<T>` for some type `T`, the `Sub` trait is public so in theory some downstream crate could implement `(): Sub<Local>`. However, the supertrait `Super` cannot be implement for the type `()` by any downstream crates as `impl Super for () {` would not pass the orphan check in downstream crates 
+
+If we were to introduce an `impl Super for () {` to this example then coherence would forbid this code.
 
 </details>